Suggest the correct array length on mismatch

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -64,6 +64,7 @@ use rustc_errors::{Applicability, DiagnosticBuilder, DiagnosticStyledString};
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::intravisit::Visitor;
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::Node;
 use rustc_middle::dep_graph::DepContext;
@@ -1975,6 +1976,70 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         (ty::Bool, ty::Tuple(list)) => if list.len() == 0 {
                             self.suggest_let_for_letchains(&mut err, &trace.cause, span);
                         }
+                        (ty::Array(_, _), ty::Array(_, _)) => 'block: {
+                            let hir = self.tcx.hir();
+                            let TypeError::FixedArraySize(sz) = terr else {
+                                break 'block;
+                            };
+                            let tykind = match hir.find_by_def_id(trace.cause.body_id) {
+                                Some(hir::Node::Item(hir::Item {
+                                    kind: hir::ItemKind::Fn(_, _, body_id),
+                                    ..
+                                })) => {
+                                    let body = hir.body(*body_id);
+                                    struct LetVisitor<'v> {
+                                        span: Span,
+                                        result: Option<&'v hir::Ty<'v>>,
+                                    }
+                                    impl<'v> Visitor<'v> for LetVisitor<'v> {
+                                        fn visit_stmt(&mut self, s: &'v hir::Stmt<'v>) {
+                                            if self.result.is_some() {
+                                                return;
+                                            }
+                                            // Find a local statement where the initializer has
+                                            // the same span as the error and the type is specified.
+                                            if let hir::Stmt {
+                                                kind: hir::StmtKind::Local(hir::Local {
+                                                    init: Some(hir::Expr {
+                                                        span: init_span,
+                                                        ..
+                                                    }),
+                                                    ty: Some(array_ty),
+                                                    ..
+                                                }),
+                                                ..
+                                            } = s
+                                            && init_span == &self.span {
+                                                self.result = Some(*array_ty);
+                                            }
+                                        }
+                                    }
+                                    let mut visitor = LetVisitor {span, result: None};
+                                    visitor.visit_body(body);
+                                    visitor.result.map(|r| &r.peel_refs().kind)
+                                }
+                                Some(hir::Node::Item(hir::Item {
+                                    kind: hir::ItemKind::Const(ty, _),
+                                    ..
+                                })) => {
+                                    Some(&ty.peel_refs().kind)
+                                }
+                                _ => None
+                            };
+
+                            if let Some(tykind) = tykind
+                                && let hir::TyKind::Array(_, length) = tykind
+                                && let hir::ArrayLen::Body(hir::AnonConst { hir_id, .. }) = length
+                                && let Some(span) = self.tcx.hir().opt_span(*hir_id)
+                            {
+                                err.span_suggestion(
+                                    span,
+                                    "consider specifying the actual array length",
+                                    sz.found,
+                                    Applicability::MaybeIncorrect,
+                                );
+                            }
+                        }
                         _ => {}
                     }
                 }

tests/ui/consts/array-literal-len-mismatch.rs

@@ -0,0 +1,4 @@
+const NUMBERS: [u8; 3] = [10, 20];
+//~^ ERROR mismatched types
+//~^^ HELP consider specifying the actual array length
+fn main() {}

tests/ui/consts/array-literal-len-mismatch.stderr

@@ -0,0 +1,11 @@
+error[E0308]: mismatched types
+  --> $DIR/array-literal-len-mismatch.rs:1:26
+   |
+LL | const NUMBERS: [u8; 3] = [10, 20];
+   |                     -    ^^^^^^^^ expected an array with a fixed size of 3 elements, found one with 2 elements
+   |                     |
+   |                     help: consider specifying the actual array length: `2`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/consts/const-array-oob-arith.stderr

@@ -2,13 +2,17 @@ error[E0308]: mismatched types
   --> $DIR/const-array-oob-arith.rs:5:45
    |
 LL | const BLUB: [i32; (ARR[0] - 40) as usize] = [5];
-   |                                             ^^^ expected an array with a fixed size of 2 elements, found one with 1 element
+   |                   ----------------------    ^^^ expected an array with a fixed size of 2 elements, found one with 1 element
+   |                   |
+   |                   help: consider specifying the actual array length: `1`
 
 error[E0308]: mismatched types
   --> $DIR/const-array-oob-arith.rs:8:44
    |
 LL | const BOO: [i32; (ARR[0] - 41) as usize] = [5, 99];
-   |                                            ^^^^^^^ expected an array with a fixed size of 1 element, found one with 2 elements
+   |                  ----------------------    ^^^^^^^ expected an array with a fixed size of 1 element, found one with 2 elements
+   |                  |
+   |                  help: consider specifying the actual array length: `2`
 
 error: aborting due to 2 previous errors
 

tests/ui/inference/array-len-mismatch.rs

@@ -0,0 +1,12 @@
+fn returns_arr() -> [u8; 2] {
+    [1, 2]
+}
+
+fn main() {
+    let wrong: [u8; 3] = [10, 20];
+    //~^ ERROR mismatched types
+    //~^^ HELP consider specifying the actual array length
+    let wrong: [u8; 3] = returns_arr();
+    //~^ ERROR mismatched types
+    //~^^ HELP consider specifying the actual array length
+}

tests/ui/inference/array-len-mismatch.stderr

@@ -0,0 +1,21 @@
+error[E0308]: mismatched types
+  --> $DIR/array-len-mismatch.rs:6:26
+   |
+LL |     let wrong: [u8; 3] = [10, 20];
+   |                -------   ^^^^^^^^ expected an array with a fixed size of 3 elements, found one with 2 elements
+   |                |    |
+   |                |    help: consider specifying the actual array length: `2`
+   |                expected due to this
+
+error[E0308]: mismatched types
+  --> $DIR/array-len-mismatch.rs:9:26
+   |
+LL |     let wrong: [u8; 3] = returns_arr();
+   |                -------   ^^^^^^^^^^^^^ expected an array with a fixed size of 3 elements, found one with 2 elements
+   |                |    |
+   |                |    help: consider specifying the actual array length: `2`
+   |                expected due to this
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.