Point at `const` definition when used instead of a binding in a `let` statement

After:

```
error[E0005]: refutable pattern in local binding
  --> $DIR/bad-pattern.rs:19:13
   |
LL |     const PAT: u32 = 0;
   |     -------------- missing patterns are not covered because `PAT` is interpreted as a constant pattern, not a new variable
...
LL |         let PAT = v1;
   |             ^^^
   |             |
   |             pattern `1_u32..=u32::MAX` not covered
   |             help: introduce a variable instead: `PAT_var`
   |
   = note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
   = note: for more information, visit https://doc.rust-lang.org/book/ch18-02-refutability.html
   = note: the matched value is of type `u32`
```

Before:

```
error[E0005]: refutable pattern in local binding
  --> $DIR/bad-pattern.rs:19:13
   |
LL |         let PAT = v1;
   |             ^^^
   |             |
   |             pattern `1_u32..=u32::MAX` not covered
   |             missing patterns are not covered because `PAT` is interpreted as a constant pattern, not a new variable
   |             help: introduce a variable instead: `PAT_var`
   |
   = note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
   = note: for more information, visit https://doc.rust-lang.org/book/ch18-02-refutability.html
   = note: the matched value is of type `u32`
```

2 files changed, 10 insertions, 3 deletions

diff --git a/compiler/rustc_middle/src/thir.rs b/compiler/rustc_middle/src/thir.rs
index 45ceb0a555d..821f8c99704 100644
--- a/compiler/rustc_middle/src/thir.rs
+++ b/compiler/rustc_middle/src/thir.rs
@@ -640,6 +640,7 @@ impl<'tcx> Pat<'tcx> {
             | Range(..)
             | Binding { subpattern: None, .. }
             | Constant { .. }
+            | NamedConstant { .. }
             | Error(_) => {}
             AscribeUserType { subpattern, .. }
             | Binding { subpattern: Some(subpattern), .. }
@@ -788,6 +789,12 @@ pub enum PatKind<'tcx> {
         value: mir::Const<'tcx>,
     },
 
+    /// Same as `Constant`, but that came from a `const` that we can point at in diagnostics.
+    NamedConstant {
+        value: mir::Const<'tcx>,
+        span: Span,
+    },
+
     /// Inline constant found while lowering a pattern.
     InlineConstant {
         /// [LocalDefId] of the constant, we need this so that we have a
@@ -1084,8 +1091,8 @@ mod size_asserts {
     static_assert_size!(Block, 48);
     static_assert_size!(Expr<'_>, 64);
     static_assert_size!(ExprKind<'_>, 40);
-    static_assert_size!(Pat<'_>, 64);
-    static_assert_size!(PatKind<'_>, 48);
+    static_assert_size!(Pat<'_>, 72);
+    static_assert_size!(PatKind<'_>, 56);
     static_assert_size!(Stmt<'_>, 48);
     static_assert_size!(StmtKind<'_>, 48);
     // tidy-alphabetical-end
diff --git a/compiler/rustc_middle/src/thir/visit.rs b/compiler/rustc_middle/src/thir/visit.rs
index 36f0e3d890c..759ed77dbcb 100644
--- a/compiler/rustc_middle/src/thir/visit.rs
+++ b/compiler/rustc_middle/src/thir/visit.rs
@@ -246,7 +246,7 @@ pub fn walk_pat<'thir, 'tcx: 'thir, V: Visitor<'thir, 'tcx>>(
                 visitor.visit_pat(&subpattern.pattern);
             }
         }
-        Constant { value: _ } => {}
+        Constant { value: _ } | NamedConstant { value: _, span: _ } => {}
         InlineConstant { def: _, subpattern } => visitor.visit_pat(subpattern),
         Range(_) => {}
         Slice { prefix, slice, suffix } | Array { prefix, slice, suffix } => {