Use trait name instead of full constraint in suggestion message

```
help: consider restricting type parameter `T` with traits `Copy` and `Trait`
   |
LL | fn duplicate_custom<T: Copy + Trait>(t: S<T>) -> (S<T>, S<T>) {
   |                      ++++++++++++++
```

```
help: consider restricting type parameter `V` with trait `Copy`
   |
LL | fn index<'a, K, V: std::marker::Copy>(map: &'a HashMap<K, V>, k: K) -> &'a V {
   |                  +++++++++++++++++++
```

Author: estebank

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -4,7 +4,9 @@ use std::fmt::Write;
 use std::ops::ControlFlow;
 
 use rustc_data_structures::fx::FxHashMap;
-use rustc_errors::{Applicability, Diag, DiagArgValue, IntoDiagArg, into_diag_arg_using_display};
+use rustc_errors::{
+    Applicability, Diag, DiagArgValue, IntoDiagArg, into_diag_arg_using_display, pluralize,
+};
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::DefId;
 use rustc_hir::{self as hir, LangItem, PredicateOrigin, WherePredicateKind};
@@ -288,7 +290,11 @@ pub fn suggest_constraining_type_params<'a>(
             None => true,
         };
         if stable || tcx.sess.is_nightly_build() {
-            grouped.entry(param_name).or_insert(Vec::new()).push((constraint, def_id));
+            grouped.entry(param_name).or_insert(Vec::new()).push((
+                constraint,
+                def_id,
+                if stable { "" } else { "unstable " },
+            ));
             if !stable {
                 unstable_suggestion = true;
             }
@@ -303,10 +309,10 @@ pub fn suggest_constraining_type_params<'a>(
         let Some(param) = param else { return false };
 
         {
-            let mut sized_constraints = constraints.extract_if(|(_, def_id)| {
+            let mut sized_constraints = constraints.extract_if(|(_, def_id, _)| {
                 def_id.is_some_and(|def_id| tcx.is_lang_item(def_id, LangItem::Sized))
             });
-            if let Some((_, def_id)) = sized_constraints.next() {
+            if let Some((_, def_id, _)) = sized_constraints.next() {
                 applicability = Applicability::MaybeIncorrect;
 
                 err.span_label(param.span, "this type parameter needs to be `Sized`");
@@ -325,15 +331,52 @@ pub fn suggest_constraining_type_params<'a>(
             .collect();
 
         constraints
-            .retain(|(_, def_id)| def_id.map_or(true, |def| !bound_trait_defs.contains(&def)));
+            .retain(|(_, def_id, _)| def_id.map_or(true, |def| !bound_trait_defs.contains(&def)));
 
         if constraints.is_empty() {
             continue;
         }
 
-        let mut constraint = constraints.iter().map(|&(c, _)| c).collect::<Vec<_>>();
+        let mut constraint = constraints.iter().map(|&(c, _, _)| c).collect::<Vec<_>>();
         constraint.sort();
         constraint.dedup();
+        let all_stable = constraints.iter().all(|&(_, _, stable)| stable.is_empty());
+        let all_unstable = constraints.iter().all(|&(_, _, stable)| !stable.is_empty());
+        let post = if all_stable || all_unstable {
+            // Don't redundantly say "trait `X`, trait `Y`", instead "traits `X` and `Y`"
+            let mut trait_names = constraints
+                .iter()
+                .map(|&(c, def_id, _)| match def_id {
+                    None => format!("`{c}`"),
+                    Some(def_id) => format!("`{}`", tcx.item_name(def_id)),
+                })
+                .collect::<Vec<_>>();
+            trait_names.sort();
+            trait_names.dedup();
+            let n = trait_names.len();
+            let stable = if all_stable { "" } else { "unstable " };
+            format!("{stable}trait{} {}", pluralize!(n), match &trait_names[..] {
+                [t] => t.to_string(),
+                [ts @ .., last] => format!("{} and {last}", ts.join(", ")),
+                [] => return false,
+            },)
+        } else {
+            // We're more explicit when there's a mix of stable and unstable traits.
+            let mut trait_names = constraints
+                .iter()
+                .map(|&(c, def_id, stable)| match def_id {
+                    None => format!("{stable}trait `{c}`"),
+                    Some(def_id) => format!("{stable}trait `{}`", tcx.item_name(def_id)),
+                })
+                .collect::<Vec<_>>();
+            trait_names.sort();
+            trait_names.dedup();
+            match &trait_names[..] {
+                [t] => t.to_string(),
+                [ts @ .., last] => format!("{} and {last}", ts.join(", ")),
+                [] => return false,
+            }
+        };
         let constraint = constraint.join(" + ");
         let mut suggest_restrict = |span, bound_list_non_empty, open_paren_sp| {
             let suggestion = if span_to_replace.is_some() {
@@ -351,18 +394,18 @@ pub fn suggest_constraining_type_params<'a>(
             if let Some(open_paren_sp) = open_paren_sp {
                 suggestions.push((
                     open_paren_sp,
-                    constraint.clone(),
+                    post.clone(),
                     "(".to_string(),
                     RestrictBoundFurther,
                 ));
                 suggestions.push((
                     span,
-                    constraint.clone(),
+                    post.clone(),
                     format!("){suggestion}"),
                     RestrictBoundFurther,
                 ));
             } else {
-                suggestions.push((span, constraint.clone(), suggestion, RestrictBoundFurther));
+                suggestions.push((span, post.clone(), suggestion, RestrictBoundFurther));
             }
         };
 
