From 71d4ef88c61ab0f4673589bb4d19e96c76f89560 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Wed, 23 Sep 2026 09:30:29 +1000
Subject: [PATCH] Remove unused `StashKey::UnderscoreForArrayLengths`

It's no longer ever used for stashing, so the code looking for it is
dead.

It became dead in PR 141610 when `generic_arg_infer` was stabilized and
`[T; _]` became valid.
---
 compiler/rustc_errors/src/lib.rs      |  1 -
 compiler/rustc_hir_typeck/src/expr.rs | 29 ---------------------------
 2 files changed, 30 deletions(-)

diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index 68e10802e21c5..25a82b22707cc 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -375,7 +375,6 @@ struct DiagCtxtInner {
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
 pub enum StashKey {
     ItemNoType,
-    UnderscoreForArrayLengths,
     EarlySyntaxWarning,
     CallIntoMethod,
     /// When an invalid lifetime e.g. `'2` should be reinterpreted
diff --git a/compiler/rustc_hir_typeck/src/expr.rs b/compiler/rustc_hir_typeck/src/expr.rs
index 04e55dd0fdd95..8944d6a47488a 100644
--- a/compiler/rustc_hir_typeck/src/expr.rs
+++ b/compiler/rustc_hir_typeck/src/expr.rs
@@ -1702,34 +1702,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             self.next_ty_var(expr.span)
         };
         let array_len = args.len() as u64;
-        self.suggest_array_len(expr, array_len);
         Ty::new_array(self.tcx, element_ty, array_len)
     }
 
-    fn suggest_array_len(&self, expr: &'tcx hir::Expr<'tcx>, array_len: u64) {
-        let parent_node = self.tcx.hir_parent_iter(expr.hir_id).find(|(_, node)| {
-            !matches!(node, hir::Node::Expr(hir::Expr { kind: hir::ExprKind::AddrOf(..), .. }))
-        });
-        let Some((_, hir::Node::LetStmt(hir::LetStmt { ty: Some(ty), .. }))) = parent_node else {
-            return;
-        };
-        if let hir::TyKind::Array(_, ct) = ty.peel_refs().kind {
-            let span = ct.span;
-            self.dcx().try_steal_modify_and_emit_err(
-                span,
-                StashKey::UnderscoreForArrayLengths,
-                |err| {
-                    err.span_suggestion_verbose(
-                        span,
-                        "consider specifying the array length",
-                        array_len,
-                        Applicability::MaybeIncorrect,
-                    );
-                },
-            );
-        }
-    }
-
     pub(super) fn check_expr_const_block(
         &self,
         block: &'tcx hir::ConstBlock,
@@ -1764,10 +1739,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             ),
         );
 
-        if let Some(count) = count.try_to_target_usize(tcx) {
-            self.suggest_array_len(expr, count);
-        }
-
         let uty = match expected {
             ExpectHasType(uty) => uty.builtin_index(),
             _ => None,