Auto merge of #45050 - petrochenkov:ambind, r=nikomatsakis

resolve: Use same rules for disambiguating fresh bindings in `match` and `let`

Resolve `Unit` as a unit struct pattern in
```rust
struct Unit;

let Unit = x;
```
consistently with
```rust
match x {
    Unit => {}
}
```
It was previously an error.
(The change also applies to unit variants and constants.)

Fixes https://users.rust-lang.org/t/e0530-cannot-shadow-unit-structs-what-in-the-earthly-what/13054
(This particular change doesn't depend on a fix for the issue mentioned in https://users.rust-lang.org/t/e0530-cannot-shadow-unit-structs-what-in-the-earthly-what/13054/4)

cc @rust-lang/lang
r? @nikomatsakis

src/librustc_const_eval/check_match.rs

@@ -260,7 +260,14 @@ impl<'a, 'tcx> MatchVisitor<'a, 'tcx> {
                 "refutable pattern in {}: `{}` not covered",
                 origin, pattern_string
             );
-            diag.span_label(pat.span, format!("pattern `{}` not covered", pattern_string));
+            let label_msg = match pat.node {
+                PatKind::Path(hir::QPath::Resolved(None, ref path))
+                        if path.segments.len() == 1 && path.segments[0].parameters.is_none() => {
+                    format!("interpreted as a {} pattern, not new variable", path.def.kind_name())
+                }
+                _ => format!("pattern `{}` not covered", pattern_string),
+            };
+            diag.span_label(pat.span, label_msg);
             diag.emit();
         });
     }