Tweak output of missing lifetime on associated type

[estebank]

Follow up to #135602.

Previously we only showed the trait's assoc item if the trait was local, because we were looking for a small span only for the generics, which we don't have for foreign traits. We now use def_span for the item, so we at least provide some context, even if its span is too wide.

error[E0195]: lifetime parameters or bounds on type `IntoIter` do not match the trait declaration
   --> tests/ui/lifetimes/missing-lifetime-in-assoc-type-4.rs:7:18
    |
7   |     type IntoIter<'a> = std::collections::btree_map::Values<'a, i32, T>;
    |                  ^^^^ lifetimes do not match type in trait
    |
   ::: /home/gh-estebank/rust/library/core/src/iter/traits/collect.rs:292:5
    |
292 |     type IntoIter: Iterator<Item = Self::Item>;
    |     ------------------------------------------ lifetimes in impl do not match this type in trait

Given an associated item that needs a named lifetime, look at the enclosing impl item for one. If there is none, look at the self type and the implemented trait to see if either of those has an anonimous lifetime. If so, suggest adding a named lifetime.

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-2.rs:5:17
   |
LL |     type Item = &T;
   |                 ^ this lifetime must come from the implemented type
   |
help: add a lifetime to the impl block and use it in the self type and associated type
   |
LL ~ impl<'a> IntoIterator for &'a S {
LL ~     type Item = &'a T;
   |

Move the previous long message to a note and use a shorter primary message:

error: missing lifetime in associated type
  --> $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
...
LL |     type Item = &T;
   |                 ^ this lifetime must come from the implemented type
   |
note: in the trait the associated type is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type
  --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL
help: consider using the lifetime from the impl block
   |
LL |     type Item = &'a T;
   |                  ++

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  • 364475b

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fmt: checked 6301 files
Diff in /checkout/compiler/rustc_resolve/src/late.rs:1963:
     }
 
     fn point_at_impl_lifetimes(&mut self, err: &mut Diag<'_>, i: usize, lifetime: Span) {
-        let Some((rib, span)) = self.lifetime_ribs[..i]
-            .iter()
-            .rev()
-            .find_map(|rib| match rib.kind {
+        let Some((rib, span)) =
+            self.lifetime_ribs[..i].iter().rev().find_map(|rib| match rib.kind {
                 LifetimeRibKind::Generics { span, kind: LifetimeBinderKind::ImplBlock, .. } => {
                     Some((rib, span))
                 }
Build completed unsuccessfully in 0:00:48
  local time: Tue Aug 12 18:04:32 UTC 2025
  network time: Tue, 12 Aug 2025 18:04:32 GMT
##[error]Process completed with exit code 1.