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

2 files changed, 24 insertions, 0 deletions

diff --git a/src/test/ui/resolve/name-clash-nullary.rs b/src/test/ui/resolve/name-clash-nullary.rs
new file mode 100644
index 00000000000..adf52c6d8e6
--- /dev/null
+++ b/src/test/ui/resolve/name-clash-nullary.rs
@@ -0,0 +1,13 @@
+// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+  let None: isize = 42; //~ ERROR mismatched types
+}
diff --git a/src/test/ui/resolve/name-clash-nullary.stderr b/src/test/ui/resolve/name-clash-nullary.stderr
new file mode 100644
index 00000000000..014b1fe1b5b
--- /dev/null
+++ b/src/test/ui/resolve/name-clash-nullary.stderr
@@ -0,0 +1,11 @@
+error[E0308]: mismatched types
+  --> $DIR/name-clash-nullary.rs:12:7
+   |
+12 |   let None: isize = 42; //~ ERROR mismatched types
+   |       ^^^^ expected isize, found enum `std::option::Option`
+   |
+   = note: expected type `isize`
+              found type `std::option::Option<_>`
+
+error: aborting due to previous error
+