E0599: use param name for lifetime arg suggestions too

It's not needed to convey it's a placeholder since it's already
surrounded by `/*` and `*/`, and `'_` wouldn't work in that position so
it's not a helpful suggestion.

Author: dianne

Diff for: compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -3878,19 +3878,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             let candidate_strs: Vec<String> = candidates
                                 .iter()
                                 .map(|cand| {
-                                    use ty::GenericParamDefKind::{Const, Lifetime, Type};
+                                    use ty::GenericParamDefKind::{Const, Type};
                                     let cand_path = self.tcx.def_path_str(cand.def_id);
                                     let cand_params = &self.tcx.generics_of(cand.def_id).own_params;
                                     let cand_args: String = cand_params
                                         .iter()
                                         .skip(1)
                                         .filter_map(|param| match param.kind {
-                                            Lifetime => Some("'_"),
                                             Type { has_default: true, .. }
                                             | Const { has_default: true, .. } => None,
-                                            // in keeping with suggestions for missing generic args,
-                                            // use their names from the trait definition as placeholders
-                                            Type { .. } | Const { .. } => Some(param.name.as_str()),
+                                            // if a placeholder is needed, use the parameter's name
+                                            _ => Some(param.name.as_str()),
                                         })
                                         .intersperse(", ")
                                         .collect();

Diff for: tests/ui/suggestions/no-method-found-suggest-trait-args.rs

@@ -11,7 +11,7 @@ trait Trait2<'a, A, const B: u8, C = (), const D: u8 = 0> {
 
 fn foo<T>(value: T) {
     //~^ SUGGESTION : Trait</* I */>
-    //~| SUGGESTION : Trait2</* '_, A, B */>
+    //~| SUGGESTION : Trait2</* 'a, A, B */>
     value.method();
     //~^ ERROR no method named `method` found for type parameter `T` in the current scope [E0599]
     value.method2();
@@ -20,7 +20,7 @@ fn foo<T>(value: T) {
 
 fn bar(value: impl Copy) {
     //~^ SUGGESTION + Trait</* I */>
-    //~| SUGGESTION + Trait2</* '_, A, B */>
+    //~| SUGGESTION + Trait2</* 'a, A, B */>
     value.method();
     //~^ ERROR no method named `method` found for type parameter `impl Copy` in the current scope [E0599]
     value.method2();

Diff for: tests/ui/suggestions/no-method-found-suggest-trait-args.stderr

@@ -25,7 +25,7 @@ LL |     value.method2();
    = help: items from traits can only be used if the type parameter is bounded by the trait
 help: the following trait defines an item `method2`, perhaps you need to restrict type parameter `T` with it:
    |
-LL | fn foo<T: Trait2</* '_, A, B */>>(value: T) {
+LL | fn foo<T: Trait2</* 'a, A, B */>>(value: T) {
    |         ++++++++++++++++++++++++
 
 error[E0599]: no method named `method` found for type parameter `impl Copy` in the current scope
@@ -55,7 +55,7 @@ LL |     value.method2();
    = help: items from traits can only be used if the type parameter is bounded by the trait
 help: the following trait defines an item `method2`, perhaps you need to restrict type parameter `impl Copy` with it:
    |
-LL | fn bar(value: impl Copy + Trait2</* '_, A, B */>) {
+LL | fn bar(value: impl Copy + Trait2</* 'a, A, B */>) {
    |                         ++++++++++++++++++++++++
 
 error: aborting due to 4 previous errors