diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index 6ee1dbe4ae3ee..75f384405bb2b 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -34,7 +34,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 }
                 ExprKind::Repeat(ref expr, ref count) => {
                     let expr = self.lower_expr(expr);
-                    let count = self.lower_anon_const(count);
+                    let count = self.lower_array_length(count);
                     hir::ExprKind::Repeat(expr, count)
                 }
                 ExprKind::Tup(ref elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 77738b2c5cc75..35eb716949a13 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -56,6 +56,7 @@ use rustc_hir::{ConstArg, GenericArg, ParamName};
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_query_system::ich::StableHashingContext;
 use rustc_session::lint::LintBuffer;
+use rustc_session::parse::feature_err;
 use rustc_session::utils::{FlattenNonterminals, NtToTokenstream};
 use rustc_session::Session;
 use rustc_span::hygiene::ExpnId;
@@ -1248,7 +1249,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 ))
             }
             TyKind::Array(ref ty, ref length) => {
-                hir::TyKind::Array(self.lower_ty(ty, itctx), self.lower_anon_const(length))
+                hir::TyKind::Array(self.lower_ty(ty, itctx), self.lower_array_length(length))
             }
             TyKind::Typeof(ref expr) => hir::TyKind::Typeof(self.lower_anon_const(expr)),
             TyKind::TraitObject(ref bounds, kind) => {
@@ -2039,6 +2040,26 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         self.expr_block(block, AttrVec::new())
     }
 
+    fn lower_array_length(&mut self, c: &AnonConst) -> hir::ArrayLen {
+        match c.value.kind {
+            ExprKind::Underscore => {
+                if self.sess.features_untracked().generic_arg_infer {
+                    hir::ArrayLen::Infer(self.lower_node_id(c.id), c.value.span)
+                } else {
+                    feature_err(
+                        &self.sess.parse_sess,
+                        sym::generic_arg_infer,
+                        c.value.span,
+                        "using `_` for array lengths is unstable",
+                    )
+                    .emit();
+                    hir::ArrayLen::Body(self.lower_anon_const(c))
+                }
+            }
+            _ => hir::ArrayLen::Body(self.lower_anon_const(c)),
+        }
+    }
+
     fn lower_anon_const(&mut self, c: &AnonConst) -> hir::AnonConst {
         self.with_new_scopes(|this| hir::AnonConst {
             hir_id: this.lower_node_id(c.id),
diff --git a/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs b/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs
index 3e7371179cc72..01f7868df3492 100644
--- a/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs
+++ b/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs
@@ -160,17 +160,17 @@ impl<'ll> DebugInfoBuilderMethods for Builder<'_, 'll, '_> {
         // the values should match the ones in the DWARF standard anyway.
         let op_deref = || unsafe { llvm::LLVMRustDIBuilderCreateOpDeref() };
         let op_plus_uconst = || unsafe { llvm::LLVMRustDIBuilderCreateOpPlusUconst() };
-        let mut addr_ops = SmallVec::<[_; 8]>::new();
+        let mut addr_ops = SmallVec::<[u64; 8]>::new();
 
         if direct_offset.bytes() > 0 {
             addr_ops.push(op_plus_uconst());
-            addr_ops.push(direct_offset.bytes() as i64);
+            addr_ops.push(direct_offset.bytes() as u64);
         }
         for &offset in indirect_offsets {
             addr_ops.push(op_deref());
             if offset.bytes() > 0 {
                 addr_ops.push(op_plus_uconst());
-                addr_ops.push(offset.bytes() as i64);
+                addr_ops.push(offset.bytes() as u64);
             }
         }
 
diff --git a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
index 6450b7490bd2b..f2782f84f557b 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
@@ -2108,7 +2108,7 @@ extern "C" {
         Builder: &DIBuilder<'a>,
         Val: &'a Value,
         VarInfo: &'a DIVariable,
-        AddrOps: *const i64,
+        AddrOps: *const u64,
         AddrOpsCount: c_uint,
         DL: &'a DILocation,
         InsertAtEnd: &'a BasicBlock,
@@ -2199,8 +2199,8 @@ extern "C" {
         Scope: &'a DIScope,
         InlinedAt: Option<&'a DILocation>,
     ) -> &'a DILocation;
-    pub fn LLVMRustDIBuilderCreateOpDeref() -> i64;
-    pub fn LLVMRustDIBuilderCreateOpPlusUconst() -> i64;
+    pub fn LLVMRustDIBuilderCreateOpDeref() -> u64;
+    pub fn LLVMRustDIBuilderCreateOpPlusUconst() -> u64;
 
     #[allow(improper_ctypes)]
     pub fn LLVMRustWriteTypeToString(Type: &Type, s: &RustString);
diff --git a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
index c5412affafe23..3ec9f3ca3b8c2 100644
--- a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
+++ b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
@@ -63,7 +63,7 @@ fn eval_body_using_ecx<'mir, 'tcx>(
         cid.instance,
         body,
         Some(&ret.into()),
-        StackPopCleanup::None { cleanup: false },
+        StackPopCleanup::Root { cleanup: false },
     )?;
 
     // The main interpreter loop.
diff --git a/compiler/rustc_const_eval/src/interpret/eval_context.rs b/compiler/rustc_const_eval/src/interpret/eval_context.rs
index 0fc3827d16c7e..0a8112da2aba8 100644
--- a/compiler/rustc_const_eval/src/interpret/eval_context.rs
+++ b/compiler/rustc_const_eval/src/interpret/eval_context.rs
@@ -156,11 +156,11 @@ pub enum StackPopCleanup {
     /// `ret` stores the block we jump to on a normal return, while `unwind`
     /// stores the block used for cleanup during unwinding.
     Goto { ret: Option<mir::BasicBlock>, unwind: StackPopUnwind },
-    /// Just do nothing: Used by Main and for TLS hooks in miri.
+    /// The root frame of the stack: nowhere else to jump to.
     /// `cleanup` says whether locals are deallocated. Static computation
     /// wants them leaked to intern what they need (and just throw away
     /// the entire `ecx` when it is done).
-    None { cleanup: bool },
+    Root { cleanup: bool },
 }
 
 /// State of a local variable including a memoized layout
@@ -849,7 +849,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         // because this is CTFE and the final value will be thoroughly validated anyway.
         let cleanup = match return_to_block {
             StackPopCleanup::Goto { .. } => true,
-            StackPopCleanup::None { cleanup, .. } => cleanup,
+            StackPopCleanup::Root { cleanup, .. } => cleanup,
         };
 
         if !cleanup {
@@ -874,8 +874,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             // Follow the unwind edge.
             let unwind = match return_to_block {
                 StackPopCleanup::Goto { unwind, .. } => unwind,
-                StackPopCleanup::None { .. } => {
-                    panic!("Encountered StackPopCleanup::None when unwinding!")
+                StackPopCleanup::Root { .. } => {
+                    panic!("encountered StackPopCleanup::Root when unwinding!")
                 }
             };
             self.unwind_to_block(unwind)
@@ -883,7 +883,13 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             // Follow the normal return edge.
             match return_to_block {
                 StackPopCleanup::Goto { ret, .. } => self.return_to_block(ret),
-                StackPopCleanup::None { .. } => Ok(()),
+                StackPopCleanup::Root { .. } => {
+                    assert!(
+                        self.stack().is_empty(),
+                        "only the topmost frame can have StackPopCleanup::Root"
+                    );
+                    Ok(())
+                }
             }
         }
     }
diff --git a/compiler/rustc_hir/src/hir.rs b/compiler/rustc_hir/src/hir.rs
index b3446e51c7381..d59756239d9da 100644
--- a/compiler/rustc_hir/src/hir.rs
+++ b/compiler/rustc_hir/src/hir.rs
@@ -1407,6 +1407,20 @@ impl fmt::Display for ConstContext {
 /// A literal.
 pub type Lit = Spanned<LitKind>;
 
