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/incremental/dirty_clean.rs b/src/test/incremental/dirty_clean.rs
index 2a1056df4ce..02c9a0c5798 100644
--- a/src/test/incremental/dirty_clean.rs
+++ b/src/test/incremental/dirty_clean.rs
@@ -27,7 +27,7 @@ mod y {
 
     #[rustc_clean(label="typeck", cfg="cfail2")]
     pub fn y() {
-        //[cfail2]~^ ERROR `typeck(y::y)` should be clean but is not
+        //[cfail2]~^ ERROR `typeck(y)` should be clean but is not
         x::x();
     }
 }
@@ -35,6 +35,6 @@ mod y {
 mod z {
     #[rustc_dirty(label="typeck", cfg="cfail2")]
     pub fn z() {
-        //[cfail2]~^ ERROR `typeck(z::z)` should be dirty but is not
+        //[cfail2]~^ ERROR `typeck(z)` should be dirty but is not
     }
 }