Slightly tweak the diagnostic for uncovered type parameters

The diagnostic is generally very noisy.
This commit tries to make it slightly less cluttered and more legible by

1. stopping to reuse the lengthy primary message as the primary label.
   This duplication didn't increase the information content and only
   turned the diagnostic into a letter and symbol soup.
   Instead, we now pick a succinct label ("uncovered type parameter")
   that immediately(!) signals to the users in red what is wrong with
   the highlighted type parameter.
2. breaking some information-dense notes into multiple lines.

Finally, this rephrases the slightly awkward and confusing
"type parameter […] must be used as a type parameter for […] type" to
"type parameter […] must be used as an argument to […] type".

Author: fmease

compiler/rustc_hir_analysis/src/diagnostics.rs

@@ -1968,28 +1968,37 @@ impl<G: EmissionGuarantee> Diagnostic<'_, G> for UncoveredTyParam<'_> {
     fn into_diag(self, dcx: DiagCtxtHandle<'_>, level: Level) -> Diag<'_, G> {
         let Self { param, local_ty } = self;
 
-        let mut diag = Diag::new(dcx, level, "").with_span(param.span);
+        let mut diag = Diag::new(dcx, level, "")
+            .with_span(param.span)
+            .with_span_label(param.span, "uncovered type parameter");
         if diag.is_error() {
             diag.code(E0210);
         }
 
-        let note = "implementing a foreign trait is only possible if at least one of the types for which it is implemented is local";
+        let note = "\
+            implementing a foreign trait is only possible if \
+            at least one of the types for which it is implemented is local";
+
         if let Some(local_ty) = local_ty {
-            let msg = format!(
-                "type parameter `{param}` must be covered by another type when it appears before the first local type (`{local_ty}`)"
-            );
-            diag.primary_message(msg.clone());
-            diag.span_label(param.span, msg);
+            diag.primary_message(format!(
+                "type parameter `{param}` must be covered by another type when \
+                 it appears before the first local type (`{local_ty}`)"
+            ));
+
             diag.note(format!(
-                "{note}, and no uncovered type parameters appear before that first local type"
+                "{note},\nand no uncovered type parameters appear before that first local type"
             ));
-            diag.note("in this case, 'before' refers to the following order: `impl<..> ForeignTrait<T1, ..., Tn> for T0`, where `T0` is the first and `Tn` is the last");
-        } else {
-            let msg = format!(
-                "type parameter `{param}` must be used as the type parameter for some local type"
+            diag.note(
+                "in this case, 'before' refers to the following order: \
+                 `impl<..> ForeignTrait<T1, ..., Tn> for T0`,\n\
+                 where `T0` is the first and `Tn` is the last",
             );
-            diag.primary_message(format!("{msg} (e.g., `MyStruct<{param}>`)"));
-            diag.span_label(param.span, msg);
+        } else {
+            diag.primary_message(format!(
+                "type parameter `{param}` must be used as an argument to \
+                 some local type (e.g., `MyStruct<{param}>`)"
+            ));
+
             diag.note(note);
             diag.note(
                 "only traits defined in the current crate can be implemented for a type parameter",

tests/ui/coercion/invalid-blanket-coerce-unsized-impl.rs

@@ -5,7 +5,7 @@
 #![feature(coerce_unsized)]
 
 impl<A> std::ops::CoerceUnsized<A> for A {}
-//~^ ERROR type parameter `A` must be used as the type parameter for some local type
+//~^ ERROR type parameter `A` must be used as an argument to some local type
 //~| ERROR the trait `CoerceUnsized` may only be implemented for a coercion between structures
 
 const C: usize = 1;

tests/ui/coercion/invalid-blanket-coerce-unsized-impl.stderr

@@ -1,8 +1,8 @@
-error[E0210]: type parameter `A` must be used as the type parameter for some local type (e.g., `MyStruct<A>`)
+error[E0210]: type parameter `A` must be used as an argument to some local type (e.g., `MyStruct<A>`)
   --> $DIR/invalid-blanket-coerce-unsized-impl.rs:7:6
    |
 LL | impl<A> std::ops::CoerceUnsized<A> for A {}
-   |      ^ type parameter `A` must be used as the type parameter for some local type
+   |      ^ uncovered type parameter
    |
    = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local
    = note: only traits defined in the current crate can be implemented for a type parameter