+#[derive(Copy, Clone, PartialEq, Eq, Encodable, Debug, HashStable_Generic)]
+pub enum ArrayLen {
+    Infer(HirId, Span),
+    Body(AnonConst),
+}
+
+impl ArrayLen {
+    pub fn hir_id(&self) -> HirId {
+        match self {
+            &ArrayLen::Infer(hir_id, _) | &ArrayLen::Body(AnonConst { hir_id, body: _ }) => hir_id,
+        }
+    }
+}
+
 /// A constant (expression) that's not an item or associated item,
 /// but needs its own `DefId` for type-checking, const-eval, etc.
 /// These are usually found nested inside types (e.g., array lengths)
@@ -1756,7 +1770,7 @@ pub enum ExprKind<'hir> {
     ///
     /// E.g., `[1; 5]`. The first expression is the element
     /// to be repeated; the second is the number of times to repeat it.
-    Repeat(&'hir Expr<'hir>, AnonConst),
+    Repeat(&'hir Expr<'hir>, ArrayLen),
 
     /// A suspension point for generators (i.e., `yield <expr>`).
     Yield(&'hir Expr<'hir>, YieldSource),
@@ -2266,7 +2280,7 @@ pub enum TyKind<'hir> {
     /// A variable length slice (i.e., `[T]`).
     Slice(&'hir Ty<'hir>),
     /// A fixed length array (i.e., `[T; n]`).
-    Array(&'hir Ty<'hir>, AnonConst),
+    Array(&'hir Ty<'hir>, ArrayLen),
     /// A raw pointer (i.e., `*const T` or `*mut T`).
     Ptr(MutTy<'hir>),
     /// A reference (i.e., `&'a T` or `&'a mut T`).
diff --git a/compiler/rustc_hir/src/intravisit.rs b/compiler/rustc_hir/src/intravisit.rs
index 0fab7cbfeea34..d0eee422202af 100644
--- a/compiler/rustc_hir/src/intravisit.rs
+++ b/compiler/rustc_hir/src/intravisit.rs
@@ -383,6 +383,9 @@ pub trait Visitor<'v>: Sized {
     fn visit_pat(&mut self, p: &'v Pat<'v>) {
         walk_pat(self, p)
     }
+    fn visit_array_length(&mut self, len: &'v ArrayLen) {
+        walk_array_len(self, len)
+    }
     fn visit_anon_const(&mut self, c: &'v AnonConst) {
         walk_anon_const(self, c)
     }
@@ -753,7 +756,7 @@ pub fn walk_ty<'v, V: Visitor<'v>>(visitor: &mut V, typ: &'v Ty<'v>) {
         }
         TyKind::Array(ref ty, ref length) => {
             visitor.visit_ty(ty);
-            visitor.visit_anon_const(length)
+            visitor.visit_array_length(length)
         }
         TyKind::TraitObject(bounds, ref lifetime, _syntax) => {
             for bound in bounds {
@@ -1124,6 +1127,13 @@ pub fn walk_stmt<'v, V: Visitor<'v>>(visitor: &mut V, statement: &'v Stmt<'v>) {
     }
 }
 
+pub fn walk_array_len<'v, V: Visitor<'v>>(visitor: &mut V, len: &'v ArrayLen) {
+    match len {
+        &ArrayLen::Infer(hir_id, _span) => visitor.visit_id(hir_id),
+        ArrayLen::Body(c) => visitor.visit_anon_const(c),
+    }
+}
+
 pub fn walk_anon_const<'v, V: Visitor<'v>>(visitor: &mut V, constant: &'v AnonConst) {
     visitor.visit_id(constant.hir_id);
     visitor.visit_nested_body(constant.body);
@@ -1147,7 +1157,7 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr<'v>)
         ExprKind::ConstBlock(ref anon_const) => visitor.visit_anon_const(anon_const),
         ExprKind::Repeat(ref element, ref count) => {
             visitor.visit_expr(element);
-            visitor.visit_anon_const(count)
+            visitor.visit_array_length(count)
         }
         ExprKind::Struct(ref qpath, fields, ref optional_base) => {
             visitor.visit_qpath(qpath, expression.hir_id, expression.span);
diff --git a/compiler/rustc_hir_pretty/src/lib.rs b/compiler/rustc_hir_pretty/src/lib.rs
index c17286dfbe38e..4c9e2d7fe42b1 100644
--- a/compiler/rustc_hir_pretty/src/lib.rs
+++ b/compiler/rustc_hir_pretty/src/lib.rs
@@ -358,7 +358,7 @@ impl<'a> State<'a> {
                 self.word("[");
                 self.print_type(&ty);
                 self.word("; ");
-                self.print_anon_const(length);
+                self.print_array_length(length);
                 self.word("]");
             }
             hir::TyKind::Typeof(ref e) => {
@@ -1065,6 +1065,13 @@ impl<'a> State<'a> {
         self.print_else(elseopt)
     }
 
+    pub fn print_array_length(&mut self, len: &hir::ArrayLen) {
+        match len {
+            hir::ArrayLen::Infer(_, _) => self.word("_"),
+            hir::ArrayLen::Body(ct) => self.print_anon_const(ct),
+        }
+    }
+
     pub fn print_anon_const(&mut self, constant: &hir::AnonConst) {
         self.ann.nested(self, Nested::Body(constant.body))
     }
@@ -1140,12 +1147,12 @@ impl<'a> State<'a> {
         self.end()
     }
 
-    fn print_expr_repeat(&mut self, element: &hir::Expr<'_>, count: &hir::AnonConst) {
+    fn print_expr_repeat(&mut self, element: &hir::Expr<'_>, count: &hir::ArrayLen) {
         self.ibox(INDENT_UNIT);
         self.word("[");
         self.print_expr(element);
         self.word_space(";");
-        self.print_anon_const(count);
+        self.print_array_length(count);
         self.word("]");
         self.end()
     }
diff --git a/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
index 025a0ee376e3e..2a0f4fdc77412 100644
--- a/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
+++ b/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
@@ -979,11 +979,11 @@ LLVMRustDIBuilderGetOrCreateArray(LLVMRustDIBuilderRef Builder,
 
 extern "C" LLVMValueRef LLVMRustDIBuilderInsertDeclareAtEnd(
     LLVMRustDIBuilderRef Builder, LLVMValueRef V, LLVMMetadataRef VarInfo,
-    int64_t *AddrOps, unsigned AddrOpsCount, LLVMMetadataRef DL,
+    uint64_t *AddrOps, unsigned AddrOpsCount, LLVMMetadataRef DL,
     LLVMBasicBlockRef InsertAtEnd) {
   return wrap(Builder->insertDeclare(
       unwrap(V), unwrap<DILocalVariable>(VarInfo),
-      Builder->createExpression(llvm::ArrayRef<int64_t>(AddrOps, AddrOpsCount)),
+      Builder->createExpression(llvm::ArrayRef<uint64_t>(AddrOps, AddrOpsCount)),
       DebugLoc(cast<MDNode>(unwrap(DL))),
       unwrap(InsertAtEnd)));
 }
@@ -1057,11 +1057,11 @@ LLVMRustDIBuilderCreateDebugLocation(unsigned Line, unsigned Column,
   return wrap(Loc);
 }
 
-extern "C" int64_t LLVMRustDIBuilderCreateOpDeref() {
+extern "C" uint64_t LLVMRustDIBuilderCreateOpDeref() {
   return dwarf::DW_OP_deref;
 }
 
-extern "C" int64_t LLVMRustDIBuilderCreateOpPlusUconst() {
+extern "C" uint64_t LLVMRustDIBuilderCreateOpPlusUconst() {
   return dwarf::DW_OP_plus_uconst;
 }
 
diff --git a/compiler/rustc_mir_build/src/thir/cx/expr.rs b/compiler/rustc_mir_build/src/thir/cx/expr.rs
index 092fe13117470..bdde6b4a356c1 100644
--- a/compiler/rustc_mir_build/src/thir/cx/expr.rs
+++ b/compiler/rustc_mir_build/src/thir/cx/expr.rs
@@ -583,9 +583,12 @@ impl<'tcx> Cx<'tcx> {
                 ExprKind::ConstBlock { value }
             }
             // Now comes the rote stuff:
-            hir::ExprKind::Repeat(ref v, ref count) => {
-                let count_def_id = self.tcx.hir().local_def_id(count.hir_id);
-                let count = ty::Const::from_anon_const(self.tcx, count_def_id);
+            hir::ExprKind::Repeat(ref v, _) => {
+                let ty = self.typeck_results().expr_ty(expr);
+                let count = match ty.kind() {
+                    ty::Array(_, ct) => ct,
+                    _ => span_bug!(expr.span, "unexpected repeat expr ty: {:?}", ty),
+                };
 
