Tweak value suggestions in borrowck and hir_analysis

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -652,6 +652,68 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         err
     }
 
+    fn ty_kind_suggestion(&self, ty: Ty<'tcx>) -> Option<String> {
+        // Keep in sync with `rustc_hir_analysis/src/check/mod.rs:ty_kind_suggestion`.
+        // FIXME: deduplicate the above.
+        let tcx = self.infcx.tcx;
+        let implements_default = |ty| {
+            let Some(default_trait) = tcx.get_diagnostic_item(sym::Default) else {
+                return false;
+            };
+            self.infcx
+                .type_implements_trait(default_trait, [ty], self.param_env)
+                .must_apply_modulo_regions()
+        };
+
+        Some(match ty.kind() {
+            ty::Never | ty::Error(_) => return None,
+            ty::Bool => "false".to_string(),
+            ty::Char => "\'x\'".to_string(),
+            ty::Int(_) | ty::Uint(_) => "42".into(),
+            ty::Float(_) => "3.14159".into(),
+            ty::Slice(_) => "[]".to_string(),
+            ty::Adt(def, _) if Some(def.did()) == tcx.get_diagnostic_item(sym::Vec) => {
+                "vec![]".to_string()
+            }
+            ty::Adt(def, _) if Some(def.did()) == tcx.get_diagnostic_item(sym::String) => {
+                "String::new()".to_string()
+            }
+            ty::Adt(def, args) if def.is_box() => {
+                format!("Box::new({})", self.ty_kind_suggestion(args[0].expect_ty())?)
+            }
+            ty::Adt(def, _) if Some(def.did()) == tcx.get_diagnostic_item(sym::Option) => {
+                "None".to_string()
+            }
+            ty::Adt(def, args) if Some(def.did()) == tcx.get_diagnostic_item(sym::Result) => {
+                format!("Ok({})", self.ty_kind_suggestion(args[0].expect_ty())?)
+            }
+            ty::Adt(_, _) if implements_default(ty) => "Default::default()".to_string(),
+            ty::Ref(_, ty, mutability) => {
+                if let (ty::Str, hir::Mutability::Not) = (ty.kind(), mutability) {
+                    "\"\"".to_string()
+                } else {
+                    let Some(ty) = self.ty_kind_suggestion(*ty) else {
+                        return None;
+                    };
+                    format!("&{}{ty}", mutability.prefix_str())
+                }
+            }
+            ty::Array(ty, len) => format!(
+                "[{}; {}]",
+                self.ty_kind_suggestion(*ty)?,
+                len.eval_target_usize(tcx, ty::ParamEnv::reveal_all()),
+            ),
+            ty::Tuple(tys) => format!(
+                "({})",
+                tys.iter()
+                    .map(|ty| self.ty_kind_suggestion(ty))
+                    .collect::<Option<Vec<String>>>()?
+                    .join(", ")
+            ),
+            _ => "value".to_string(),
+        })
+    }
+
     fn suggest_assign_value(
         &self,
         err: &mut Diag<'_>,
@@ -661,34 +723,16 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         let ty = moved_place.ty(self.body, self.infcx.tcx).ty;
         debug!("ty: {:?}, kind: {:?}", ty, ty.kind());
 
-        let tcx = self.infcx.tcx;
-        let implements_default = |ty, param_env| {
-            let Some(default_trait) = tcx.get_diagnostic_item(sym::Default) else {
-                return false;
-            };
-            self.infcx
-                .type_implements_trait(default_trait, [ty], param_env)
-                .must_apply_modulo_regions()
-        };
-
-        let assign_value = match ty.kind() {
-            ty::Bool => "false",
-            ty::Float(_) => "0.0",
-            ty::Int(_) | ty::Uint(_) => "0",
-            ty::Never | ty::Error(_) => "",
-            ty::Adt(def, _) if Some(def.did()) == tcx.get_diagnostic_item(sym::Vec) => "vec![]",
-            ty::Adt(_, _) if implements_default(ty, self.param_env) => "Default::default()",
-            _ => "todo!()",
+        let Some(assign_value) = self.ty_kind_suggestion(ty) else {
+            return;
         };
 
