Avoid repeating self type in placeholder error

Author: matthewjasper

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/placeholder_error.rs

@@ -370,6 +370,8 @@ impl NiceRegionError<'me, 'tcx> {
             value: trait_ref,
         };
 
+        let same_self_type = actual_trait_ref.self_ty() == expected_trait_ref.self_ty();
+
         let mut expected_trait_ref = highlight_trait_ref(expected_trait_ref);
         expected_trait_ref.highlight.maybe_highlighting_region(sub_placeholder, has_sub);
         expected_trait_ref.highlight.maybe_highlighting_region(sup_placeholder, has_sup);
@@ -385,7 +387,16 @@ impl NiceRegionError<'me, 'tcx> {
                 }
             };
 
-            let mut note = if passive_voice {
+            let mut note = if same_self_type {
+                let mut self_ty = expected_trait_ref.map(|tr| tr.self_ty());
+                self_ty.highlight.maybe_highlighting_region(vid, actual_has_vid);
+                format!(
+                    "{}`{}` must implement `{}`",
+                    if leading_ellipsis { "..." } else { "" },
+                    self_ty,
+                    expected_trait_ref.map(|tr| tr.print_only_trait_path()),
+                )
+            } else if passive_voice {
                 format!(
                     "{}`{}` would have to be implemented for the type `{}`",
                     if leading_ellipsis { "..." } else { "" },
@@ -431,7 +442,12 @@ impl NiceRegionError<'me, 'tcx> {
                 None => true,
             };
 
-            let mut note = if passive_voice {
+            let mut note = if same_self_type {
+                format!(
+                    "...but it actually implements `{}`",
+                    actual_trait_ref.map(|tr| tr.print_only_trait_path()),
+                )
+            } else if passive_voice {
                 format!(
                     "...but `{}` is actually implemented for the type `{}`",
                     actual_trait_ref.map(|tr| tr.print_only_trait_path()),

src/test/ui/associated-types/associated-types-eq-hr.stderr

@@ -35,7 +35,7 @@ LL |     tuple_one::<Tuple>();
    |     ^^^^^^^^^^^^^^^^^^ implementation of `TheTrait` is not general enough
    |
    = note: `Tuple` must implement `TheTrait<(&'0 isize, &'1 isize)>`, for any two lifetimes `'0` and `'1`...
-   = note: ...but `Tuple` actually implements `TheTrait<(&'2 isize, &'2 isize)>`, for some specific lifetime `'2`
+   = note: ...but it actually implements `TheTrait<(&'2 isize, &'2 isize)>`, for some specific lifetime `'2`
 
 error: implementation of `TheTrait` is not general enough
   --> $DIR/associated-types-eq-hr.rs:96:5
@@ -44,7 +44,7 @@ LL |     tuple_one::<Tuple>();
    |     ^^^^^^^^^^^^^^^^^^ implementation of `TheTrait` is not general enough
    |
    = note: `Tuple` must implement `TheTrait<(&'0 isize, &'1 isize)>`, for any two lifetimes `'0` and `'1`...
-   = note: ...but `Tuple` actually implements `TheTrait<(&'2 isize, &'2 isize)>`, for some specific lifetime `'2`
+   = note: ...but it actually implements `TheTrait<(&'2 isize, &'2 isize)>`, for some specific lifetime `'2`
 
 error: implementation of `TheTrait` is not general enough
   --> $DIR/associated-types-eq-hr.rs:102:5
@@ -53,7 +53,7 @@ LL |     tuple_two::<Tuple>();
    |     ^^^^^^^^^^^^^^^^^^ implementation of `TheTrait` is not general enough
    |
    = note: `Tuple` must implement `TheTrait<(&'0 isize, &'1 isize)>`, for any two lifetimes `'0` and `'1`...
-   = note: ...but `Tuple` actually implements `TheTrait<(&'2 isize, &'2 isize)>`, for some specific lifetime `'2`
+   = note: ...but it actually implements `TheTrait<(&'2 isize, &'2 isize)>`, for some specific lifetime `'2`
 