                 ExprKind::Repeat { value: self.mirror_expr(v), count }
             }
diff --git a/compiler/rustc_mir_transform/src/const_prop.rs b/compiler/rustc_mir_transform/src/const_prop.rs
index ecc8e7a6f1a39..e3ff6ad45490d 100644
--- a/compiler/rustc_mir_transform/src/const_prop.rs
+++ b/compiler/rustc_mir_transform/src/const_prop.rs
@@ -406,7 +406,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
             Instance::new(def_id, substs),
             dummy_body,
             ret.as_ref(),
-            StackPopCleanup::None { cleanup: false },
+            StackPopCleanup::Root { cleanup: false },
         )
         .expect("failed to push initial stack frame");
 
diff --git a/compiler/rustc_save_analysis/src/dump_visitor.rs b/compiler/rustc_save_analysis/src/dump_visitor.rs
index f1a5282b08871..23f5b17fa7889 100644
--- a/compiler/rustc_save_analysis/src/dump_visitor.rs
+++ b/compiler/rustc_save_analysis/src/dump_visitor.rs
@@ -1326,12 +1326,18 @@ impl<'tcx> Visitor<'tcx> for DumpVisitor<'tcx> {
                 }
                 intravisit::walk_qpath(self, path, t.hir_id, t.span);
             }
-            hir::TyKind::Array(ref ty, ref anon_const) => {
+            hir::TyKind::Array(ref ty, ref length) => {
                 self.visit_ty(ty);
                 let map = self.tcx.hir();
-                self.nest_typeck_results(self.tcx.hir().local_def_id(anon_const.hir_id), |v| {
-                    v.visit_expr(&map.body(anon_const.body).value)
-                });
+                match length {
+                    // FIXME(generic_arg_infer): We probably want to
+                    // output the inferred type here? :shrug:
+                    hir::ArrayLen::Infer(..) => {}
+                    hir::ArrayLen::Body(anon_const) => self
+                        .nest_typeck_results(self.tcx.hir().local_def_id(anon_const.hir_id), |v| {
+                            v.visit_expr(&map.body(anon_const.body).value)
+                        }),
+                }
             }
             hir::TyKind::OpaqueDef(item_id, _) => {
                 let item = self.tcx.hir().item(item_id);
@@ -1390,12 +1396,18 @@ impl<'tcx> Visitor<'tcx> for DumpVisitor<'tcx> {
                     v.visit_expr(&body.value)
                 });
             }
-            hir::ExprKind::Repeat(ref expr, ref anon_const) => {
+            hir::ExprKind::Repeat(ref expr, ref length) => {
                 self.visit_expr(expr);
                 let map = self.tcx.hir();
-                self.nest_typeck_results(self.tcx.hir().local_def_id(anon_const.hir_id), |v| {
-                    v.visit_expr(&map.body(anon_const.body).value)
-                });
+                match length {
+                    // FIXME(generic_arg_infer): We probably want to
+                    // output the inferred type here? :shrug:
+                    hir::ArrayLen::Infer(..) => {}
+                    hir::ArrayLen::Body(anon_const) => self
+                        .nest_typeck_results(self.tcx.hir().local_def_id(anon_const.hir_id), |v| {
+                            v.visit_expr(&map.body(anon_const.body).value)
+                        }),
+                }
             }
             // In particular, we take this branch for call and path expressions,
             // where we'll index the idents involved just by continuing to walk.
diff --git a/compiler/rustc_save_analysis/src/sig.rs b/compiler/rustc_save_analysis/src/sig.rs
index e43344ad6d9ef..4971bb6d1aad7 100644
--- a/compiler/rustc_save_analysis/src/sig.rs
+++ b/compiler/rustc_save_analysis/src/sig.rs
@@ -310,9 +310,9 @@ impl<'hir> Sig for hir::Ty<'hir> {
                 let nested = bounds_to_string(&bounds);
                 Ok(text_sig(nested))
             }
-            hir::TyKind::Array(ref ty, ref anon_const) => {
+            hir::TyKind::Array(ref ty, ref length) => {
                 let nested_ty = ty.make(offset + 1, id, scx)?;
-                let expr = id_to_string(&scx.tcx.hir(), anon_const.body.hir_id).replace('\n', " ");
+                let expr = id_to_string(&scx.tcx.hir(), length.hir_id()).replace('\n', " ");
                 let text = format!("[{}; {}]", nested_ty.text, expr);
                 Ok(replace_text(nested_ty, text))
             }
diff --git a/compiler/rustc_typeck/src/astconv/generics.rs b/compiler/rustc_typeck/src/astconv/generics.rs
index caa5c71e21cd9..956696546da8e 100644
--- a/compiler/rustc_typeck/src/astconv/generics.rs
+++ b/compiler/rustc_typeck/src/astconv/generics.rs
@@ -104,7 +104,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 GenericArg::Type(hir::Ty { kind: hir::TyKind::Array(_, len), .. }),
                 GenericParamDefKind::Const { .. },
             ) if tcx.type_of(param.def_id) == tcx.types.usize => {
-                let snippet = sess.source_map().span_to_snippet(tcx.hir().span(len.hir_id));
+                let snippet = sess.source_map().span_to_snippet(tcx.hir().span(len.hir_id()));
                 if let Ok(snippet) = snippet {
                     err.span_suggestion(
                         arg.span(),
diff --git a/compiler/rustc_typeck/src/astconv/mod.rs b/compiler/rustc_typeck/src/astconv/mod.rs
index 8db706c3709c1..8226ffbccc431 100644
--- a/compiler/rustc_typeck/src/astconv/mod.rs
+++ b/compiler/rustc_typeck/src/astconv/mod.rs
@@ -2363,8 +2363,14 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 self.normalize_ty(span, tcx.at(span).type_of(def_id).subst(tcx, substs))
             }
             hir::TyKind::Array(ref ty, ref length) => {
-                let length_def_id = tcx.hir().local_def_id(length.hir_id);
-                let length = ty::Const::from_anon_const(tcx, length_def_id);
+                let length = match length {
+                    &hir::ArrayLen::Infer(_, span) => self.ct_infer(tcx.types.usize, None, span),
+                    hir::ArrayLen::Body(constant) => {
+                        let length_def_id = tcx.hir().local_def_id(constant.hir_id);
+                        ty::Const::from_anon_const(tcx, length_def_id)
+                    }
+                };
+
                 let array_ty = tcx.mk_ty(ty::Array(self.ast_ty_to_ty(ty), length));
                 self.normalize_ty(ast_ty.span, array_ty)
             }
diff --git a/compiler/rustc_typeck/src/check/expr.rs b/compiler/rustc_typeck/src/check/expr.rs
index 8d370e440eaf1..621938c9b783d 100644
--- a/compiler/rustc_typeck/src/check/expr.rs
+++ b/compiler/rustc_typeck/src/check/expr.rs
@@ -1238,12 +1238,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     fn check_expr_repeat(
         &self,
         element: &'tcx hir::Expr<'tcx>,
-        count: &'tcx hir::AnonConst,
+        count: &'tcx hir::ArrayLen,
         expected: Expectation<'tcx>,
         _expr: &'tcx hir::Expr<'tcx>,
     ) -> Ty<'tcx> {
         let tcx = self.tcx;
-        let count = self.to_const(count);
+        let count = self.array_length_to_const(count);
 
         let uty = match expected {
             ExpectHasType(uty) => match *uty.kind() {
diff --git a/compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs b/compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs
index 67630fd4e582b..1aca2911533ad 100644
--- a/compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs
+++ b/compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs
@@ -498,6 +498,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         ty
     }
 
+    pub fn array_length_to_const(&self, length: &hir::ArrayLen) -> &'tcx ty::Const<'tcx> {
+        match length {
+            &hir::ArrayLen::Infer(_, span) => self.ct_infer(self.tcx.types.usize, None, span),
+            hir::ArrayLen::Body(anon_const) => self.to_const(anon_const),
+        }
+    }
+
     pub fn to_const(&self, ast_c: &hir::AnonConst) -> &'tcx ty::Const<'tcx> {
         let const_def_id = self.tcx.hir().local_def_id(ast_c.hir_id);
         let c = ty::Const::from_anon_const(self.tcx, const_def_id);
diff --git a/compiler/rustc_typeck/src/collect.rs b/compiler/rustc_typeck/src/collect.rs
index 2fb5590016ef8..e1fef84d9d929 100644
--- a/compiler/rustc_typeck/src/collect.rs
+++ b/compiler/rustc_typeck/src/collect.rs
@@ -182,7 +182,7 @@ crate fn placeholder_type_error<'tcx>(
         sugg.push((span, format!(", {}", type_name)));
     }
 
-    let mut err = bad_placeholder_type(tcx, placeholder_types, kind);
+    let mut err = bad_placeholder(tcx, "type", placeholder_types, kind);
 
     // Suggest, but only if it is not a function in const or static
     if suggest {
@@ -314,8 +314,9 @@ impl<'tcx> Visitor<'tcx> for CollectItemTypesVisitor<'tcx> {
 ///////////////////////////////////////////////////////////////////////////
 // Utility types and common code for the above passes.
 
