From a4f6d3e5c27e09f9fab591c19391b53a4175b666 Mon Sep 17 00:00:00 2001
From: Mara Bos <m-ou.se@m-ou.se>
Date: Mon, 30 Aug 2021 19:01:34 +0200
Subject: [PATCH] Fix lifetime generics in <T as Trait<..>>::try_from
 suggestion.

---
 .../src/check/method/prelude2021.rs           | 27 ++++++++++++-------
 1 file changed, 17 insertions(+), 10 deletions(-)

diff --git a/compiler/rustc_typeck/src/check/method/prelude2021.rs b/compiler/rustc_typeck/src/check/method/prelude2021.rs
index b5bc9d3599ac..0ab64170e4c2 100644
--- a/compiler/rustc_typeck/src/check/method/prelude2021.rs
+++ b/compiler/rustc_typeck/src/check/method/prelude2021.rs
@@ -239,16 +239,23 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let trait_path = self.trait_path_or_bare_name(span, expr_id, pick.item.container.id());
             let trait_generics = self.tcx.generics_of(pick.item.container.id());
 
-            let parameter_count = trait_generics.count() - (trait_generics.has_self as usize);
-            let trait_name = if parameter_count == 0 {
-                trait_path
-            } else {
-                format!(
-                    "{}<{}>",
-                    trait_path,
-                    std::iter::repeat("_").take(parameter_count).collect::<Vec<_>>().join(", ")
-                )
-            };
+            let trait_name =
+                if trait_generics.params.len() <= trait_generics.has_self as usize {
+                    trait_path
+                } else {
+                    let counts = trait_generics.own_counts();
+                    format!(
+                        "{}<{}>",
+                        trait_path,
+                        std::iter::repeat("'_")
+                            .take(counts.lifetimes)
+                            .chain(std::iter::repeat("_").take(
+                                counts.types + counts.consts - trait_generics.has_self as usize
+                            ))
+                            .collect::<Vec<_>>()
+                            .join(", ")
+                    )
+                };
 
             let mut lint = lint.build(&format!(
                 "trait-associated function `{}` will become ambiguous in Rust 2021",