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`
```

Author: estebank

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

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 } => {

compiler/rustc_mir_build/src/build/custom/parse/instruction.rs

@@ -144,7 +144,9 @@ impl<'a, 'tcx> ParseCtxt<'a, 'tcx> {
         let mut targets = Vec::new();
         for arm in rest {
             let arm = &self.thir[*arm];
-            let PatKind::Constant { value } = arm.pattern.kind else {
+            let (PatKind::Constant { value } | PatKind::NamedConstant { value, span: _ }) =
+                arm.pattern.kind
+            else {
                 return Err(ParseError {
                     span: arm.pattern.span,
                     item_description: format!("{:?}", arm.pattern.kind),

compiler/rustc_mir_build/src/build/matches/match_pair.rs

@@ -129,7 +129,9 @@ impl<'pat, 'tcx> MatchPairTree<'pat, 'tcx> {
                 }
             }
 
-            PatKind::Constant { value } => TestCase::Constant { value },
+            PatKind::Constant { value } | PatKind::NamedConstant { value, span: _ } => {
+                TestCase::Constant { value }
+            }
 
             PatKind::AscribeUserType {
                 ascription: thir::Ascription { ref annotation, variance },

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -882,6 +882,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
 
             PatKind::Constant { .. }
+            | PatKind::NamedConstant { .. }
             | PatKind::Range { .. }
             | PatKind::Wild
             | PatKind::Never

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -316,6 +316,7 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                 PatKind::Binding { .. }
                 // match is conditional on having this value
                 | PatKind::Constant { .. }
+                | PatKind::NamedConstant { .. }
                 | PatKind::Variant { .. }
                 | PatKind::Leaf { .. }
                 | PatKind::Deref { .. }

compiler/rustc_mir_build/src/errors.rs

@@ -860,8 +860,10 @@ pub(crate) struct PatternNotCovered<'s, 'tcx> {
     pub(crate) uncovered: Uncovered,
     #[subdiagnostic]
     pub(crate) inform: Option<Inform>,
+    #[label(mir_build_confused)]
+    pub(crate) interpreted_as_const: Option<Span>,
     #[subdiagnostic]
-    pub(crate) interpreted_as_const: Option<InterpretedAsConst>,
+    pub(crate) interpreted_as_const_sugg: Option<InterpretedAsConst>,
     #[subdiagnostic]
     pub(crate) adt_defined_here: Option<AdtDefinedHere<'tcx>>,
     #[note(mir_build_privately_uninhabited)]
@@ -913,7 +915,6 @@ impl<'tcx> Subdiagnostic for AdtDefinedHere<'tcx> {
     code = "{variable}_var",
     applicability = "maybe-incorrect"
 )]
-#[label(mir_build_confused)]
 pub(crate) struct InterpretedAsConst {
     #[primary_span]
     pub(crate) span: Span,

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -668,8 +668,20 @@ impl<'p, 'tcx> MatchVisitor<'p, 'tcx> {
         let mut let_suggestion = None;
         let mut misc_suggestion = None;
         let mut interpreted_as_const = None;
+        let mut interpreted_as_const_sugg = None;
 
-        if let PatKind::Constant { .. }
+        if let PatKind::NamedConstant { span, .. }
+        | PatKind::AscribeUserType {
+            subpattern: box Pat { kind: PatKind::NamedConstant { span, .. }, .. },
+            ..
+        } = pat.kind
+            && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(pat.span)
+        {
+            // When we encounter a constant as the binding name, point at the `const` definition.
+            interpreted_as_const = Some(span);
+            interpreted_as_const_sugg =
+                Some(InterpretedAsConst { span: pat.span, variable: snippet });
+        } else if let PatKind::Constant { .. }
         | PatKind::AscribeUserType {
             subpattern: box Pat { kind: PatKind::Constant { .. }, .. },
             ..
@@ -683,7 +695,8 @@ impl<'p, 'tcx> MatchVisitor<'p, 'tcx> {
                     start_span: pat.span.shrink_to_lo(),
                 });
             } else if snippet.chars().all(|c| c.is_alphanumeric() || c == '_') {
-                interpreted_as_const =
+                interpreted_as_const = Some(pat.span);
+                interpreted_as_const_sugg =
                     Some(InterpretedAsConst { span: pat.span, variable: snippet });
             }
         }
@@ -733,6 +746,7 @@ impl<'p, 'tcx> MatchVisitor<'p, 'tcx> {
             uncovered: Uncovered::new(pat.span, &cx, witnesses),
             inform,
             interpreted_as_const,
+            interpreted_as_const_sugg,
             witness_1_is_privately_uninhabited,
             _p: (),
             pattern_ty,

compiler/rustc_mir_build/src/thir/pattern/mod.rs

@@ -157,7 +157,9 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
                     }
                     kind => (kind, None, None),
                 };
-                let value = if let PatKind::Constant { value } = kind {
+                let value = if let PatKind::Constant { value }
+                | PatKind::NamedConstant { value, span: _ } = kind
+                {
                     value
                 } else {
                     let msg = format!(
@@ -560,9 +562,15 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
             _ => return pat_from_kind(self.lower_variant_or_leaf(res, id, span, ty, vec![])),
         };
 
+        // HERE
         let args = self.typeck_results.node_args(id);
         let c = ty::Const::new_unevaluated(self.tcx, ty::UnevaluatedConst { def: def_id, args });
-        let pattern = self.const_to_pat(c, ty, id, span);
+        let def_span = self.tcx.def_span(def_id);
+        let mut pattern = self.const_to_pat(c, ty, id, span);
+        if let PatKind::Constant { value } = pattern.kind {
+            pattern.kind = PatKind::NamedConstant { value, span: def_span };
+        }
+        tracing::info!("pattern {pattern:#?} {c:?} {ty:?} {id:?}");
 
         if !is_associated_const {
             return pattern;

compiler/rustc_mir_build/src/thir/print.rs

@@ -702,7 +702,7 @@ impl<'a, 'tcx> ThirPrinter<'a, 'tcx> {
                 self.print_pat(subpattern, depth_lvl + 2);
                 print_indented!(self, "}", depth_lvl + 1);
             }
-            PatKind::Constant { value } => {
+            PatKind::Constant { value } | PatKind::NamedConstant { value, span: _ } => {
                 print_indented!(self, "Constant {", depth_lvl + 1);
                 print_indented!(self, format!("value: {:?}", value), depth_lvl + 2);
                 print_indented!(self, "}", depth_lvl + 1);