-fn bad_placeholder_type<'tcx>(
+fn bad_placeholder<'tcx>(
     tcx: TyCtxt<'tcx>,
+    placeholder_kind: &'static str,
     mut spans: Vec<Span>,
     kind: &'static str,
 ) -> rustc_errors::DiagnosticBuilder<'tcx> {
@@ -326,7 +327,8 @@ fn bad_placeholder_type<'tcx>(
         tcx.sess,
         spans.clone(),
         E0121,
-        "the type placeholder `_` is not allowed within types on item signatures for {}",
+        "the {} placeholder `_` is not allowed within types on item signatures for {}",
+        placeholder_kind,
         kind
     );
     for span in spans {
@@ -393,7 +395,7 @@ impl<'tcx> AstConv<'tcx> for ItemCtxt<'tcx> {
         _: Option<&ty::GenericParamDef>,
         span: Span,
     ) -> &'tcx Const<'tcx> {
-        bad_placeholder_type(self.tcx(), vec![span], "generic").emit();
+        bad_placeholder(self.tcx(), "const", vec![span], "generic").emit();
         // Typeck doesn't expect erased regions to be returned from `type_of`.
         let ty = self.tcx.fold_regions(ty, &mut false, |r, _| match r {
             ty::ReErased => self.tcx.lifetimes.re_static,
@@ -1482,7 +1484,11 @@ fn generics_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::Generics {
                     // `enum` discriminants (i.e. `D` in `enum Foo { Bar = D }`),
                     // as they shouldn't be able to cause query cycle errors.
                     Node::Expr(&Expr { kind: ExprKind::Repeat(_, ref constant), .. })
-                    | Node::Variant(Variant { disr_expr: Some(ref constant), .. })
+                        if constant.hir_id() == hir_id =>
+                    {
+                        Some(parent_def_id.to_def_id())
+                    }
+                    Node::Variant(Variant { disr_expr: Some(ref constant), .. })
                         if constant.hir_id == hir_id =>
                     {
                         Some(parent_def_id.to_def_id())
@@ -1788,7 +1794,7 @@ fn fn_sig(tcx: TyCtxt<'_>, def_id: DefId) -> ty::PolyFnSig<'_> {
 
                     let mut visitor = PlaceholderHirTyCollector::default();
                     visitor.visit_ty(ty);
-                    let mut diag = bad_placeholder_type(tcx, visitor.0, "return type");
+                    let mut diag = bad_placeholder(tcx, "type", visitor.0, "return type");
                     let ret_ty = fn_sig.skip_binder().output();
                     if !ret_ty.references_error() {
                         if !ret_ty.is_closure() {
diff --git a/compiler/rustc_typeck/src/collect/type_of.rs b/compiler/rustc_typeck/src/collect/type_of.rs
index 99fddcb00ceca..04e887bf7420f 100644
--- a/compiler/rustc_typeck/src/collect/type_of.rs
+++ b/compiler/rustc_typeck/src/collect/type_of.rs
@@ -13,7 +13,7 @@ use rustc_span::symbol::Ident;
 use rustc_span::{Span, DUMMY_SP};
 
 use super::ItemCtxt;
-use super::{bad_placeholder_type, is_suggestable_infer_ty};
+use super::{bad_placeholder, is_suggestable_infer_ty};
 
 /// Computes the relevant generic parameter for a potential generic const argument.
 ///
@@ -490,7 +490,7 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
             match parent_node {
                 Node::Ty(&Ty { kind: TyKind::Array(_, ref constant), .. })
                 | Node::Expr(&Expr { kind: ExprKind::Repeat(_, ref constant), .. })
-                    if constant.hir_id == hir_id =>
+                    if constant.hir_id() == hir_id =>
                 {
                     tcx.types.usize
                 }
@@ -788,7 +788,7 @@ fn infer_placeholder_type<'a>(
             err.emit();
         }
         None => {
-            let mut diag = bad_placeholder_type(tcx, vec![span], kind);
+            let mut diag = bad_placeholder(tcx, "type", vec![span], kind);
 
             if !ty.references_error() {
                 let mut mk_nameable = MakeNameable::new(tcx);
diff --git a/library/core/src/ops/unsize.rs b/library/core/src/ops/unsize.rs
index 483362023b22c..a920b9165c18e 100644
--- a/library/core/src/ops/unsize.rs
+++ b/library/core/src/ops/unsize.rs
@@ -68,7 +68,38 @@ impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *mut T {}
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *const T {}
 
-/// This is used for object safety, to check that a method's receiver type can be dispatched on.
+/// `DispatchFromDyn` is used in the implementation of object safety checks (specifically allowing
+/// arbitrary self types), to guarantee that a method's receiver type can be dispatched on.
+///
+/// Note: `DispatchFromDyn` was briefly named `CoerceSized` (and had a slightly different
+/// interpretation).
+///
+/// Imagine we have a trait object `t` with type `&dyn Tr`, where `Tr` is some trait with a method
+/// `m` defined as `fn m(&self);`. When calling `t.m()`, the receiver `t` is a wide pointer, but an
+/// implementation of `m` will expect a narrow pointer as `&self` (a reference to the concrete
+/// type). The compiler must generate an implicit conversion from the trait object/wide pointer to
+/// the concrete reference/narrow pointer. Implementing `DispatchFromDyn` indicates that that
+/// conversion is allowed and thus that the type implementing `DispatchFromDyn` is safe to use as
+/// the self type in an object-safe method. (in the above example, the compiler will require
+/// `DispatchFromDyn` is implemented for `&'a U`).
+///
+/// `DispatchFromDyn` does not specify the conversion from wide pointer to narrow pointer; the
+/// conversion is hard-wired into the compiler. For the conversion to work, the following
+/// properties must hold (i.e., it is only safe to implement `DispatchFromDyn` for types which have
+/// these properties, these are also checked by the compiler):
+///
+/// * EITHER `Self` and `T` are either both references or both raw pointers; in either case, with
+///   the same mutability.
+/// * OR, all of the following hold
+///   - `Self` and `T` must have the same type constructor, and only vary in a single type parameter
+///     formal (the *coerced type*, e.g., `impl DispatchFromDyn<Rc<T>> for Rc<U>` is ok and the
+///     single type parameter (instantiated with `T` or `U`) is the coerced type,
+///     `impl DispatchFromDyn<Arc<T>> for Rc<U>` is not ok).
+///   - The definition for `Self` must be a struct.
+///   - The definition for `Self` must not be `#[repr(packed)]` or `#[repr(C)]`.
+///   - Other than one-aligned, zero-sized fields, the definition for `Self` must have exactly one
+///     field and that field's type must be the coerced type. Furthermore, `Self`'s field type must
+///     implement `DispatchFromDyn<F>` where `F` is the type of `T`'s field type.
 ///
 /// An example implementation of the trait:
 ///
diff --git a/library/core/src/option.rs b/library/core/src/option.rs
index 0022df4f65ff7..1ec119a71e42c 100644
--- a/library/core/src/option.rs
+++ b/library/core/src/option.rs
@@ -571,36 +571,6 @@ impl<T> Option<T> {
         !self.is_some()
     }
 
