Detect non-lifetime binder params shadowing item params

compiler/rustc_resolve/src/late.rs

@@ -2671,17 +2671,17 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
         // Store all seen lifetimes names from outer scopes.
         let mut seen_lifetimes = FxHashSet::default();
 
-        // We also can't shadow bindings from the parent item
-        if let RibKind::AssocItem = kind {
-            let mut add_bindings_for_ns = |ns| {
-                let parent_rib = self.ribs[ns]
-                    .iter()
-                    .rfind(|r| matches!(r.kind, RibKind::Item(..)))
-                    .expect("associated item outside of an item");
+        // We also can't shadow bindings from associated parent items.
+        for ns in [ValueNS, TypeNS] {
+            for parent_rib in self.ribs[ns].iter().rev() {
                 seen_bindings.extend(parent_rib.bindings.keys().map(|ident| (*ident, ident.span)));
-            };
-            add_bindings_for_ns(ValueNS);
-            add_bindings_for_ns(TypeNS);
+
+                // Break at mod level, to account for nested items which are
+                // allowed to shadow generic param names.
+                if matches!(parent_rib.kind, RibKind::Module(..)) {
+                    break;
+                }
+            }
         }
 
         // Forbid shadowing lifetime bindings

tests/crashes/119716-2.rs

@@ -1,4 +1,4 @@
 //@ known-bug: #119716
 #![feature(non_lifetime_binders)]
 trait Trait<T> {}
-fn f<T>() -> impl for<T> Trait<impl Trait<T>> {}
+fn f() -> impl for<T> Trait<impl Trait<T>> {}

tests/ui/const-generics/generic_const_exprs/ice-predicates-of-no-entry-found-for-key-119275.rs

@@ -9,9 +9,10 @@ fn bug<const N: Nat>(&self)
 where
     for<const N: usize = 3, T = u32> [(); COT::BYTES]:,
     //~^ ERROR only lifetime parameters can be used in this context
-    //~^^ ERROR defaults for generic parameters are not allowed in `for<...>` binders
-    //~^^^ ERROR defaults for generic parameters are not allowed in `for<...>` binders
-    //~^^^^ ERROR failed to resolve: use of undeclared type `COT`
+    //~| ERROR defaults for generic parameters are not allowed in `for<...>` binders
+    //~| ERROR defaults for generic parameters are not allowed in `for<...>` binders
+    //~| ERROR failed to resolve: use of undeclared type `COT`
+    //~| ERROR  the name `N` is already used for a generic parameter in this item's generic parameters
 {
 }
 

tests/ui/const-generics/generic_const_exprs/ice-predicates-of-no-entry-found-for-key-119275.stderr

@@ -6,6 +6,15 @@ LL | fn bug<const N: Nat>(&self)
    |
    = note: associated functions are those in `impl` or `trait` definitions
 
+error[E0403]: the name `N` is already used for a generic parameter in this item's generic parameters
+  --> $DIR/ice-predicates-of-no-entry-found-for-key-119275.rs:10:15
+   |
+LL | fn bug<const N: Nat>(&self)
+   |              - first use of `N`
+...
+LL |     for<const N: usize = 3, T = u32> [(); COT::BYTES]:,
+   |               ^ already used
+
 error[E0412]: cannot find type `Nat` in this scope
   --> $DIR/ice-predicates-of-no-entry-found-for-key-119275.rs:6:17
    |
@@ -40,7 +49,7 @@ error[E0433]: failed to resolve: use of undeclared type `COT`
 LL |     for<const N: usize = 3, T = u32> [(); COT::BYTES]:,
    |                                           ^^^ use of undeclared type `COT`
 
-error: aborting due to 6 previous errors
+error: aborting due to 7 previous errors
 
-Some errors have detailed explanations: E0412, E0433, E0658.
-For more information about an error, try `rustc --explain E0412`.
+Some errors have detailed explanations: E0403, E0412, E0433, E0658.
+For more information about an error, try `rustc --explain E0403`.

tests/ui/traits/non_lifetime_binders/shadowed.rs

@@ -0,0 +1,18 @@
+#![feature(non_lifetime_binders)]
+//~^ WARN the feature `non_lifetime_binders` is incomplete
+
+fn function<T>() where for<T> (): Sized {}
+//~^ ERROR the name `T` is already used for a generic parameter
+
+struct Struct<T>(T) where for<T> (): Sized;
+//~^ ERROR the name `T` is already used for a generic parameter
+
+impl<T> Struct<T> {
+    fn method() where for<T> (): Sized {}
+    //~^ ERROR the name `T` is already used for a generic parameter
+}
+
+fn repeated() where for<T, T> (): Sized {}
+//~^ ERROR the name `T` is already used for a generic parameter
+
+fn main() {}

tests/ui/traits/non_lifetime_binders/shadowed.stderr

@@ -0,0 +1,44 @@
+error[E0403]: the name `T` is already used for a generic parameter in this item's generic parameters
+  --> $DIR/shadowed.rs:4:28
+   |
+LL | fn function<T>() where for<T> (): Sized {}
+   |             -              ^ already used
+   |             |
+   |             first use of `T`
+
+error[E0403]: the name `T` is already used for a generic parameter in this item's generic parameters
+  --> $DIR/shadowed.rs:7:31
+   |
+LL | struct Struct<T>(T) where for<T> (): Sized;
+   |               -               ^ already used
+   |               |
+   |               first use of `T`
+
+error[E0403]: the name `T` is already used for a generic parameter in this item's generic parameters
+  --> $DIR/shadowed.rs:11:27
+   |
+LL | impl<T> Struct<T> {
+   |      - first use of `T`
+LL |     fn method() where for<T> (): Sized {}
+   |                           ^ already used
+
+error[E0403]: the name `T` is already used for a generic parameter in this item's generic parameters
+  --> $DIR/shadowed.rs:15:28
+   |
+LL | fn repeated() where for<T, T> (): Sized {}
+   |                         -  ^ already used
+   |                         |
+   |                         first use of `T`
+
+warning: the feature `non_lifetime_binders` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/shadowed.rs:1:12
+   |
+LL | #![feature(non_lifetime_binders)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #108185 <https://github.com/rust-lang/rust/issues/108185> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: aborting due to 4 previous errors; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0403`.