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, 2 deletions

diff --git a/src/test/ui/missing_debug_impls.rs b/src/test/ui/missing_debug_impls.rs
index 72fcba51588..dc4dacfc468 100644
--- a/src/test/ui/missing_debug_impls.rs
+++ b/src/test/ui/missing_debug_impls.rs
@@ -4,7 +4,7 @@
 
 use std::fmt;
 
-pub enum A {} //~ ERROR type does not implement `std::fmt::Debug`
+pub enum A {} //~ ERROR type does not implement `Debug`
 
 #[derive(Debug)]
 pub enum B {}
@@ -17,7 +17,7 @@ impl fmt::Debug for C {
     }
 }
 
-pub struct Foo; //~ ERROR type does not implement `std::fmt::Debug`
+pub struct Foo; //~ ERROR type does not implement `Debug`
 
 #[derive(Debug)]
 pub struct Bar;