tests/ui/coherence/coherence-all-remote.stderr

@@ -1,8 +1,8 @@
-error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
+error[E0210]: type parameter `T` must be used as an argument to some local type (e.g., `MyStruct<T>`)
   --> $DIR/coherence-all-remote.rs:6:6
    |
 LL | impl<T> Remote1<T> for isize { }
-   |      ^ type parameter `T` must be used as the type parameter for some local type
+   |      ^ uncovered type parameter
    |
    = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local
    = note: only traits defined in the current crate can be implemented for a type parameter

tests/ui/coherence/coherence-bigint-param.stderr

@@ -2,10 +2,12 @@ error[E0210]: type parameter `T` must be covered by another type when it appears
   --> $DIR/coherence-bigint-param.rs:8:6
    |
 LL | impl<T> Remote1<BigInt> for T { }
-   |      ^ type parameter `T` must be covered by another type when it appears before the first local type (`BigInt`)
+   |      ^ uncovered type parameter
    |
-   = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local, and no uncovered type parameters appear before that first local type
-   = note: in this case, 'before' refers to the following order: `impl<..> ForeignTrait<T1, ..., Tn> for T0`, where `T0` is the first and `Tn` is the last
+   = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local,
+           and no uncovered type parameters appear before that first local type
+   = note: in this case, 'before' refers to the following order: `impl<..> ForeignTrait<T1, ..., Tn> for T0`,
+           where `T0` is the first and `Tn` is the last
 
 error: aborting due to 1 previous error
 

tests/ui/coherence/coherence-cross-crate-conflict.stderr

@@ -1,8 +1,8 @@
-error[E0210]: type parameter `A` must be used as the type parameter for some local type (e.g., `MyStruct<A>`)
+error[E0210]: type parameter `A` must be used as an argument to some local type (e.g., `MyStruct<A>`)
   --> $DIR/coherence-cross-crate-conflict.rs:9:6
    |
 LL | impl<A> Foo for A {
-   |      ^ type parameter `A` must be used as the type parameter for some local type
+   |      ^ uncovered type parameter
    |
    = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local
    = note: only traits defined in the current crate can be implemented for a type parameter

tests/ui/coherence/coherence-lone-type-parameter.stderr

@@ -1,8 +1,8 @@
-error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
+error[E0210]: type parameter `T` must be used as an argument to some local type (e.g., `MyStruct<T>`)
   --> $DIR/coherence-lone-type-parameter.rs:6:6
    |
 LL | impl<T> Remote for T { }
-   |      ^ type parameter `T` must be used as the type parameter for some local type
+   |      ^ uncovered type parameter
    |
    = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local
    = note: only traits defined in the current crate can be implemented for a type parameter

tests/ui/coherence/impl[t]-foreign-for-fundamental[t].rs

@@ -8,8 +8,7 @@ use std::rc::Rc;
 struct Local;
 
 impl<T> Remote for Box<T> {
-    //~^ ERROR type parameter `T` must be used as the type parameter for
-    // | some local type (e.g., `MyStruct<T>`)
+    //~^ ERROR type parameter `T` must be used as an argument to some local type
 }
 
 fn main() {}

tests/ui/coherence/impl[t]-foreign-for-fundamental[t].stderr

@@ -1,8 +1,8 @@
-error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
+error[E0210]: type parameter `T` must be used as an argument to some local type (e.g., `MyStruct<T>`)
   --> $DIR/impl[t]-foreign-for-fundamental[t].rs:10:6
    |
 LL | impl<T> Remote for Box<T> {
-   |      ^ type parameter `T` must be used as the type parameter for some local type
+   |      ^ uncovered type parameter
    |
    = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local
    = note: only traits defined in the current crate can be implemented for a type parameter

tests/ui/coherence/impl[t]-foreign[foreign]-for-fundamental[t].rs

@@ -8,11 +8,11 @@ use std::rc::Rc;
 struct Local;
 
 impl<T> Remote1<u32> for Box<T> {
-    //~^ ERROR type parameter `T` must be used as the type parameter for some local type
+    //~^ ERROR type parameter `T` must be used as an argument to some local type
 }
 
 impl<'a, T> Remote1<u32> for &'a T {
-    //~^ ERROR type parameter `T` must be used as the type parameter for some local type
+    //~^ ERROR type parameter `T` must be used as an argument to some local type
 }
 
 fn main() {}