-    /// Returns `true` if the option is a [`Some`] value containing the given value.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// #![feature(option_result_contains)]
-    ///
-    /// let x: Option<u32> = Some(2);
-    /// assert_eq!(x.contains(&2), true);
-    ///
-    /// let x: Option<u32> = Some(3);
-    /// assert_eq!(x.contains(&2), false);
-    ///
-    /// let x: Option<u32> = None;
-    /// assert_eq!(x.contains(&2), false);
-    /// ```
-    #[must_use]
-    #[inline]
-    #[unstable(feature = "option_result_contains", issue = "62358")]
-    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
-    pub const fn contains<U>(&self, x: &U) -> bool
-    where
-        U: ~const PartialEq<T>,
-    {
-        match self {
-            Some(y) => x.eq(y),
-            None => false,
-        }
-    }
-
     /////////////////////////////////////////////////////////////////////////
     // Adapter for working with references
     /////////////////////////////////////////////////////////////////////////
@@ -1573,6 +1543,36 @@ impl<T> Option<T> {
         mem::replace(self, Some(value))
     }
 
+    /// Returns `true` if the option is a [`Some`] value containing the given value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(option_result_contains)]
+    ///
+    /// let x: Option<u32> = Some(2);
+    /// assert_eq!(x.contains(&2), true);
+    ///
+    /// let x: Option<u32> = Some(3);
+    /// assert_eq!(x.contains(&2), false);
+    ///
+    /// let x: Option<u32> = None;
+    /// assert_eq!(x.contains(&2), false);
+    /// ```
+    #[must_use]
+    #[inline]
+    #[unstable(feature = "option_result_contains", issue = "62358")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn contains<U>(&self, x: &U) -> bool
+    where
+        U: ~const PartialEq<T>,
+    {
+        match self {
+            Some(y) => x.eq(y),
+            None => false,
+        }
+    }
+
     /// Zips `self` with another `Option`.
     ///
     /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
diff --git a/library/core/src/result.rs b/library/core/src/result.rs
index f46632e7a8d20..504a01813ac84 100644
--- a/library/core/src/result.rs
+++ b/library/core/src/result.rs
@@ -563,64 +563,6 @@ impl<T, E> Result<T, E> {
         !self.is_ok()
     }
 
-    /// Returns `true` if the result is an [`Ok`] value containing the given value.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// #![feature(option_result_contains)]
-    ///
-    /// let x: Result<u32, &str> = Ok(2);
-    /// assert_eq!(x.contains(&2), true);
-    ///
-    /// let x: Result<u32, &str> = Ok(3);
-    /// assert_eq!(x.contains(&2), false);
-    ///
-    /// let x: Result<u32, &str> = Err("Some error message");
-    /// assert_eq!(x.contains(&2), false);
-    /// ```
-    #[must_use]
-    #[inline]
-    #[unstable(feature = "option_result_contains", issue = "62358")]
-    pub fn contains<U>(&self, x: &U) -> bool
-    where
-        U: PartialEq<T>,
-    {
-        match self {
-            Ok(y) => x == y,
-            Err(_) => false,
-        }
-    }
-
-    /// Returns `true` if the result is an [`Err`] value containing the given value.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// #![feature(result_contains_err)]
-    ///
-    /// let x: Result<u32, &str> = Ok(2);
-    /// assert_eq!(x.contains_err(&"Some error message"), false);
-    ///
-    /// let x: Result<u32, &str> = Err("Some error message");
-    /// assert_eq!(x.contains_err(&"Some error message"), true);
-    ///
-    /// let x: Result<u32, &str> = Err("Some other error message");
-    /// assert_eq!(x.contains_err(&"Some error message"), false);
-    /// ```
-    #[must_use]
-    #[inline]
-    #[unstable(feature = "result_contains_err", issue = "62358")]
-    pub fn contains_err<F>(&self, f: &F) -> bool
-    where
-        F: PartialEq<E>,
-    {
-        match self {
-            Ok(_) => false,
-            Err(e) => f == e,
-        }
-    }
-
     /////////////////////////////////////////////////////////////////////////
     // Adapter for each variant
     /////////////////////////////////////////////////////////////////////////
@@ -1491,6 +1433,68 @@ impl<T, E> Result<T, E> {
             Err(e) => e,
         }
     }
+
+    /////////////////////////////////////////////////////////////////////////
+    // Misc or niche
+    /////////////////////////////////////////////////////////////////////////
+
+    /// Returns `true` if the result is an [`Ok`] value containing the given value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(option_result_contains)]
+    ///
+    /// let x: Result<u32, &str> = Ok(2);
+    /// assert_eq!(x.contains(&2), true);
+    ///
+    /// let x: Result<u32, &str> = Ok(3);
+    /// assert_eq!(x.contains(&2), false);
+    ///
+    /// let x: Result<u32, &str> = Err("Some error message");
+    /// assert_eq!(x.contains(&2), false);
+    /// ```
+    #[must_use]
+    #[inline]
+    #[unstable(feature = "option_result_contains", issue = "62358")]
+    pub fn contains<U>(&self, x: &U) -> bool
+    where
+        U: PartialEq<T>,
+    {
+        match self {
+            Ok(y) => x == y,
+            Err(_) => false,
+        }
+    }
+
+    /// Returns `true` if the result is an [`Err`] value containing the given value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(result_contains_err)]
+    ///
+    /// let x: Result<u32, &str> = Ok(2);
+    /// assert_eq!(x.contains_err(&"Some error message"), false);
+    ///
+    /// let x: Result<u32, &str> = Err("Some error message");
+    /// assert_eq!(x.contains_err(&"Some error message"), true);
+    ///
+    /// let x: Result<u32, &str> = Err("Some other error message");
+    /// assert_eq!(x.contains_err(&"Some error message"), false);
+    /// ```
+    #[must_use]
+    #[inline]
+    #[unstable(feature = "result_contains_err", issue = "62358")]
+    pub fn contains_err<F>(&self, f: &F) -> bool
+    where
+        F: PartialEq<E>,
+    {
+        match self {
+            Ok(_) => false,
+            Err(e) => f == e,
+        }
+    }
 }
 
 impl<T, E> Result<&T, E> {
diff --git a/src/bootstrap/builder.rs b/src/bootstrap/builder.rs
index 917abde9de1ce..6ccf8b1d5221c 100644
--- a/src/bootstrap/builder.rs
+++ b/src/bootstrap/builder.rs
@@ -988,10 +988,20 @@ impl<'a> Builder<'a> {
             }
         };
 
-        if use_new_symbol_mangling {
-            rustflags.arg("-Zsymbol-mangling-version=v0");
+        // cfg(bootstrap) -- drop the compiler.stage == 0 branch.
+        if compiler.stage == 0 {
+            if use_new_symbol_mangling {
+                rustflags.arg("-Zsymbol-mangling-version=v0");
+            } else {
+                rustflags.arg("-Zsymbol-mangling-version=legacy");
+            }
         } else {
-            rustflags.arg("-Zsymbol-mangling-version=legacy");
+            if use_new_symbol_mangling {
+                rustflags.arg("-Csymbol-mangling-version=v0");
+            } else {
+                rustflags.arg("-Csymbol-mangling-version=legacy");
+                rustflags.arg("-Zunstable-options");
+            }
         }
 
