Only ban duplication across parameters.

compiler/rustc_resolve/src/late.rs

@@ -30,6 +30,7 @@ use rustc_span::{BytePos, Span};
 use smallvec::{smallvec, SmallVec};
 
 use rustc_span::source_map::{respan, Spanned};
+use std::assert_matches::debug_assert_matches;
 use std::collections::{hash_map::Entry, BTreeSet};
 use std::mem::{replace, take};
 
@@ -1852,12 +1853,25 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
         has_self: bool,
         inputs: impl Iterator<Item = (Option<&'ast Pat>, &'ast Ty)>,
     ) -> Result<LifetimeRes, (Vec<MissingLifetime>, Vec<ElisionFnParameter>)> {
-        let outer_candidates =
-            replace(&mut self.lifetime_elision_candidates, Some(Default::default()));
+        enum Elision {
+            /// We have not found any candidate.
+            None,
+            /// We have a candidate bound to `self`.
+            Self_(LifetimeRes),
+            /// We have a candidate bound to a parameter.
+            Param(LifetimeRes),
+            /// We failed elision.
+            Err,
+        }
 
-        let mut elision_lifetime = None;
-        let mut lifetime_count = 0;
+        // Save elision state to reinstate it later.
+        let outer_candidates = self.lifetime_elision_candidates.take();
+
+        // Result of elision.
+        let mut elision_lifetime = Elision::None;
+        // Information for diagnostics.
         let mut parameter_info = Vec::new();
+        let mut all_candidates = Vec::new();
 
         let mut bindings = smallvec![(PatBoundCtx::Product, Default::default())];
         for (index, (pat, ty)) in inputs.enumerate() {
@@ -1867,61 +1881,82 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                     this.resolve_pattern(pat, PatternSource::FnParam, &mut bindings);
                 }
             });
+
+            // Record elision candidates only for this parameter.
+            debug_assert_matches!(self.lifetime_elision_candidates, None);
+            self.lifetime_elision_candidates = Some(Default::default());
             self.visit_ty(ty);
+            let local_candidates = self.lifetime_elision_candidates.take();
 
-            if let Some(ref candidates) = self.lifetime_elision_candidates {
-                let new_count = candidates.len();
-                let local_count = new_count - lifetime_count;
-                if local_count != 0 {
+            if let Some(candidates) = local_candidates {
+                let distinct: FxHashSet<_> = candidates.iter().map(|(res, _)| *res).collect();
+                let lifetime_count = distinct.len();
+                if lifetime_count != 0 {
                     parameter_info.push(ElisionFnParameter {
                         index,
                         ident: if let Some(pat) = pat && let PatKind::Ident(_, ident, _) = pat.kind {
                             Some(ident)
                         } else {
                             None
                         },
-                        lifetime_count: local_count,
+                        lifetime_count,
                         span: ty.span,
                     });
+                    all_candidates.extend(candidates.into_iter().filter_map(|(_, candidate)| {
+                        match candidate {
+                            LifetimeElisionCandidate::Ignore | LifetimeElisionCandidate::Named => {
+                                None
+                            }
+                            LifetimeElisionCandidate::Missing(missing) => Some(missing),
+                        }
+                    }));
+                }
+                let mut distinct_iter = distinct.into_iter();
+                if let Some(res) = distinct_iter.next() {
+                    match elision_lifetime {
+                        // We are the first parameter to bind lifetimes.
+                        Elision::None => {
+                            if distinct_iter.next().is_none() {
+                                // We have a single lifetime => success.
+                                elision_lifetime = Elision::Param(res)
+                            } else {
+                                // We have have multiple lifetimes => error.
+                                elision_lifetime = Elision::Err;
+                            }
+                        }
+                        // We have 2 parameters that bind lifetimes => error.
+                        Elision::Param(_) => elision_lifetime = Elision::Err,
+                        // `self` elision takes precedence over everything else.
+                        Elision::Self_(_) | Elision::Err => {}
+                    }
                 }
-                lifetime_count = new_count;
             }
 
             // Handle `self` specially.
             if index == 0 && has_self {
                 let self_lifetime = self.find_lifetime_for_self(ty);
                 if let Set1::One(lifetime) = self_lifetime {
-                    elision_lifetime = Some(lifetime);
-                    self.lifetime_elision_candidates = None;
+                    // We found `self` elision.
+                    elision_lifetime = Elision::Self_(lifetime);
                 } else {
-                    self.lifetime_elision_candidates = Some(Default::default());
-                    lifetime_count = 0;
+                    // We do not have `self` elision: disregard the `Elision::Param` that we may
+                    // have found.
+                    elision_lifetime = Elision::None;
                 }
             }
             debug!("(resolving function / closure) recorded parameter");
         }
 
-        let all_candidates = replace(&mut self.lifetime_elision_candidates, outer_candidates);
-        debug!(?all_candidates);
+        // Reinstate elision state.
+        debug_assert_matches!(self.lifetime_elision_candidates, None);
+        self.lifetime_elision_candidates = outer_candidates;
 
-        if let Some(res) = elision_lifetime {
+        if let Elision::Param(res) | Elision::Self_(res) = elision_lifetime {
             return Ok(res);
         }
 
-        // We do not have a `self` candidate, look at the full list.
-        let all_candidates = all_candidates.unwrap();
-        if let [(res, _)] = &all_candidates[..] {
-            Ok(*res)
-        } else {
-            let all_candidates = all_candidates
-                .into_iter()
-                .filter_map(|(_, candidate)| match candidate {
-                    LifetimeElisionCandidate::Ignore | LifetimeElisionCandidate::Named => None,
-                    LifetimeElisionCandidate::Missing(missing) => Some(missing),
-                })
-                .collect();
-            Err((all_candidates, parameter_info))
-        }
+        // We do not have a candidate.
+        Err((all_candidates, parameter_info))
     }
 
     /// List all the lifetimes that appear in the provided type.

compiler/rustc_resolve/src/lib.rs

@@ -7,6 +7,7 @@
 //! Type-relative name resolution (methods, fields, associated items) happens in `rustc_hir_analysis`.
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(assert_matches)]
 #![feature(box_patterns)]
 #![feature(drain_filter)]
 #![feature(if_let_guard)]

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

@@ -45,4 +45,7 @@ fn k<'a, T: WithLifetime<'a>>(_x: T::Output) -> &isize {
 fn l<'a>(_: &'a str, _: &'a str) -> &str { "" }
 //~^ ERROR missing lifetime specifier
 
+// This is ok because both `'a` are for the same parameter.
+fn m<'a>(_: &'a Foo<'a>) -> &str { "" }
+
 fn main() {}