@@ -420,8 +463,8 @@ pub fn suggest_constraining_type_params<'a>(
             //                                           - insert: `, X: Bar`
             suggestions.push((
                 generics.tail_span_for_predicate_suggestion(),
-                constraint.clone(),
-                constraints.iter().fold(String::new(), |mut string, &(constraint, _)| {
+                post,
+                constraints.iter().fold(String::new(), |mut string, &(constraint, _, _)| {
                     write!(string, ", {param_name}: {constraint}").unwrap();
                     string
                 }),
@@ -450,7 +493,7 @@ pub fn suggest_constraining_type_params<'a>(
             // default (`<T=Foo>`), so we suggest adding `where T: Bar`.
             suggestions.push((
                 generics.tail_span_for_predicate_suggestion(),
-                constraint.clone(),
+                post,
                 format!("{where_prefix} {param_name}: {constraint}"),
                 SuggestChangingConstraintsMessage::RestrictTypeFurther { ty: param_name },
             ));
@@ -464,7 +507,7 @@ pub fn suggest_constraining_type_params<'a>(
         if let Some(colon_span) = param.colon_span {
             suggestions.push((
                 colon_span.shrink_to_hi(),
-                constraint.clone(),
+                post,
                 format!(" {constraint}"),
                 SuggestChangingConstraintsMessage::RestrictType { ty: param_name },
             ));
@@ -477,7 +520,7 @@ pub fn suggest_constraining_type_params<'a>(
         //          - help: consider restricting this type parameter with `T: Foo`
         suggestions.push((
             param.span.shrink_to_hi(),
-            constraint.clone(),
+            post,
             format!(": {constraint}"),
             SuggestChangingConstraintsMessage::RestrictType { ty: param_name },
         ));
@@ -490,21 +533,16 @@ pub fn suggest_constraining_type_params<'a>(
         .collect::<Vec<_>>();
     let suggested = !suggestions.is_empty();
     if suggestions.len() == 1 {
-        let (span, constraint, suggestion, msg) = suggestions.pop().unwrap();
-        let post = format!(
-            " with {}trait{} `{constraint}`",
-            if unstable_suggestion { "unstable " } else { "" },
-            if constraint.contains('+') { "s" } else { "" },
-        );
+        let (span, post, suggestion, msg) = suggestions.pop().unwrap();
         let msg = match msg {
             SuggestChangingConstraintsMessage::RestrictBoundFurther => {
-                format!("consider further restricting this bound{post}")
+                format!("consider further restricting this bound with {post}")
             }
             SuggestChangingConstraintsMessage::RestrictType { ty } => {
-                format!("consider restricting type parameter `{ty}`{post}")
+                format!("consider restricting type parameter `{ty}` with {post}")
             }
             SuggestChangingConstraintsMessage::RestrictTypeFurther { ty } => {
-                format!("consider further restricting type parameter `{ty}`{post}")
+                format!("consider further restricting type parameter `{ty}` with {post}")
             }
             SuggestChangingConstraintsMessage::RemoveMaybeUnsized => {
                 format!("consider removing the `?Sized` bound to make the type parameter `Sized`")

tests/rustdoc-ui/synthetic-auto-trait-impls/projections-in-super-trait-bound-unsatisfied.stderr

@@ -4,7 +4,7 @@ error[E0277]: the trait bound `C: Bar<5>` is not satisfied
 LL | pub struct Structure<C: Tec> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Bar<5>` is not implemented for `C`
    |
-help: consider further restricting this bound with trait `Bar<5>`
+help: consider further restricting this bound with trait `Bar`
    |
 LL | pub struct Structure<C: Tec + Bar<5>> {
    |                             ++++++++
@@ -15,7 +15,7 @@ error[E0277]: the trait bound `C: Bar<5>` is not satisfied
 LL |     _field: C::BarType,
    |             ^^^^^^^^^^ the trait `Bar<5>` is not implemented for `C`
    |
-help: consider further restricting this bound with trait `Bar<5>`
+help: consider further restricting this bound with trait `Bar`
    |
 LL | pub struct Structure<C: Tec + Bar<5>> {
    |                             ++++++++