         // FIXME: It might be better to use the same value for both `RUSTFLAGS` and `RUSTDOCFLAGS`,
diff --git a/src/doc/book b/src/doc/book
index 8a0bb3c96e719..d3740fb7aad0e 160000
--- a/src/doc/book
+++ b/src/doc/book
@@ -1 +1 @@
-Subproject commit 8a0bb3c96e71927b80fa2286d7a5a5f2547c6aa4
+Subproject commit d3740fb7aad0ea4a80ae20f64dee3a8cfc0c5c3c
diff --git a/src/doc/reference b/src/doc/reference
index 06f9e61931bcf..f8ba2f12df60e 160000
--- a/src/doc/reference
+++ b/src/doc/reference
@@ -1 +1 @@
-Subproject commit 06f9e61931bcf58b91dfe6c924057e42ce273ee1
+Subproject commit f8ba2f12df60ee19b96de24ae5b73af3de8a446b
diff --git a/src/doc/rustc-dev-guide b/src/doc/rustc-dev-guide
index 9bf0028b55779..875464457c410 160000
--- a/src/doc/rustc-dev-guide
+++ b/src/doc/rustc-dev-guide
@@ -1 +1 @@
-Subproject commit 9bf0028b557798ddd07a6f652e4d0c635d3d6620
+Subproject commit 875464457c4104686faf667f47848aa7b0f0a744
diff --git a/src/librustdoc/clean/mod.rs b/src/librustdoc/clean/mod.rs
index 959be5478b4ec..d80e79d164a78 100644
--- a/src/librustdoc/clean/mod.rs
+++ b/src/librustdoc/clean/mod.rs
@@ -1350,17 +1350,23 @@ impl Clean<Type> for hir::Ty<'_> {
             }
             TyKind::Slice(ref ty) => Slice(box ty.clean(cx)),
             TyKind::Array(ref ty, ref length) => {
-                let def_id = cx.tcx.hir().local_def_id(length.hir_id);
-                // NOTE(min_const_generics): We can't use `const_eval_poly` for constants
-                // as we currently do not supply the parent generics to anonymous constants
-                // but do allow `ConstKind::Param`.
-                //
-                // `const_eval_poly` tries to to first substitute generic parameters which
-                // results in an ICE while manually constructing the constant and using `eval`
-                // does nothing for `ConstKind::Param`.
-                let ct = ty::Const::from_anon_const(cx.tcx, def_id);
-                let param_env = cx.tcx.param_env(def_id);
-                let length = print_const(cx, ct.eval(cx.tcx, param_env));
+                let length = match length {
+                    hir::ArrayLen::Infer(_, _) => "_".to_string(),
+                    hir::ArrayLen::Body(anon_const) => {
+                        let def_id = cx.tcx.hir().local_def_id(anon_const.hir_id);
+                        // NOTE(min_const_generics): We can't use `const_eval_poly` for constants
+                        // as we currently do not supply the parent generics to anonymous constants
+                        // but do allow `ConstKind::Param`.
+                        //
+                        // `const_eval_poly` tries to to first substitute generic parameters which
+                        // results in an ICE while manually constructing the constant and using `eval`
+                        // does nothing for `ConstKind::Param`.
+                        let ct = ty::Const::from_anon_const(cx.tcx, def_id);
+                        let param_env = cx.tcx.param_env(def_id);
+                        print_const(cx, ct.eval(cx.tcx, param_env))
+                    }
+                };
+
                 Array(box ty.clean(cx), length)
             }
             TyKind::Tup(tys) => Tuple(tys.iter().map(|x| x.clean(cx)).collect()),
diff --git a/src/test/ui/const-generics/generic_arg_infer/array-in-sig.rs b/src/test/ui/const-generics/generic_arg_infer/array-in-sig.rs
new file mode 100644
index 0000000000000..56b88a426a1a8
--- /dev/null
+++ b/src/test/ui/const-generics/generic_arg_infer/array-in-sig.rs
@@ -0,0 +1,12 @@
+// To avoid having to `or` gate `_` as an expr.
+#![feature(generic_arg_infer)]
+
+fn foo() -> [u8; _] {
+    //~^ ERROR the const placeholder `_` is not allowed within types on item signatures for generics
+    // FIXME(generic_arg_infer): this error message should say in the return type or sth like that.
+    [0; 3]
+}
+
+fn main() {
+    foo();
+}
diff --git a/src/test/ui/const-generics/generic_arg_infer/array-in-sig.stderr b/src/test/ui/const-generics/generic_arg_infer/array-in-sig.stderr
new file mode 100644
index 0000000000000..eaa12b4192dc6
--- /dev/null
+++ b/src/test/ui/const-generics/generic_arg_infer/array-in-sig.stderr
@@ -0,0 +1,9 @@
+error[E0121]: the const placeholder `_` is not allowed within types on item signatures for generics
+  --> $DIR/array-in-sig.rs:4:18
+   |
+LL | fn foo() -> [u8; _] {
+   |                  ^ not allowed in type signatures
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0121`.
diff --git a/src/test/ui/const-generics/generic_arg_infer/array-repeat-expr.rs b/src/test/ui/const-generics/generic_arg_infer/array-repeat-expr.rs
new file mode 100644
index 0000000000000..d3e53d7a89267
--- /dev/null
+++ b/src/test/ui/const-generics/generic_arg_infer/array-repeat-expr.rs
@@ -0,0 +1,13 @@
+// run-pass
+
+// To avoid having to `or` gate `_` as an expr.
+#![feature(generic_arg_infer)]
+
+fn foo() -> [u8; 3] {
+    let x: [u8; _] = [0; _];
+    x
+}
+
+fn main() {
+    assert_eq!([0; _], foo());
+}
diff --git a/src/test/ui/inference/infer-arg-test.rs b/src/test/ui/const-generics/generic_arg_infer/infer-arg-test.rs
similarity index 86%
rename from src/test/ui/inference/infer-arg-test.rs
rename to src/test/ui/const-generics/generic_arg_infer/infer-arg-test.rs
index 1b67ccd6c4337..29aa0f59d7438 100644
--- a/src/test/ui/inference/infer-arg-test.rs
+++ b/src/test/ui/const-generics/generic_arg_infer/infer-arg-test.rs
@@ -18,7 +18,5 @@ fn main() {
   let a: All<_, _, _>;
   all_fn();
   let v: [u8; _];
-  //~^ ERROR in expressions
   let v: [u8; 10] = [0; _];
-  //~^ ERROR in expressions
 }
diff --git a/src/test/ui/inference/infer-arg-test.stderr b/src/test/ui/const-generics/generic_arg_infer/infer-arg-test.stderr
similarity index 64%
rename from src/test/ui/inference/infer-arg-test.stderr
rename to src/test/ui/const-generics/generic_arg_infer/infer-arg-test.stderr
index 30e171eac2121..e6d0c743d01b9 100644
--- a/src/test/ui/inference/infer-arg-test.stderr
+++ b/src/test/ui/const-generics/generic_arg_infer/infer-arg-test.stderr
@@ -10,18 +10,6 @@ error: expected identifier, found reserved identifier `_`
 LL | fn bad_infer_fn<_>() {}
    |                 ^ expected identifier, found reserved identifier
 
-error: in expressions, `_` can only be used on the left-hand side of an assignment
-  --> $DIR/infer-arg-test.rs:20:15
-   |
-LL |   let v: [u8; _];
-   |               ^ `_` not allowed here
-
-error: in expressions, `_` can only be used on the left-hand side of an assignment
-  --> $DIR/infer-arg-test.rs:22:25
-   |
-LL |   let v: [u8; 10] = [0; _];
-   |                         ^ `_` not allowed here
-
 error[E0392]: parameter `_` is never used
   --> $DIR/infer-arg-test.rs:7:17
    |
@@ -31,6 +19,6 @@ LL | struct BadInfer<_>;
    = help: consider removing `_`, referring to it in a field, or using a marker such as `PhantomData`
    = help: if you intended `_` to be a const parameter, use `const _: usize` instead
 
