Use item_name instead of a span snippet when talking about const pattern

Author: estebank

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

@@ -675,13 +675,12 @@ impl<'p, 'tcx> MatchVisitor<'p, 'tcx> {
             subpattern: box Pat { kind: PatKind::Constant { opt_def: Some(def_id), .. }, .. },
             ..
         } = pat.kind
-            && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(pat.span)
         {
             let span = self.tcx.def_span(def_id);
+            let variable = self.tcx.item_name(def_id).to_string();
             // 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 });
+            interpreted_as_const_sugg = Some(InterpretedAsConst { span: pat.span, variable });
         } else if let PatKind::Constant { .. }
         | PatKind::AscribeUserType {
             subpattern: box Pat { kind: PatKind::Constant { .. }, .. },

tests/ui/consts/const-pattern-irrefutable.rs

@@ -1,6 +1,6 @@
 mod foo {
     pub const b: u8 = 2;
-    //~^ missing patterns are not covered because `c` is interpreted as a constant pattern, not a new variable
+    //~^ missing patterns are not covered because `b` is interpreted as a constant pattern, not a new variable
     pub const d: u8 = 2;
     //~^ missing patterns are not covered because `d` is interpreted as a constant pattern, not a new variable
 }

tests/ui/consts/const-pattern-irrefutable.stderr

@@ -18,13 +18,13 @@ error[E0005]: refutable pattern in local binding
   --> $DIR/const-pattern-irrefutable.rs:19:9
    |
 LL |     pub const b: u8 = 2;
-   |     --------------- missing patterns are not covered because `c` is interpreted as a constant pattern, not a new variable
+   |     --------------- missing patterns are not covered because `b` is interpreted as a constant pattern, not a new variable
 ...
 LL |     let c = 4;
    |         ^
    |         |
    |         patterns `0_u8..=1_u8` and `3_u8..=u8::MAX` not covered
-   |         help: introduce a variable instead: `c_var`
+   |         help: introduce a variable instead: `b_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