 error: implementation of `TheTrait` is not general enough
   --> $DIR/associated-types-eq-hr.rs:102:5
@@ -62,7 +62,7 @@ LL |     tuple_two::<Tuple>();
    |     ^^^^^^^^^^^^^^^^^^ implementation of `TheTrait` is not general enough
    |
    = note: `Tuple` must implement `TheTrait<(&'0 isize, &'1 isize)>`, for any two lifetimes `'0` and `'1`...
-   = note: ...but `Tuple` actually implements `TheTrait<(&'2 isize, &'2 isize)>`, for some specific lifetime `'2`
+   = note: ...but it actually implements `TheTrait<(&'2 isize, &'2 isize)>`, for some specific lifetime `'2`
 
 error: implementation of `TheTrait` is not general enough
   --> $DIR/associated-types-eq-hr.rs:112:5
@@ -71,7 +71,7 @@ LL |     tuple_four::<Tuple>();
    |     ^^^^^^^^^^^^^^^^^^^ implementation of `TheTrait` is not general enough
    |
    = note: `Tuple` must implement `TheTrait<(&'0 isize, &'1 isize)>`, for any two lifetimes `'0` and `'1`...
-   = note: ...but `Tuple` actually implements `TheTrait<(&'2 isize, &'2 isize)>`, for some specific lifetime `'2`
+   = note: ...but it actually implements `TheTrait<(&'2 isize, &'2 isize)>`, for some specific lifetime `'2`
 
 error: aborting due to 7 previous errors
 

src/test/ui/hrtb/hrtb-conflate-regions.stderr

@@ -5,7 +5,7 @@ LL | fn b() { want_foo2::<SomeStruct>(); }
    |          ^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
    |
    = note: `SomeStruct` must implement `Foo<(&'0 isize, &'1 isize)>`, for any two lifetimes `'0` and `'1`...
-   = note: ...but `SomeStruct` actually implements `Foo<(&'2 isize, &'2 isize)>`, for some specific lifetime `'2`
+   = note: ...but it actually implements `Foo<(&'2 isize, &'2 isize)>`, for some specific lifetime `'2`
 
 error: aborting due to previous error
 

src/test/ui/hrtb/hrtb-exists-forall-trait-contravariant.stderr

@@ -5,7 +5,7 @@ LL |     foo::<()>();
    |     ^^^^^^^^^ implementation of `Trait` is not general enough
    |
    = note: `()` must implement `Trait<for<'b> fn(&'b u32)>`
-   = note: ...but `()` actually implements `Trait<fn(&'0 u32)>`, for some specific lifetime `'0`
+   = note: ...but it actually implements `Trait<fn(&'0 u32)>`, for some specific lifetime `'0`
 
 error: aborting due to previous error
 

src/test/ui/hrtb/hrtb-exists-forall-trait-invariant.stderr

@@ -5,7 +5,7 @@ LL |     foo::<()>();
    |     ^^^^^^^^^ implementation of `Trait` is not general enough
    |
    = note: `()` must implement `Trait<for<'b> fn(Cell<&'b u32>)>`
-   = note: ...but `()` actually implements `Trait<fn(Cell<&'0 u32>)>`, for some specific lifetime `'0`
+   = note: ...but it actually implements `Trait<fn(Cell<&'0 u32>)>`, for some specific lifetime `'0`
 
 error: aborting due to previous error
 

src/test/ui/hrtb/hrtb-just-for-static.stderr

@@ -5,7 +5,7 @@ LL |     want_hrtb::<StaticInt>()
    |     ^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
    |
    = note: `StaticInt` must implement `Foo<&'0 isize>`, for any lifetime `'0`...
-   = note: ...but `StaticInt` actually implements `Foo<&'1 isize>`, for some specific lifetime `'1`
+   = note: ...but it actually implements `Foo<&'1 isize>`, for some specific lifetime `'1`
 
 error: implementation of `Foo` is not general enough
   --> $DIR/hrtb-just-for-static.rs:30:5