Auto merge of #65454 - tmandry:rollup-0k6jiik, r=tmandry

Rollup of 14 pull requests

Successful merges:

 - #64603 (Reducing spurious unused lifetime warnings.)
 - #64623 (Remove last uses of gensyms)
 - #65235 (don't assume we can *always* find a return type hint in async fn)
 - #65242 (Fix suggestion to constrain trait for method to be found)
 - #65265 (Cleanup librustc mir err codes)
 - #65293 (Optimize `try_expand_impl_trait_type`)
 - #65307 (Try fix incorrect "explicit lifetime name needed")
 - #65308 (Add long error explanation for E0574)
 - #65353 (save-analysis: Don't ICE when resolving qualified type paths in struct members)
 - #65389 (Return `false` from `needs_drop` for all zero-sized arrays.)
 - #65402 (Add troubleshooting section to PGO chapter in rustc book.)
 - #65425 (Optimize `BitIter`)
 - #65438 (Organize `never_type`  tests)
 - #65444 (Implement AsRef<[T]> for List<T>)

Failed merges:

 - #65390 (Add long error explanation for E0576)

r? @ghost

Author: bors

src/doc/rustc/src/profile-guided-optimization.md

@@ -125,6 +125,17 @@ RUSTFLAGS="-Cprofile-use=/tmp/pgo-data/merged.profdata" \
     cargo build --release --target=x86_64-unknown-linux-gnu
 ```
 
+### Troubleshooting
+
+- It is recommended to pass `-Cllvm-args=-pgo-warn-missing-function` during the
+  `-Cprofile-use` phase. LLVM by default does not warn if it cannot find
+  profiling data for a given function. Enabling this warning will make it
+  easier to spot errors in your setup.
+
+- There is a [known issue](https://github.com/rust-lang/cargo/issues/7416) in
+  Cargo prior to version 1.39 that will prevent PGO from working correctly. Be
+  sure to use Cargo 1.39 or newer when doing PGO.
+
 ## Further Reading
 
 `rustc`'s PGO support relies entirely on LLVM's implementation of the feature

src/librustc/hir/lowering.rs

@@ -3291,10 +3291,14 @@ impl<'a> LoweringContext<'a> {
                 let id = self.sess.next_node_id();
                 self.new_named_lifetime(id, span, hir::LifetimeName::Error)
             }
-            // This is the normal case.
-            AnonymousLifetimeMode::PassThrough => self.new_implicit_lifetime(span),
-
-            AnonymousLifetimeMode::ReportError => self.new_error_lifetime(None, span),
+            // `PassThrough` is the normal case.
+            // `new_error_lifetime`, which would usually be used in the case of `ReportError`,
+            // is unsuitable here, as these can occur from missing lifetime parameters in a
+            // `PathSegment`, for which there is no associated `'_` or `&T` with no explicit
+            // lifetime. Instead, we simply create an implicit lifetime, which will be checked
+            // later, at which point a suitable error will be emitted.
+          | AnonymousLifetimeMode::PassThrough
+          | AnonymousLifetimeMode::ReportError => self.new_implicit_lifetime(span),
         }
     }