-        if !assign_value.is_empty() {
-            err.span_suggestion_verbose(
-                sugg_span.shrink_to_hi(),
-                "consider assigning a value",
-                format!(" = {assign_value}"),
-                Applicability::MaybeIncorrect,
-            );
-        }
+        err.span_suggestion_verbose(
+            sugg_span.shrink_to_hi(),
+            "consider assigning a value",
+            format!(" = {assign_value}"),
+            Applicability::MaybeIncorrect,
+        );
     }
 
     fn suggest_borrow_fn_like(

compiler/rustc_hir_analysis/src/check/check.rs

@@ -22,7 +22,6 @@ use rustc_middle::ty::{
     AdtDef, ParamEnv, RegionKind, TypeSuperVisitable, TypeVisitable, TypeVisitableExt,
 };
 use rustc_session::lint::builtin::{UNINHABITED_STATIC, UNSUPPORTED_CALLING_CONVENTIONS};
-use rustc_span::symbol::sym;
 use rustc_target::abi::FieldIdx;
 use rustc_trait_selection::traits::error_reporting::on_unimplemented::OnUnimplementedDirective;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -81,6 +81,7 @@ use rustc_errors::ErrorGuaranteed;
 use rustc_errors::{pluralize, struct_span_code_err, Diag};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::intravisit::Visitor;
+use rustc_hir::Mutability;
 use rustc_index::bit_set::BitSet;
 use rustc_infer::infer::error_reporting::ObligationCauseExt as _;
 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
@@ -91,10 +92,11 @@ use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_middle::ty::{GenericArgs, GenericArgsRef};
 use rustc_session::parse::feature_err;
-use rustc_span::symbol::{kw, Ident};
-use rustc_span::{self, def_id::CRATE_DEF_ID, BytePos, Span, Symbol, DUMMY_SP};
+use rustc_span::symbol::{kw, sym, Ident};
+use rustc_span::{def_id::CRATE_DEF_ID, BytePos, Span, Symbol, DUMMY_SP};
 use rustc_target::abi::VariantIdx;
 use rustc_target::spec::abi::Abi;
+use rustc_trait_selection::infer::InferCtxtExt;
 use rustc_trait_selection::traits::error_reporting::suggestions::ReturnsVisitor;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;
 use rustc_trait_selection::traits::ObligationCtxt;
@@ -466,14 +468,64 @@ fn fn_sig_suggestion<'tcx>(
     )
 }
 
