Do not consider repeated lifetime params for elision.

compiler/rustc_resolve/src/late.rs

@@ -565,7 +565,7 @@ struct LateResolutionVisitor<'a, 'b, 'ast> {
     /// They will be used to determine the correct lifetime for the fn return type.
     /// The `LifetimeElisionCandidate` is used for diagnostics, to suggest introducing named
     /// lifetimes.
-    lifetime_elision_candidates: Option<FxIndexMap<LifetimeRes, LifetimeElisionCandidate>>,
+    lifetime_elision_candidates: Option<Vec<(LifetimeRes, LifetimeElisionCandidate)>>,
 
     /// The trait that the current context can refer to.
     current_trait_ref: Option<(Module<'a>, TraitRef)>,
@@ -1799,7 +1799,7 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
         match res {
             LifetimeRes::Param { .. } | LifetimeRes::Fresh { .. } | LifetimeRes::Static => {
                 if let Some(ref mut candidates) = self.lifetime_elision_candidates {
-                    candidates.insert(res, candidate);
+                    candidates.push((res, candidate));
                 }
             }
             LifetimeRes::Infer | LifetimeRes::Error | LifetimeRes::ElidedAnchor { .. } => {}
@@ -1910,8 +1910,8 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
 
         // We do not have a `self` candidate, look at the full list.
         let all_candidates = all_candidates.unwrap();
-        if all_candidates.len() == 1 {
+        if let [(res, _)] = &all_candidates[..] {
-            Ok(*all_candidates.first().unwrap().0)
+            Ok(*res)
         } else {
             let all_candidates = all_candidates
                 .into_iter()
@@ -2391,7 +2391,7 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
         // Do not account for the parameters we just bound for function lifetime elision.
         if let Some(ref mut candidates) = self.lifetime_elision_candidates {
             for (_, res) in function_lifetime_rib.bindings.values() {
-                candidates.remove(res);
+                candidates.retain(|(r, _)| r != res);
             }
         }
 

src/test/ui/lifetimes/lifetime-elision-return-type-requires-explicit-lifetime.rs

@@ -42,4 +42,7 @@ fn k<'a, T: WithLifetime<'a>>(_x: T::Output) -> &isize {
     panic!()
 }
 
+fn l<'a>(_: &'a str, _: &'a str) -> &str { "" }
+//~^ ERROR missing lifetime specifier
+
 fn main() {}

src/test/ui/lifetimes/lifetime-elision-return-type-requires-explicit-lifetime.stderr

@@ -70,6 +70,18 @@ help: consider using the `'a` lifetime
 LL | fn k<'a, T: WithLifetime<'a>>(_x: T::Output) -> &'a isize {
    |                                                  ++
 
-error: aborting due to 6 previous errors
+error[E0106]: missing lifetime specifier
+  --> $DIR/lifetime-elision-return-type-requires-explicit-lifetime.rs:45:37
+   |
+LL | fn l<'a>(_: &'a str, _: &'a str) -> &str { "" }
+   |             -------     -------     ^ expected named lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value with an elided lifetime, but the lifetime cannot be derived from the arguments
+help: consider using the `'a` lifetime
+   |
+LL | fn l<'a>(_: &'a str, _: &'a str) -> &'a str { "" }
+   |                                      ++
+
+error: aborting due to 7 previous errors
 
 For more information about this error, try `rustc --explain E0106`.