-error: aborting due to 5 previous errors
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0392`.
diff --git a/src/test/ui/feature-gates/feature-gate-generic_arg_infer.normal.stderr b/src/test/ui/feature-gates/feature-gate-generic_arg_infer.normal.stderr
index c17f02d58f325..49eede4794b37 100644
--- a/src/test/ui/feature-gates/feature-gate-generic_arg_infer.normal.stderr
+++ b/src/test/ui/feature-gates/feature-gate-generic_arg_infer.normal.stderr
@@ -1,5 +1,35 @@
+error[E0658]: using `_` for array lengths is unstable
+  --> $DIR/feature-gate-generic_arg_infer.rs:11:27
+   |
+LL |     let _x: [u8; 3] = [0; _];
+   |                           ^
+   |
+   = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information
+   = help: add `#![feature(generic_arg_infer)]` to the crate attributes to enable
+
+error: in expressions, `_` can only be used on the left-hand side of an assignment
+  --> $DIR/feature-gate-generic_arg_infer.rs:11:27
+   |
+LL |     let _x: [u8; 3] = [0; _];
+   |                           ^ `_` not allowed here
+
+error[E0658]: using `_` for array lengths is unstable
+  --> $DIR/feature-gate-generic_arg_infer.rs:14:18
+   |
+LL |     let _y: [u8; _] = [0; 3];
+   |                  ^
+   |
+   = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information
+   = help: add `#![feature(generic_arg_infer)]` to the crate attributes to enable
+
+error: in expressions, `_` can only be used on the left-hand side of an assignment
+  --> $DIR/feature-gate-generic_arg_infer.rs:14:18
+   |
+LL |     let _y: [u8; _] = [0; 3];
+   |                  ^ `_` not allowed here
+
 error[E0747]: type provided when a constant was expected
-  --> $DIR/feature-gate-generic_arg_infer.rs:11:20
+  --> $DIR/feature-gate-generic_arg_infer.rs:20:20
    |
 LL |     let _x = foo::<_>([1,2]);
    |                    ^
@@ -7,6 +37,7 @@ LL |     let _x = foo::<_>([1,2]);
    = help: const arguments cannot yet be inferred with `_`
    = help: add `#![feature(generic_arg_infer)]` to the crate attributes to enable
 
-error: aborting due to previous error
+error: aborting due to 5 previous errors
 
-For more information about this error, try `rustc --explain E0747`.
+Some errors have detailed explanations: E0658, E0747.
+For more information about an error, try `rustc --explain E0658`.
diff --git a/src/test/ui/feature-gates/feature-gate-generic_arg_infer.rs b/src/test/ui/feature-gates/feature-gate-generic_arg_infer.rs
index 4729773b12ef0..afd14b7843e20 100644
--- a/src/test/ui/feature-gates/feature-gate-generic_arg_infer.rs
+++ b/src/test/ui/feature-gates/feature-gate-generic_arg_infer.rs
@@ -7,7 +7,17 @@ fn foo<const N: usize>(_: [u8; N]) -> [u8; N] {
   [0; N]
 }
 
+fn bar() {
+    let _x: [u8; 3] = [0; _];
+    //[normal]~^ ERROR: using `_` for array lengths is unstable
+    //[normal]~| ERROR: in expressions, `_` can only be used on the left-hand side of an assignment
+    let _y: [u8; _] = [0; 3];
+    //[normal]~^ ERROR: using `_` for array lengths is unstable
+    //[normal]~| ERROR: in expressions, `_` can only be used on the left-hand side of an assignment
+}
+
 fn main() {
     let _x = foo::<_>([1,2]);
     //[normal]~^ ERROR: type provided when a constant was expected
+    let _y = bar();
 }
diff --git a/src/tools/clippy/clippy_lints/src/trailing_empty_array.rs b/src/tools/clippy/clippy_lints/src/trailing_empty_array.rs
index 47c0a84cd4630..af36f7267004d 100644
--- a/src/tools/clippy/clippy_lints/src/trailing_empty_array.rs
+++ b/src/tools/clippy/clippy_lints/src/trailing_empty_array.rs
@@ -59,6 +59,7 @@ fn is_struct_with_trailing_zero_sized_array(cx: &LateContext<'tcx>, item: &'tcx
         if let ItemKind::Struct(data, _) = &item.kind;
         if let Some(last_field) = data.fields().last();
         if let rustc_hir::TyKind::Array(_, length) = last_field.ty.kind;
+        if let rustc_hir::ArrayLen::Body(length) = length;
 
         // Then check if that that array zero-sized
         let length_ldid = cx.tcx.hir().local_def_id(length.hir_id);
diff --git a/src/tools/clippy/clippy_lints/src/utils/author.rs b/src/tools/clippy/clippy_lints/src/utils/author.rs
index c1b811c217440..9b06ca4e82493 100644
--- a/src/tools/clippy/clippy_lints/src/utils/author.rs
+++ b/src/tools/clippy/clippy_lints/src/utils/author.rs
@@ -6,7 +6,7 @@ use rustc_ast::ast::{LitFloatType, LitKind};
 use rustc_ast::LitIntType;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_hir as hir;
-use rustc_hir::{ExprKind, FnRetTy, HirId, Lit, PatKind, QPath, StmtKind, TyKind};
+use rustc_hir::{ArrayLen, ExprKind, FnRetTy, HirId, Lit, PatKind, QPath, StmtKind, TyKind};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::symbol::{Ident, Symbol};
@@ -567,7 +567,14 @@ impl<'a, 'tcx> PrintVisitor<'a, 'tcx> {
                 bind!(self, value, length);
                 kind!("Repeat({value}, {length})");
                 self.expr(value);
-                self.body(field!(length.body));
+                match length.value {
+                    ArrayLen::Infer(..) => out!("if let ArrayLen::Infer(..) = length;"),
+                    ArrayLen::Body(anon_const) => {
+                        bind!(self, anon_const);
+                        out!("if let ArrayLen::Body({anon_const}) = {length};");
+                        self.body(field!(anon_const.body));
+                    }
+                }
             },
             ExprKind::Err => kind!("Err"),
             ExprKind::DropTemps(expr) => {
diff --git a/src/tools/clippy/clippy_lints/src/utils/inspector.rs b/src/tools/clippy/clippy_lints/src/utils/inspector.rs
index abf4826a06917..c96766e56784f 100644
--- a/src/tools/clippy/clippy_lints/src/utils/inspector.rs
+++ b/src/tools/clippy/clippy_lints/src/utils/inspector.rs
@@ -334,12 +334,17 @@ fn print_expr(cx: &LateContext<'_>, expr: &hir::Expr<'_>, indent: usize) {
             println!("{}anon_const:", ind);
             print_expr(cx, &cx.tcx.hir().body(anon_const.body).value, indent + 1);
         },
-        hir::ExprKind::Repeat(val, ref anon_const) => {
+        hir::ExprKind::Repeat(val, length) => {
             println!("{}Repeat", ind);
             println!("{}value:", ind);
             print_expr(cx, val, indent + 1);
             println!("{}repeat count:", ind);
-            print_expr(cx, &cx.tcx.hir().body(anon_const.body).value, indent + 1);
+            match length {
+                hir::ArrayLen::Infer(_, _) => println!("{}repeat count: _", ind),
+                hir::ArrayLen::Body(anon_const) => {
+                    print_expr(cx, &cx.tcx.hir().body(anon_const.body).value, indent + 1)
+                }
+            }
         },
         hir::ExprKind::Err => {
             println!("{}Err", ind);
diff --git a/src/tools/clippy/clippy_utils/src/hir_utils.rs b/src/tools/clippy/clippy_utils/src/hir_utils.rs
index ad50759effaf8..ac2b1a0259e3e 100644
--- a/src/tools/clippy/clippy_utils/src/hir_utils.rs
+++ b/src/tools/clippy/clippy_utils/src/hir_utils.rs
@@ -8,7 +8,7 @@ use rustc_hir::HirIdMap;
 use rustc_hir::{
     BinOpKind, Block, BodyId, Expr, ExprField, ExprKind, FnRetTy, GenericArg, GenericArgs, Guard, HirId,
     InlineAsmOperand, Let, Lifetime, LifetimeName, ParamName, Pat, PatField, PatKind, Path, PathSegment, QPath, Stmt,
-    StmtKind, Ty, TyKind, TypeBinding,
+    StmtKind, Ty, TyKind, TypeBinding, ArrayLen
 };
 use rustc_lexer::{tokenize, TokenKind};
 use rustc_lint::LateContext;
@@ -170,6 +170,14 @@ impl HirEqInterExpr<'_, '_, '_> {
         }
     }
 