tests/ui/associated-types/defaults-suitability.current.stderr

@@ -47,7 +47,7 @@ note: required by a bound in `Foo::Bar`
    |
 LL |     type Bar: Clone = Vec<T>;
    |               ^^^^^ required by this bound in `Foo::Bar`
-help: consider restricting type parameter `T` with trait `std::clone::Clone`
+help: consider restricting type parameter `T` with trait `Clone`
    |
 LL | trait Foo<T: std::clone::Clone> {
    |            +++++++++++++++++++
@@ -132,7 +132,7 @@ LL |     Self::Baz: Clone,
 ...
 LL |     type Baz = T;
    |          --- required by a bound in this associated type
-help: consider further restricting type parameter `T` with trait `std::clone::Clone`
+help: consider further restricting type parameter `T` with trait `Clone`
    |
 LL |     Self::Baz: Clone, T: std::clone::Clone
    |                     ~~~~~~~~~~~~~~~~~~~~~~

tests/ui/associated-types/defaults-suitability.next.stderr

@@ -47,7 +47,7 @@ note: required by a bound in `Foo::Bar`
    |
 LL |     type Bar: Clone = Vec<T>;
    |               ^^^^^ required by this bound in `Foo::Bar`
-help: consider restricting type parameter `T` with trait `std::clone::Clone`
+help: consider restricting type parameter `T` with trait `Clone`
    |
 LL | trait Foo<T: std::clone::Clone> {
    |            +++++++++++++++++++
@@ -132,7 +132,7 @@ LL |     Self::Baz: Clone,
 ...
 LL |     type Baz = T;
    |          --- required by a bound in this associated type
-help: consider further restricting type parameter `T` with trait `std::clone::Clone`
+help: consider further restricting type parameter `T` with trait `Clone`
    |
 LL |     Self::Baz: Clone, T: std::clone::Clone
    |                     ~~~~~~~~~~~~~~~~~~~~~~

tests/ui/associated-types/hr-associated-type-bound-param-6.stderr

@@ -4,7 +4,7 @@ error[E0277]: the trait bound `for<'b> T: X<'b, T>` is not satisfied
 LL | impl<S, T> X<'_, T> for (S,) {
    |            ^^^^^^^^ the trait `for<'b> X<'b, T>` is not implemented for `T`
    |
-help: consider restricting type parameter `T` with trait `for<'b> X<'b, T>`
+help: consider restricting type parameter `T` with trait `X`
    |
 LL | impl<S, T: for<'b> X<'b, T>> X<'_, T> for (S,) {
    |          ++++++++++++++++++

tests/ui/associated-types/issue-27675-unchecked-bounds.stderr

@@ -9,7 +9,7 @@ note: required by a bound in `copy`
    |
 LL | fn copy<U: Setup + ?Sized>(from: &U::From) -> U::From {
    |            ^^^^^ required by this bound in `copy`
-help: consider restricting type parameter `T` with trait `std::marker::Copy`
+help: consider restricting type parameter `T` with trait `Copy`
    |
 LL | pub fn copy_any<T: std::marker::Copy>(t: &T) -> T {
    |                  +++++++++++++++++++

tests/ui/associated-types/issue-43784-associated-type.stderr

@@ -14,7 +14,7 @@ note: required by a bound in `Complete::Assoc`
    |
 LL |     type Assoc: Partial<Self>;
    |                 ^^^^^^^^^^^^^ required by this bound in `Complete::Assoc`
-help: consider restricting type parameter `T` with trait `std::marker::Copy`
+help: consider restricting type parameter `T` with trait `Copy`
    |
 LL | impl<T: std::marker::Copy> Complete for T {
    |       +++++++++++++++++++

tests/ui/async-await/issue-70818.stderr

@@ -9,7 +9,7 @@ note: captured value is not `Send`
    |
 LL |     async { (ty, ty1) }
    |                  ^^^ has type `U` which is not `Send`
-help: consider restricting type parameter `U` with trait `std::marker::Send`
+help: consider restricting type parameter `U` with trait `Send`
    |
 LL | fn foo<T: Send, U: std::marker::Send>(ty: T, ty1: U) -> impl Future<Output = (T, U)> + Send {
    |                  +++++++++++++++++++

tests/ui/async-await/issue-86507.stderr

@@ -14,7 +14,7 @@ note: captured value is not `Send` because `&` references cannot be sent unless
 LL |                     let x = x;
    |                             ^ has type `&T` which is not `Send`, because `T` is not `Sync`
    = note: required for the cast from `Pin<Box<{async block@$DIR/issue-86507.rs:18:17: 18:27}>>` to `Pin<Box<(dyn Future<Output = ()> + Send + 'async_trait)>>`
-help: consider further restricting this bound with trait `std::marker::Sync`
+help: consider further restricting this bound with trait `Sync`
    |
 LL |     fn bar<'me, 'async_trait, T: Send + std::marker::Sync>(x: &'me T)
    |                                       +++++++++++++++++++

tests/ui/binop/issue-93927.stderr

@@ -6,7 +6,7 @@ LL |     val == val
    |     |
    |     MyType<T>
    |
-help: consider further restricting this bound with trait `std::cmp::Eq`
+help: consider further restricting this bound with trait `Eq`
    |
 LL | fn cond<T: PartialEq + std::cmp::Eq>(val: MyType<T>) -> bool {
    |                      ++++++++++++++