-pub fn ty_kind_suggestion(ty: Ty<'_>) -> Option<&'static str> {
+pub fn ty_kind_suggestion<'tcx>(ty: Ty<'tcx>, tcx: TyCtxt<'tcx>) -> Option<String> {
+    // Keep in sync with `rustc_borrowck/src/diagnostics/conflict_errors.rs:ty_kind_suggestion`.
+    // FIXME: deduplicate the above.
+    let implements_default = |ty| {
+        let Some(default_trait) = tcx.get_diagnostic_item(sym::Default) else {
+            return false;
+        };
+        let infcx = tcx.infer_ctxt().build();
+        infcx
+            .type_implements_trait(default_trait, [ty], ty::ParamEnv::reveal_all())
+            .must_apply_modulo_regions()
+    };
     Some(match ty.kind() {
-        ty::Bool => "true",
-        ty::Char => "'a'",
-        ty::Int(_) | ty::Uint(_) => "42",
-        ty::Float(_) => "3.14159",
-        ty::Error(_) | ty::Never => return None,
-        _ => "value",
+        ty::Never | ty::Error(_) => return None,
+        ty::Bool => "false".to_string(),
+        ty::Char => "\'x\'".to_string(),
+        ty::Int(_) | ty::Uint(_) => "42".into(),
+        ty::Float(_) => "3.14159".into(),
+        ty::Slice(_) => "[]".to_string(),
+        ty::Adt(def, _) if Some(def.did()) == tcx.get_diagnostic_item(sym::Vec) => {
+            "vec![]".to_string()
+        }
+        ty::Adt(def, _) if Some(def.did()) == tcx.get_diagnostic_item(sym::String) => {
+            "String::new()".to_string()
+        }
+        ty::Adt(def, args) if def.is_box() => {
+            format!("Box::new({})", ty_kind_suggestion(args[0].expect_ty(), tcx)?)
+        }
+        ty::Adt(def, _) if Some(def.did()) == tcx.get_diagnostic_item(sym::Option) => {
+            "None".to_string()
+        }
+        ty::Adt(def, args) if Some(def.did()) == tcx.get_diagnostic_item(sym::Result) => {
+            format!("Ok({})", ty_kind_suggestion(args[0].expect_ty(), tcx)?)
+        }
+        ty::Adt(_, _) if implements_default(ty) => "Default::default()".to_string(),
+        ty::Ref(_, ty, mutability) => {
+            if let (ty::Str, Mutability::Not) = (ty.kind(), mutability) {
+                "\"\"".to_string()
+            } else {
+                let Some(ty) = ty_kind_suggestion(*ty, tcx) else {
+                    return None;
+                };
+                format!("&{}{ty}", mutability.prefix_str())
+            }
+        }
+        ty::Array(ty, len) => format!(
+            "[{}; {}]",
+            ty_kind_suggestion(*ty, tcx)?,
+            len.eval_target_usize(tcx, ty::ParamEnv::reveal_all()),
+        ),
+        ty::Tuple(tys) => format!(
+            "({})",
+            tys.iter()
+                .map(|ty| ty_kind_suggestion(ty, tcx))
+                .collect::<Option<Vec<String>>>()?
+                .join(", ")
+        ),
+        _ => "value".to_string(),
     })
 }
 
@@ -511,7 +563,7 @@ fn suggestion_signature<'tcx>(
         }
         ty::AssocKind::Const => {
             let ty = tcx.type_of(assoc.def_id).instantiate_identity();
-            let val = ty_kind_suggestion(ty).unwrap_or("todo!()");
+            let val = ty_kind_suggestion(ty, tcx).unwrap_or_else(|| "value".to_string());
             format!("const {}: {} = {};", assoc.name, ty, val)
         }
     }

compiler/rustc_hir_typeck/src/expr.rs

@@ -694,10 +694,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             );
                             let error = Some(Sorts(ExpectedFound { expected: ty, found: e_ty }));
                             self.annotate_loop_expected_due_to_inference(err, expr, error);
-                            if let Some(val) = ty_kind_suggestion(ty) {
+                            if let Some(val) = ty_kind_suggestion(ty, tcx) {
                                 err.span_suggestion_verbose(
                                     expr.span.shrink_to_hi(),
-                                    "give it a value of the expected type",
+                                    "give the `break` a value of the expected type",
                                     format!(" {val}"),
                                     Applicability::HasPlaceholders,
                                 );

tests/ui/asm/aarch64/type-check-2-2.stderr

@@ -8,8 +8,8 @@ LL |         asm!("{}", in(reg) x);
    |
 help: consider assigning a value
    |
-LL |         let x: u64 = 0;
-   |                    +++
+LL |         let x: u64 = 42;
+   |                    ++++
 
 error[E0381]: used binding `y` isn't initialized
   --> $DIR/type-check-2-2.rs:22:9
@@ -21,8 +21,8 @@ LL |         asm!("{}", inout(reg) y);
    |
 help: consider assigning a value
    |
-LL |         let mut y: u64 = 0;
-   |                        +++
+LL |         let mut y: u64 = 42;
+   |                        ++++
 
 error[E0596]: cannot borrow `v` as mutable, as it is not declared as mutable
   --> $DIR/type-check-2-2.rs:28:13

tests/ui/asm/x86_64/type-check-5.stderr

@@ -8,8 +8,8 @@ LL |         asm!("{}", in(reg) x);
    |
 help: consider assigning a value
    |
-LL |         let x: u64 = 0;
-   |                    +++
+LL |         let x: u64 = 42;
+   |                    ++++
 
