Rollup merge of #85478 - FabianWolff:issue-85348, r=petrochenkov

Disallow shadowing const parameters

This pull request fixes #85348. Trying to shadow a `const` parameter as follows:
```rust
fn foo<const N: i32>() {
    let N @ _ = 0;
}
```
currently causes an ICE. With my changes, I get:
```
error[E0530]: let bindings cannot shadow const parameters
 --> test.rs:2:9
  |
1 | fn foo<const N: i32>() {
  |              - the const parameter `N` is defined here
2 |     let N @ _ = 0;
  |         ^ cannot be named the same as a const parameter

error: aborting due to previous error
```
This is the same error you get when trying to shadow a constant:
```rust
const N: i32 = 0;
let N @ _ = 0;
```
```
error[E0530]: let bindings cannot shadow constants
 --> src/lib.rs:3:5
  |
2 | const N: i32 = 0;
  | ----------------- the constant `N` is defined here
3 | let N @ _ = 0;
  |     ^ cannot be named the same as a constant

error: aborting due to previous error
```
The reason for disallowing shadowing in both cases is described [here](#33118 (comment)) (the comment there only talks about constants, but the same reasoning applies to `const` parameters).

compiler/rustc_resolve/src/diagnostics.rs

@@ -425,24 +425,29 @@ impl<'a> Resolver<'a> {
                 }
                 err
             }
-            ResolutionError::BindingShadowsSomethingUnacceptable(what_binding, name, binding) => {
-                let res = binding.res();
-                let shadows_what = res.descr();
+            ResolutionError::BindingShadowsSomethingUnacceptable {
+                shadowing_binding_descr,
+                name,
+                participle,
+                article,
+                shadowed_binding_descr,
+                shadowed_binding_span,
+            } => {
                 let mut err = struct_span_err!(
                     self.session,
                     span,
                     E0530,
                     "{}s cannot shadow {}s",
-                    what_binding,
-                    shadows_what
+                    shadowing_binding_descr,
+                    shadowed_binding_descr,
                 );
                 err.span_label(
                     span,
-                    format!("cannot be named the same as {} {}", res.article(), shadows_what),
+                    format!("cannot be named the same as {} {}", article, shadowed_binding_descr),
                 );
-                let participle = if binding.is_import() { "imported" } else { "defined" };
-                let msg = format!("the {} `{}` is {} here", shadows_what, name, participle);
-                err.span_label(binding.span, msg);
+                let msg =
+                    format!("the {} `{}` is {} here", shadowed_binding_descr, name, participle);
+                err.span_label(shadowed_binding_span, msg);
                 err
             }
             ResolutionError::ForwardDeclaredTyParam => {

compiler/rustc_resolve/src/late.rs

@@ -1763,13 +1763,33 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                 // to something unusable as a pattern (e.g., constructor function),
                 // but we still conservatively report an error, see
                 // issues/33118#issuecomment-233962221 for one reason why.
+                let binding = binding.expect("no binding for a ctor or static");
                 self.report_error(
                     ident.span,
-                    ResolutionError::BindingShadowsSomethingUnacceptable(
-                        pat_src.descr(),
-                        ident.name,
-                        binding.expect("no binding for a ctor or static"),
-                    ),
+                    ResolutionError::BindingShadowsSomethingUnacceptable {
+                        shadowing_binding_descr: pat_src.descr(),
+                        name: ident.name,
+                        participle: if binding.is_import() { "imported" } else { "defined" },
+                        article: binding.res().article(),
+                        shadowed_binding_descr: binding.res().descr(),
+                        shadowed_binding_span: binding.span,
+                    },
+                );
+                None
+            }
+            Res::Def(DefKind::ConstParam, def_id) => {
+                // Same as for DefKind::Const above, but here, `binding` is `None`, so we
+                // have to construct the error differently
+                self.report_error(
+                    ident.span,
+                    ResolutionError::BindingShadowsSomethingUnacceptable {
+                        shadowing_binding_descr: pat_src.descr(),
+                        name: ident.name,
+                        participle: "defined",
+                        article: res.article(),
+                        shadowed_binding_descr: res.descr(),
+                        shadowed_binding_span: self.r.opt_span(def_id).expect("const parameter defined outside of local crate"),
+                    }
                 );
                 None
             }

