Rollup merge of rust-lang#128377 - veera-sivarajan:fix-128249, r=davidtwco

Fix ICE Caused by Incorrectly Delaying E0107

Fixes  rust-lang#128249

For the following code:
```rust
trait Foo<T> {}
impl Foo<T: Default> for u8 {}
```
rust-lang#126054 added some logic to delay emitting E0107 as the names of associated type `T` in the impl header and generic parameter `T` in `trait Foo` match.

But it failed to ensure whether such unexpected associated type bounds are coming from a impl block header. This caused an ICE as the compiler was delaying E0107 for code like:
```rust
trait Trait<Type> {
    type Type;

    fn method(&self) -> impl Trait<Type: '_>;
}
```
because it assumed the associated type bound `Type: '_` is for the generic parameter `Type` in `trait Trait` since the names are same.

This PR adds a check to ensure that E0107 is delayed only in the context of impl block header.

Author: matthiaskrgr

compiler/rustc_hir/src/hir.rs

@@ -3700,6 +3700,11 @@ impl<'hir> OwnerNode<'hir> {
         }
     }
 
+    /// Check if node is an impl block.
+    pub fn is_impl_block(&self) -> bool {
+        matches!(self, OwnerNode::Item(Item { kind: ItemKind::Impl(_), .. }))
+    }
+
     expect_methods_self! {
         expect_item,         &'hir Item<'hir>,        OwnerNode::Item(n),        n;
         expect_foreign_item, &'hir ForeignItem<'hir>, OwnerNode::ForeignItem(n), n;

compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs

@@ -552,21 +552,34 @@ pub(crate) fn check_generic_arg_count(
                 synth_provided,
             }
         } else {
-            let num_missing_args = expected_max - provided;
+            // Check if associated type bounds are incorrectly written in impl block header like:
+            // ```
+            // trait Foo<T> {}
+            // impl Foo<T: Default> for u8 {}
+            // ```
+            let parent_is_impl_block = cx
+                .tcx()
+                .hir()
+                .parent_owner_iter(seg.hir_id)
+                .next()
+                .is_some_and(|(_, owner_node)| owner_node.is_impl_block());
+            if parent_is_impl_block {
+                let constraint_names: Vec<_> =
+                    gen_args.constraints.iter().map(|b| b.ident.name).collect();
+                let param_names: Vec<_> = gen_params
+                    .own_params
+                    .iter()
+                    .filter(|param| !has_self || param.index != 0) // Assumes `Self` will always be the first parameter
+                    .map(|param| param.name)
+                    .collect();
+                if constraint_names == param_names {
+                    // We set this to true and delay emitting `WrongNumberOfGenericArgs`
+                    // to provide a succinct error for cases like issue #113073
+                    all_params_are_binded = true;
+                };
+            }
 
-            let constraint_names: Vec<_> =
-                gen_args.constraints.iter().map(|b| b.ident.name).collect();
-            let param_names: Vec<_> = gen_params
-                .own_params
-                .iter()
-                .filter(|param| !has_self || param.index != 0) // Assumes `Self` will always be the first parameter
-                .map(|param| param.name)
-                .collect();
-            if constraint_names == param_names {
-                // We set this to true and delay emitting `WrongNumberOfGenericArgs`
-                // to provide a succinct error for cases like issue #113073
-                all_params_are_binded = true;
-            };
+            let num_missing_args = expected_max - provided;
 
             GenericArgsInfo::MissingTypesOrConsts {
                 num_missing_args,

tests/ui/associated-type-bounds/name-same-as-generic-type-issue-128249.rs

@@ -0,0 +1,15 @@
+trait Trait<Type> {
+    type Type;
+
+    fn one(&self, val:  impl Trait<Type: Default>);
+    //~^ ERROR trait takes 1 generic argument but 0 generic arguments were supplied
+
+    fn two<T: Trait<Type: Default>>(&self) -> T;
+    //~^ ERROR trait takes 1 generic argument but 0 generic arguments were supplied
+
+    fn three<T>(&self) -> T where
+        T: Trait<Type: Default>,;
+    //~^ ERROR trait takes 1 generic argument but 0 generic arguments were supplied
+}
+
+fn main() {}

tests/ui/associated-type-bounds/name-same-as-generic-type-issue-128249.stderr

@@ -0,0 +1,51 @@
+error[E0107]: trait takes 1 generic argument but 0 generic arguments were supplied
+  --> $DIR/name-same-as-generic-type-issue-128249.rs:4:30
+   |
+LL |     fn one(&self, val:  impl Trait<Type: Default>);
+   |                              ^^^^^ expected 1 generic argument
+   |
+note: trait defined here, with 1 generic parameter: `Type`
+  --> $DIR/name-same-as-generic-type-issue-128249.rs:1:7
+   |
+LL | trait Trait<Type> {
+   |       ^^^^^ ----
+help: add missing generic argument
+   |
+LL |     fn one(&self, val:  impl Trait<Type, Type: Default>);
+   |                                    +++++
+
+error[E0107]: trait takes 1 generic argument but 0 generic arguments were supplied
+  --> $DIR/name-same-as-generic-type-issue-128249.rs:7:15
+   |
+LL |     fn two<T: Trait<Type: Default>>(&self) -> T;
+   |               ^^^^^ expected 1 generic argument
+   |
+note: trait defined here, with 1 generic parameter: `Type`
+  --> $DIR/name-same-as-generic-type-issue-128249.rs:1:7
+   |
+LL | trait Trait<Type> {
+   |       ^^^^^ ----
+help: add missing generic argument
+   |
+LL |     fn two<T: Trait<Type, Type: Default>>(&self) -> T;
+   |                     +++++
+
+error[E0107]: trait takes 1 generic argument but 0 generic arguments were supplied
+  --> $DIR/name-same-as-generic-type-issue-128249.rs:11:12
+   |
+LL |         T: Trait<Type: Default>,;
+   |            ^^^^^ expected 1 generic argument
+   |
+note: trait defined here, with 1 generic parameter: `Type`
+  --> $DIR/name-same-as-generic-type-issue-128249.rs:1:7
+   |
+LL | trait Trait<Type> {
+   |       ^^^^^ ----
+help: add missing generic argument
+   |
+LL |         T: Trait<Type, Type: Default>,;
+   |                  +++++
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0107`.