+    pub fn eq_array_length(&mut self, left: ArrayLen, right: ArrayLen) -> bool {
+            match (left, right) {
+                (ArrayLen::Infer(..), ArrayLen::Infer(..)) => true,
+                (ArrayLen::Body(l_ct), ArrayLen::Body(r_ct)) => self.eq_body(l_ct.body, r_ct.body),
+                (_, _) => false,
+            }
+    }
+
     pub fn eq_body(&mut self, left: BodyId, right: BodyId) -> bool {
         let cx = self.inner.cx;
         let eval_const = |body| constant_context(cx, cx.tcx.typeck_body(body)).expr(&cx.tcx.hir().body(body).value);
@@ -194,8 +202,8 @@ impl HirEqInterExpr<'_, '_, '_> {
         }
 
         let is_eq = match (
-            &reduce_exprkind(self.inner.cx, &left.kind),
-            &reduce_exprkind(self.inner.cx, &right.kind),
+            reduce_exprkind(self.inner.cx, &left.kind),
+            reduce_exprkind(self.inner.cx, &right.kind),
         ) {
             (&ExprKind::AddrOf(lb, l_mut, le), &ExprKind::AddrOf(rb, r_mut, re)) => {
                 lb == rb && l_mut == r_mut && self.eq_expr(le, re)
@@ -232,7 +240,7 @@ impl HirEqInterExpr<'_, '_, '_> {
             },
             (&ExprKind::Index(la, li), &ExprKind::Index(ra, ri)) => self.eq_expr(la, ra) && self.eq_expr(li, ri),
             (&ExprKind::If(lc, lt, ref le), &ExprKind::If(rc, rt, ref re)) => {
-                self.eq_expr(lc, rc) && self.eq_expr(&**lt, &**rt) && both(le, re, |l, r| self.eq_expr(l, r))
+                self.eq_expr(lc, rc) && self.eq_expr(lt, rt) && both(le, re, |l, r| self.eq_expr(l, r))
             },
             (&ExprKind::Let(l), &ExprKind::Let(r)) => {
                 self.eq_pat(l.pat, r.pat) && both(&l.ty, &r.ty, |l, r| self.eq_ty(l, r)) && self.eq_expr(l.init, r.init)
@@ -253,8 +261,8 @@ impl HirEqInterExpr<'_, '_, '_> {
             (&ExprKind::MethodCall(l_path, _, l_args, _), &ExprKind::MethodCall(r_path, _, r_args, _)) => {
                 self.inner.allow_side_effects && self.eq_path_segment(l_path, r_path) && self.eq_exprs(l_args, r_args)
             },
-            (&ExprKind::Repeat(le, ref ll_id), &ExprKind::Repeat(re, ref rl_id)) => {
-                self.eq_expr(le, re) && self.eq_body(ll_id.body, rl_id.body)
+            (&ExprKind::Repeat(le, ll), &ExprKind::Repeat(re, rl)) => {
+                self.eq_expr(le, re) && self.eq_array_length(ll, rl)
             },
             (&ExprKind::Ret(ref l), &ExprKind::Ret(ref r)) => both(l, r, |l, r| self.eq_expr(l, r)),
             (&ExprKind::Path(ref l), &ExprKind::Path(ref r)) => self.eq_qpath(l, r),
@@ -391,8 +399,8 @@ impl HirEqInterExpr<'_, '_, '_> {
     fn eq_ty(&mut self, left: &Ty<'_>, right: &Ty<'_>) -> bool {
         match (&left.kind, &right.kind) {
             (&TyKind::Slice(l_vec), &TyKind::Slice(r_vec)) => self.eq_ty(l_vec, r_vec),
-            (&TyKind::Array(lt, ref ll_id), &TyKind::Array(rt, ref rl_id)) => {
-                self.eq_ty(lt, rt) && self.eq_body(ll_id.body, rl_id.body)
+            (&TyKind::Array(lt, ll), &TyKind::Array(rt, rl)) => {
+                self.eq_ty(lt, rt) && self.eq_array_length(ll, rl)
             },
             (&TyKind::Ptr(ref l_mut), &TyKind::Ptr(ref r_mut)) => {
                 l_mut.mutbl == r_mut.mutbl && self.eq_ty(&*l_mut.ty, &*r_mut.ty)
@@ -714,9 +722,9 @@ impl<'a, 'tcx> SpanlessHash<'a, 'tcx> {
             ExprKind::ConstBlock(ref l_id) => {
                 self.hash_body(l_id.body);
             },
-            ExprKind::Repeat(e, ref l_id) => {
+            ExprKind::Repeat(e, len) => {
                 self.hash_expr(e);
-                self.hash_body(l_id.body);
+                self.hash_array_length(len);
             },
             ExprKind::Ret(ref e) => {
                 if let Some(e) = *e {
@@ -906,9 +914,9 @@ impl<'a, 'tcx> SpanlessHash<'a, 'tcx> {
             TyKind::Slice(ty) => {
                 self.hash_ty(ty);
             },
-            TyKind::Array(ty, anon_const) => {
+            &TyKind::Array(ty, len) => {
                 self.hash_ty(ty);
-                self.hash_body(anon_const.body);
+                self.hash_array_length(len);
             },
             TyKind::Ptr(ref mut_ty) => {
                 self.hash_ty(mut_ty.ty);
@@ -953,6 +961,13 @@ impl<'a, 'tcx> SpanlessHash<'a, 'tcx> {
         }
     }
 
+    pub fn hash_array_length(&mut self, length: ArrayLen) {
+        match length {
+            ArrayLen::Infer(..) => {}
+            ArrayLen::Body(anon_const) => self.hash_body(anon_const.body),
+        }
+    }
+
     pub fn hash_body(&mut self, body_id: BodyId) {
         // swap out TypeckResults when hashing a body
         let old_maybe_typeck_results = self.maybe_typeck_results.replace(self.cx.tcx.typeck_body(body_id));
diff --git a/src/tools/clippy/clippy_utils/src/lib.rs b/src/tools/clippy/clippy_utils/src/lib.rs
index 54d470ca73820..9179e67c4f4ee 100644
--- a/src/tools/clippy/clippy_utils/src/lib.rs
+++ b/src/tools/clippy/clippy_utils/src/lib.rs
@@ -79,7 +79,7 @@ use rustc_hir::{
     def, Arm, BindingAnnotation, Block, BlockCheckMode, Body, Constness, Destination, Expr, ExprKind, FnDecl,
     ForeignItem, GenericArgs, HirId, Impl, ImplItem, ImplItemKind, IsAsync, Item, ItemKind, LangItem, Local,
     MatchSource, Mutability, Node, Param, Pat, PatKind, Path, PathSegment, PrimTy, QPath, Stmt, StmtKind, TraitItem,
-    TraitItemKind, TraitRef, TyKind, UnOp,
+    TraitItemKind, TraitRef, TyKind, UnOp, ArrayLen
 };
 use rustc_lint::{LateContext, Level, Lint, LintContext};
 use rustc_middle::hir::exports::Export;
@@ -703,8 +703,9 @@ pub fn is_default_equivalent(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
             _ => false,
         },
         ExprKind::Tup(items) | ExprKind::Array(items) => items.iter().all(|x| is_default_equivalent(cx, x)),
-        ExprKind::Repeat(x, y) => if_chain! {
-            if let ExprKind::Lit(ref const_lit) = cx.tcx.hir().body(y.body).value.kind;
+        ExprKind::Repeat(x, len) => if_chain! {
+            if let ArrayLen::Body(len) = len;
+            if let ExprKind::Lit(ref const_lit) = cx.tcx.hir().body(len.body).value.kind;
             if let LitKind::Int(v, _) = const_lit.node;
             if v <= 32 && is_default_equivalent(cx, x);
             then {
diff --git a/src/tools/clippy/tests/ui/author/repeat.stdout b/src/tools/clippy/tests/ui/author/repeat.stdout
index f16350e4b5e6d..471bbce4f4185 100644
--- a/src/tools/clippy/tests/ui/author/repeat.stdout
+++ b/src/tools/clippy/tests/ui/author/repeat.stdout
@@ -2,7 +2,8 @@ if_chain! {
     if let ExprKind::Repeat(value, length) = expr.kind;
     if let ExprKind::Lit(ref lit) = value.kind;
     if let LitKind::Int(1, LitIntType::Unsigned(UintTy::U8)) = lit.node;
-    let expr1 = &cx.tcx.hir().body(length.body).value;
+    if let ArrayLen::Body(anon_const) = length;
+    let expr1 = &cx.tcx.hir().body(anon_const.body).value;
     if let ExprKind::Lit(ref lit1) = expr1.kind;
     if let LitKind::Int(5, LitIntType::Unsuffixed) = lit1.node;
     then {