compiler/rustc_resolve/src/lib.rs

@@ -233,7 +233,14 @@ enum ResolutionError<'a> {
         /* current */ &'static str,
     ),
     /// Error E0530: `X` bindings cannot shadow `Y`s.
-    BindingShadowsSomethingUnacceptable(&'static str, Symbol, &'a NameBinding<'a>),
+    BindingShadowsSomethingUnacceptable {
+        shadowing_binding_descr: &'static str,
+        name: Symbol,
+        participle: &'static str,
+        article: &'static str,
+        shadowed_binding_descr: &'static str,
+        shadowed_binding_span: Span,
+    },
     /// Error E0128: generic parameters with a default cannot use forward-declared identifiers.
     ForwardDeclaredTyParam, // FIXME(const_generics_defaults)
     /// ERROR E0770: the type of const parameters must not depend on other generic parameters.

src/test/ui/resolve/issue-85348.rs

@@ -0,0 +1,12 @@
+// Checks whether shadowing a const parameter leads to an ICE (#85348).
+
+impl<const N: usize> ArrayWindowsExample {
+//~^ ERROR: cannot find type `ArrayWindowsExample` in this scope [E0412]
+    fn next() {
+        let mut N;
+        //~^ ERROR: let bindings cannot shadow const parameters [E0530]
+        //~| ERROR: type annotations needed [E0282]
+    }
+}
+
+fn main() {}

src/test/ui/resolve/issue-85348.stderr

@@ -0,0 +1,25 @@
+error[E0530]: let bindings cannot shadow const parameters
+  --> $DIR/issue-85348.rs:6:17
+   |
+LL | impl<const N: usize> ArrayWindowsExample {
+   |            - the const parameter `N` is defined here
+...
+LL |         let mut N;
+   |                 ^ cannot be named the same as a const parameter
+
+error[E0412]: cannot find type `ArrayWindowsExample` in this scope
+  --> $DIR/issue-85348.rs:3:22
+   |
+LL | impl<const N: usize> ArrayWindowsExample {
+   |                      ^^^^^^^^^^^^^^^^^^^ not found in this scope
+
+error[E0282]: type annotations needed
+  --> $DIR/issue-85348.rs:6:13
+   |
+LL |         let mut N;
+   |             ^^^^^ consider giving `N` a type
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0282, E0412, E0530.
+For more information about an error, try `rustc --explain E0282`.

src/test/ui/resolve/shadow-const-param.rs

@@ -0,0 +1,20 @@
+// Checks that const parameters cannot be shadowed with fresh bindings
+// even in syntactically unambiguous contexts. See
+// https://github.com/rust-lang/rust/issues/33118#issuecomment-233962221
+
+fn foo<const N: i32>(i: i32) -> bool {
+    match i {
+        N @ _ => true,
+        //~^ ERROR: match bindings cannot shadow const parameters [E0530]
+    }
+}
+
+fn bar<const N: i32>(i: i32) -> bool {
+    let N @ _ = 0;
+    //~^ ERROR: let bindings cannot shadow const parameters [E0530]
+    match i {
+        N @ _ => true,
+    }
+}
+
+fn main() {}

src/test/ui/resolve/shadow-const-param.stderr

@@ -0,0 +1,20 @@
+error[E0530]: match bindings cannot shadow const parameters
+  --> $DIR/shadow-const-param.rs:7:9
+   |
+LL | fn foo<const N: i32>(i: i32) -> bool {
+   |              - the const parameter `N` is defined here
+LL |     match i {
+LL |         N @ _ => true,
+   |         ^ cannot be named the same as a const parameter
+
+error[E0530]: let bindings cannot shadow const parameters
+  --> $DIR/shadow-const-param.rs:13:9
+   |
+LL | fn bar<const N: i32>(i: i32) -> bool {
+   |              - the const parameter `N` is defined here
+LL |     let N @ _ = 0;
+   |         ^ cannot be named the same as a const parameter
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0530`.