Do not favor projection type when pointing out arg causing fulfillment error

Author: compiler-errors

compiler/rustc_typeck/src/check/fn_ctxt/checks.rs

@@ -1742,7 +1742,25 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             .inputs()
             .iter()
             .enumerate()
-            .filter(|(_, ty)| ty.walk().any(|arg| arg == param_to_point_at))
+            .filter(|(_, ty)| {
+                let mut walk = ty.walk();
+                while let Some(arg) = walk.next() {
+                    if arg == param_to_point_at {
+                        return true;
+                    } else if let ty::GenericArgKind::Type(ty) = arg.unpack()
+                        && let ty::Projection(..) = ty.kind()
+                    {
+                        // This logic may seem a bit strange, but typically when
+                        // we have a projection type in a function signature, the
+                        // argument that's being passed into that signature is
+                        // not actually constraining that projection in a meaningful
+                        // way. So we skip it, and see improvements in some UI tests
+                        // due to it.
+                        walk.skip_current_subtree();
+                    }
+                }
+                false
+            })
             .collect();
 
         if let [(idx, _)] = args_referencing_param.as_slice()

src/test/ui/associated-types/associated-types-path-2.stderr

@@ -17,10 +17,12 @@ LL |     f1(2i32, 4u32);
    |               ~~~
 
 error[E0277]: the trait bound `u32: Foo` is not satisfied
-  --> $DIR/associated-types-path-2.rs:29:5
+  --> $DIR/associated-types-path-2.rs:29:8
    |
 LL |     f1(2u32, 4u32);
-   |     ^^ the trait `Foo` is not implemented for `u32`
+   |     -- ^^^^ the trait `Foo` is not implemented for `u32`
+   |     |
+   |     required by a bound introduced by this call
    |
    = help: the trait `Foo` is implemented for `i32`
 note: required by a bound in `f1`
@@ -38,10 +40,12 @@ LL |     f1(2u32, 4u32);
    = help: the trait `Foo` is implemented for `i32`
 
 error[E0277]: the trait bound `u32: Foo` is not satisfied
-  --> $DIR/associated-types-path-2.rs:35:5
+  --> $DIR/associated-types-path-2.rs:35:8
    |
 LL |     f1(2u32, 4i32);
-   |     ^^ the trait `Foo` is not implemented for `u32`
+   |     -- ^^^^ the trait `Foo` is not implemented for `u32`
+   |     |
+   |     required by a bound introduced by this call
    |
    = help: the trait `Foo` is implemented for `i32`
 note: required by a bound in `f1`

src/test/ui/associated-types/issue-27675-unchecked-bounds.stderr

@@ -1,10 +1,8 @@
 error[E0277]: the trait bound `T: Copy` is not satisfied
-  --> $DIR/issue-27675-unchecked-bounds.rs:15:31
+  --> $DIR/issue-27675-unchecked-bounds.rs:15:12
    |
 LL |     copy::<dyn Setup<From=T>>(t)
-   |     ------------------------- ^ the trait `Copy` is not implemented for `T`
-   |     |
-   |     required by a bound introduced by this call
+   |            ^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `T`
    |
 note: required by a bound in `copy`
   --> $DIR/issue-27675-unchecked-bounds.rs:10:12

src/test/ui/issues/issue-69683.stderr

@@ -14,7 +14,7 @@ error[E0283]: type annotations needed
   --> $DIR/issue-69683.rs:30:10
    |
 LL |     0u16.foo(b);
-   |          ^^^ - type must be known at this point
+   |          ^^^
    |
 note: multiple `impl`s satisfying `u8: Element<_>` found
   --> $DIR/issue-69683.rs:5:1

src/test/ui/traits/object/enforce-supertrait-projection.stderr

@@ -1,14 +1,12 @@
 error[E0271]: type mismatch resolving `<dyn Trait<B = B, A = A> as SuperTrait>::A == B`
-  --> $DIR/enforce-supertrait-projection.rs:9:42
+  --> $DIR/enforce-supertrait-projection.rs:9:17
    |
 LL | fn transmute<A, B>(x: A) -> B {
    |              -  - expected type parameter
    |              |
    |              found type parameter
 LL |     foo::<A, B, dyn Trait<A = A, B = B>>(x)
-   |     ------------------------------------ ^ expected type parameter `B`, found type parameter `A`
-   |     |
-   |     required by a bound introduced by this call
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^ expected type parameter `B`, found type parameter `A`
    |
    = note: expected type parameter `B`
               found type parameter `A`