On unconstrained lifetime on impl block, suggest using it if there's an implicit borrow in the self type

```
error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
  --> $DIR/missing-lifetime-in-assoc-type-1.rs:4:6
   |
LL | impl<'a> IntoIterator for &S {
   |      ^^ unconstrained lifetime parameter
   |
help: consider using the named lifetime here instead of an implict lifetime
   |
LL | impl<'a> IntoIterator for &'a S {
   |                            ++
```

Author: estebank

compiler/rustc_hir_analysis/src/impl_wf_check.rs

@@ -12,11 +12,12 @@ use std::assert_matches::debug_assert_matches;
 
 use min_specialization::check_min_specialization;
 use rustc_data_structures::fx::FxHashSet;
+use rustc_errors::Applicability;
 use rustc_errors::codes::*;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::LocalDefId;
 use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
-use rustc_span::ErrorGuaranteed;
+use rustc_span::{ErrorGuaranteed, kw};
 
 use crate::constrained_generic_params as cgp;
 use crate::errors::UnconstrainedGenericParameter;
@@ -158,6 +159,27 @@ pub(crate) fn enforce_impl_lifetime_params_are_constrained(
                         const_param_note2: false,
                     });
                     diag.code(E0207);
+                    for p in &impl_generics.own_params {
+                        if p.name == kw::UnderscoreLifetime {
+                            let span = tcx.def_span(p.def_id);
+                            let Ok(snippet) = tcx.sess.source_map().span_to_snippet(span) else {
+                                continue;
+                            };
+
+                            let (span, sugg) = if &snippet == "'_" {
+                                (span, param.name.to_string())
+                            } else {
+                                (span.shrink_to_hi(), format!("{} ", param.name))
+                            };
+                            diag.span_suggestion_verbose(
+                                span,
+                                "consider using the named lifetime here instead of an implict \
+                                 lifetime",
+                                sugg,
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
+                    }
                     res = Err(diag.emit());
                 }
             }

tests/ui/lifetimes/missing-lifetime-in-assoc-type-1.rs

@@ -5,6 +5,7 @@ impl<'a> IntoIterator for &S {
     //~^ ERROR E0207
     //~| NOTE there is a named lifetime specified on the impl block you could use
     //~| NOTE unconstrained lifetime parameter
+    //~| HELP consider using the named lifetime here instead of an implict lifetime
     type Item = &T;
     //~^ ERROR in the trait associated type
     //~| HELP consider using the lifetime from the impl block

tests/ui/lifetimes/missing-lifetime-in-assoc-type-1.stderr

@@ -1,5 +1,5 @@
 error: in the trait associated type is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type
-  --> $DIR/missing-lifetime-in-assoc-type-1.rs:8:17
+  --> $DIR/missing-lifetime-in-assoc-type-1.rs:9:17
    |
 LL | impl<'a> IntoIterator for &S {
    |     ---- there is a named lifetime specified on the impl block you could use
@@ -17,6 +17,11 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a> IntoIterator for &S {
    |      ^^ unconstrained lifetime parameter
+   |
+help: consider using the named lifetime here instead of an implict lifetime
+   |
+LL | impl<'a> IntoIterator for &'a S {
+   |                            ++
 
 error: aborting due to 2 previous errors
 

tests/ui/lifetimes/missing-lifetime-in-assoc-type-5.rs

@@ -0,0 +1,19 @@
+struct S;
+struct T;
+
+impl<'a> IntoIterator for &'_ S {
+    //~^ ERROR E0207
+    //~| NOTE there is a named lifetime specified on the impl block you could use
+    //~| NOTE unconstrained lifetime parameter
+    //~| HELP consider using the named lifetime here instead of an implict lifetime
+    type Item = &T;
+    //~^ ERROR in the trait associated type
+    //~| HELP consider using the lifetime from the impl block
+    //~| NOTE this lifetime must come from the implemented type
+    type IntoIter = std::collections::btree_map::Values<'a, i32, T>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        todo!()
+    }
+}
+fn main() {}

tests/ui/lifetimes/missing-lifetime-in-assoc-type-5.stderr

@@ -0,0 +1,28 @@
+error: in the trait associated type is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type
+  --> $DIR/missing-lifetime-in-assoc-type-5.rs:9:17
+   |
+LL | impl<'a> IntoIterator for &'_ S {
+   |     ---- there is a named lifetime specified on the impl block you could use
+...
+LL |     type Item = &T;
+   |                 ^ this lifetime must come from the implemented type
+   |
+help: consider using the lifetime from the impl block
+   |
+LL |     type Item = &'a T;
+   |                  ++
+
+error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/missing-lifetime-in-assoc-type-5.rs:4:6
+   |
+LL | impl<'a> IntoIterator for &'_ S {
+   |      ^^ unconstrained lifetime parameter
+   |
+help: consider using the named lifetime here instead of an implict lifetime
+   |
+LL | impl<'a> IntoIterator for &'a S {
+   |                            ~~
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0207`.