say "the lifetime" instead of "some lifetime" when it feels right

In particular, when we want to indicate that there is a connection
between the self type and the other types.

Author: nikomatsakis

src/librustc/infer/error_reporting/nice_region_error/placeholder_error.rs

@@ -95,7 +95,7 @@ impl NiceRegionError<'me, 'gcx, 'tcx> {
                 ty::RePlaceholder(_),
             )) => {
                 // I actually can't see why this would be the case ever.
-            },
+            }
 
             Some(RegionResolutionError::ConcreteFailure(
                 SubregionOrigin::Subtype(TypeTrace {
@@ -219,6 +219,10 @@ impl NiceRegionError<'me, 'gcx, 'tcx> {
             }
         });
 
+        let self_ty_has_vid = self
+            .tcx
+            .any_free_region_meets(&actual_trait_ref.self_ty(), |r| Some(r) == vid);
+
         RegionHighlightMode::maybe_highlighting_region(sub_placeholder, has_sub, || {
             RegionHighlightMode::maybe_highlighting_region(sup_placeholder, has_sup, || {
                 match (has_sub, has_sup) {
@@ -254,12 +258,21 @@ impl NiceRegionError<'me, 'gcx, 'tcx> {
 
         RegionHighlightMode::maybe_highlighting_region(vid, has_vid, || match has_vid {
             Some(n) => {
-                err.note(&format!(
-                    "but `{}` only implements `{}` for some lifetime `'{}`",
-                    actual_trait_ref.self_ty(),
-                    actual_trait_ref,
-                    n
-                ));
+                if self_ty_has_vid {
+                    err.note(&format!(
+                        "but `{}` only implements `{}` for the lifetime `'{}`",
+                        actual_trait_ref.self_ty(),
+                        actual_trait_ref,
+                        n
+                    ));
+                } else {
+                    err.note(&format!(
+                        "but `{}` only implements `{}` for some lifetime `'{}`",
+                        actual_trait_ref.self_ty(),
+                        actual_trait_ref,
+                        n
+                    ));
+                }
             }
             None => {
                 err.note(&format!(

src/test/ui/generator/auto-trait-regions.stderr

@@ -5,7 +5,7 @@ LL |     assert_foo(gen);
    |     ^^^^^^^^^^
    |
    = note: `&'0 OnlyFooIfStaticRef` must implement `Foo` for any lifetime `'0`
-   = note: but `&'1 OnlyFooIfStaticRef` only implements `Foo` for some lifetime `'1`
+   = note: but `&'1 OnlyFooIfStaticRef` only implements `Foo` for the lifetime `'1`
 
 error: implementation of `Foo` is not general enough
   --> $DIR/auto-trait-regions.rs:48:5
@@ -14,7 +14,7 @@ LL |     assert_foo(gen);
    |     ^^^^^^^^^^
    |
    = note: `A<'0, '1>` must implement `Foo` for any two lifetimes `'0` and `'1`
-   = note: but `A<'_, '2>` only implements `Foo` for some lifetime `'2`
+   = note: but `A<'_, '2>` only implements `Foo` for the lifetime `'2`
 
 error: aborting due to 2 previous errors
 

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

@@ -16,7 +16,7 @@ LL |     want_hrtb::<&'a u32>() //~ ERROR
    |
    = note: Due to a where-clause on `want_hrtb`,
    = note: `&'a u32` must implement `Foo<&'0 isize>` for any lifetime `'0`
-   = note: but `&'1 u32` only implements `Foo<&'1 isize>` for some lifetime `'1`
+   = note: but `&'1 u32` only implements `Foo<&'1 isize>` for the lifetime `'1`
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-54302-cases.stderr

@@ -5,7 +5,7 @@ LL |     <u32 as RefFoo<u32>>::ref_foo(a)
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `&'0 u32` must implement `Foo<'static, u32>` for any lifetime `'0`
-   = note: but `&'1 _` only implements `Foo<'_, _>` for some lifetime `'1`
+   = note: but `&'1 _` only implements `Foo<'_, _>` for the lifetime `'1`
 
 error: implementation of `Foo` is not general enough
   --> $DIR/issue-54302-cases.rs:69:5
@@ -14,7 +14,7 @@ LL |     <i32 as RefFoo<i32>>::ref_foo(a)
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `&'0 i32` must implement `Foo<'static, i32>` for any lifetime `'0`
-   = note: but `&'1 _` only implements `Foo<'_, _>` for some lifetime `'1`
+   = note: but `&'1 _` only implements `Foo<'_, _>` for the lifetime `'1`
 
 error: implementation of `Foo` is not general enough
   --> $DIR/issue-54302-cases.rs:75:5
@@ -23,7 +23,7 @@ LL |     <u64 as RefFoo<u64>>::ref_foo(a)
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `&'0 u64` must implement `Foo<'static, u64>` for any lifetime `'0`
-   = note: but `&'1 _` only implements `Foo<'_, _>` for some lifetime `'1`
+   = note: but `&'1 _` only implements `Foo<'_, _>` for the lifetime `'1`
 
 error: implementation of `Foo` is not general enough
   --> $DIR/issue-54302-cases.rs:81:5
@@ -32,7 +32,7 @@ LL |     <i64 as RefFoo<i64>>::ref_foo(a)
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `&'0 i64` must implement `Foo<'static, i64>` for any lifetime `'0`
-   = note: but `&'1 _` only implements `Foo<'_, _>` for some lifetime `'1`
+   = note: but `&'1 _` only implements `Foo<'_, _>` for the lifetime `'1`
 
 error: aborting due to 4 previous errors
 

src/test/ui/where-clauses/where-for-self-2.stderr

@@ -6,7 +6,7 @@ LL |     foo(&X); //~ ERROR implementation of `Bar` is not general enough
    |
    = note: Due to a where-clause on `foo`,
    = note: `&'0 _` must implement `Bar` for any lifetime `'0`
-   = note: but `&'1 u32` only implements `Bar` for some lifetime `'1`
+   = note: but `&'1 u32` only implements `Bar` for the lifetime `'1`
 
 error: aborting due to previous error