Rollup merge of #127692 - veera-sivarajan:bugfix-125139, r=estebank

Suggest `impl Trait` for References to Bare Trait in Function Header

Fixes #125139

This PR suggests `impl Trait` when `&Trait` is found as a function parameter type or return type. This makes use of existing diagnostics by adding `peel_refs()` when checking for type equality.

Additionaly, it makes a few other improvements:
1. Checks if functions inside impl blocks have bare trait in their headers.
2. Introduces a trait `NextLifetimeParamName` similar to the existing `NextTypeParamName` for suggesting a lifetime name. Also, abstracts out the common logic between the two trait impls.

### Related Issues
I ran into a bunch of related diagnostic issues but couldn't fix them within the scope of this PR. So, I have created the following issues:
1. [Misleading Suggestion when Returning a Reference to a Bare Trait from a Function](https://github.com/rust-lang/rust/issues/127689)
2. [Verbose Error When a Function Takes a Bare Trait as Parameter](https://github.com/rust-lang/rust/issues/127690)
3. [Incorrect Suggestion when Returning a Bare Trait from a Function](https://github.com/rust-lang/rust/issues/127691)

r​? ```@estebank``` since you implemented  #119148

1 files changed, 12 insertions, 4 deletions

diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
index 5c663e0bf4b..45e157b1080 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
@@ -5023,24 +5023,32 @@ impl<'v> Visitor<'v> for AwaitsVisitor {
     }
 }
 
+/// Suggest a new type parameter name for diagnostic purposes.
+///
+/// `name` is the preferred name you'd like to suggest if it's not in use already.
 pub trait NextTypeParamName {
     fn next_type_param_name(&self, name: Option<&str>) -> String;
 }
 
 impl NextTypeParamName for &[hir::GenericParam<'_>] {
     fn next_type_param_name(&self, name: Option<&str>) -> String {
-        // This is the list of possible parameter names that we might suggest.
+        // Type names are usually single letters in uppercase. So convert the first letter of input string to uppercase.
         let name = name.and_then(|n| n.chars().next()).map(|c| c.to_uppercase().to_string());
         let name = name.as_deref();
+
+        // This is the list of possible parameter names that we might suggest.
         let possible_names = [name.unwrap_or("T"), "T", "U", "V", "X", "Y", "Z", "A", "B", "C"];
-        let used_names = self
+
+        // Filter out used names based on `filter_fn`.
+        let used_names: Vec<Symbol> = self
             .iter()
-            .filter_map(|p| match p.name {
+            .filter_map(|param| match param.name {
                 hir::ParamName::Plain(ident) => Some(ident.name),
                 _ => None,
             })
-            .collect::<Vec<_>>();
+            .collect();
 
+        // Find a name from `possible_names` that is not in `used_names`.
         possible_names
             .iter()
             .find(|n| !used_names.contains(&Symbol::intern(n)))