pretty: trim paths of unique symbols

If a symbol name can only be imported from one place for a type, and
as long as it was not glob-imported anywhere in the current crate, we
can trim its printed path and print only the name.

This has wide implications on error messages with types, for example,
shortening `std::vec::Vec` to just `Vec`, as long as there is no other
`Vec` importable anywhere.

This adds a new '-Z trim-diagnostic-paths=false' option to control this
feature.

On the good path, with no diagnosis printed, we should try to avoid
issuing this query, so we need to prevent trimmed_def_paths query on
several cases.

This change also relies on a previous commit that differentiates
between `Debug` and `Display` on various rustc types, where the latter
is trimmed and presented to the user and the former is not.

1 files changed, 2 insertions, 3 deletions

diff --git a/src/test/ui/async-await/async-block-control-flow-static-semantics.rs b/src/test/ui/async-await/async-block-control-flow-static-semantics.rs
index 971d4476334..5bc7069ff89 100644
--- a/src/test/ui/async-await/async-block-control-flow-static-semantics.rs
+++ b/src/test/ui/async-await/async-block-control-flow-static-semantics.rs
@@ -4,7 +4,6 @@
 // 3. get targeted by `?` and not the parent function.
 //
 // edition:2018
-// ignore-tidy-linelength
 
 fn main() {}
 
@@ -16,7 +15,7 @@ fn return_targets_async_block_not_fn() -> u8 {
         return 0u8;
     };
     let _: &dyn Future<Output = ()> = &block;
-    //~^ ERROR type mismatch resolving `<impl std::future::Future as std::future::Future>::Output == ()`
+    //~^ ERROR type mismatch resolving `<impl Future as Future>::Output == ()`
 }
 
 async fn return_targets_async_block_not_async_fn() -> u8 {
@@ -25,7 +24,7 @@ async fn return_targets_async_block_not_async_fn() -> u8 {
         return 0u8;
     };
     let _: &dyn Future<Output = ()> = &block;
-    //~^ ERROR type mismatch resolving `<impl std::future::Future as std::future::Future>::Output == ()`
+    //~^ ERROR type mismatch resolving `<impl Future as Future>::Output == ()`
 }
 
 fn no_break_in_async_block() {