 error[E0381]: used binding `y` isn't initialized
   --> $DIR/type-check-5.rs:18:9
@@ -21,8 +21,8 @@ LL |         asm!("{}", inout(reg) y);
    |
 help: consider assigning a value
    |
-LL |         let mut y: u64 = 0;
-   |                        +++
+LL |         let mut y: u64 = 42;
+   |                        ++++
 
 error[E0596]: cannot borrow `v` as mutable, as it is not declared as mutable
   --> $DIR/type-check-5.rs:24:13

tests/ui/binop/issue-77910-1.stderr

@@ -32,8 +32,8 @@ LL |     xs
    |
 help: consider assigning a value
    |
-LL |     let xs = todo!();
-   |            +++++++++
+LL |     let xs = &42;
+   |            +++++
 
 error: aborting due to 3 previous errors
 

tests/ui/binop/issue-77910-2.stderr

@@ -21,8 +21,8 @@ LL |     xs
    |
 help: consider assigning a value
    |
-LL |     let xs = todo!();
-   |            +++++++++
+LL |     let xs = &42;
+   |            +++++
 
 error: aborting due to 2 previous errors
 

tests/ui/borrowck/borrowck-block-uninit.stderr

@@ -10,8 +10,8 @@ LL |         println!("{}", x);
    |
 help: consider assigning a value
    |
-LL |     let x: isize = 0;
-   |                  +++
+LL |     let x: isize = 42;
+   |                  ++++
 
 error: aborting due to 1 previous error
 

tests/ui/borrowck/borrowck-break-uninit-2.stderr

@@ -10,8 +10,8 @@ LL |     println!("{}", x);
    = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: consider assigning a value
    |
-LL |     let x: isize = 0;
-   |                  +++
+LL |     let x: isize = 42;
+   |                  ++++
 
 error: aborting due to 1 previous error
 

tests/ui/borrowck/borrowck-break-uninit.stderr

@@ -10,8 +10,8 @@ LL |     println!("{}", x);
    = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: consider assigning a value
    |
-LL |     let x: isize = 0;
-   |                  +++
+LL |     let x: isize = 42;
+   |                  ++++
 
 error: aborting due to 1 previous error
 

tests/ui/borrowck/borrowck-init-in-called-fn-expr.stderr

@@ -8,8 +8,8 @@ LL |         i
    |
 help: consider assigning a value
    |
-LL |         let i: isize = 0;
-   |                      +++
+LL |         let i: isize = 42;
+   |                      ++++
 
 error: aborting due to 1 previous error
 

tests/ui/borrowck/borrowck-init-in-fn-expr.stderr

@@ -8,8 +8,8 @@ LL |         i
    |
 help: consider assigning a value
    |
-LL |         let i: isize = 0;
-   |                      +++
+LL |         let i: isize = 42;
+   |                      ++++
 
 error: aborting due to 1 previous error
 

tests/ui/borrowck/borrowck-init-in-fru.stderr

@@ -8,8 +8,8 @@ LL |     origin = Point { x: 10, ..origin };
    |
 help: consider assigning a value
    |
-LL |     let mut origin: Point = todo!();
-   |                           +++++++++
+LL |     let mut origin: Point = value;
+   |                           +++++++
 
 error: aborting due to 1 previous error
 

tests/ui/borrowck/borrowck-init-op-equal.stderr

@@ -8,8 +8,8 @@ LL |     v += 1;
    |
 help: consider assigning a value
    |
-LL |     let v: isize = 0;
-   |                  +++
+LL |     let v: isize = 42;
+   |                  ++++
 
 error: aborting due to 1 previous error
 

tests/ui/borrowck/borrowck-init-plus-equal.stderr

@@ -8,8 +8,8 @@ LL |     v = v + 1;
    |
 help: consider assigning a value
    |
-LL |     let mut v: isize = 0;
-   |                      +++
+LL |     let mut v: isize = 42;
+   |                      ++++
 
 error: aborting due to 1 previous error