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.stderr b/src/test/ui/missing_debug_impls.stderr
index 51c65589b0c..0538f207b44 100644
--- a/src/test/ui/missing_debug_impls.stderr
+++ b/src/test/ui/missing_debug_impls.stderr
@@ -1,4 +1,4 @@
-error: type does not implement `std::fmt::Debug`; consider adding `#[derive(Debug)]` or a manual implementation
+error: type does not implement `Debug`; consider adding `#[derive(Debug)]` or a manual implementation
   --> $DIR/missing_debug_impls.rs:7:1
    |
 LL | pub enum A {}
@@ -10,7 +10,7 @@ note: the lint level is defined here
 LL | #![deny(missing_debug_implementations)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: type does not implement `std::fmt::Debug`; consider adding `#[derive(Debug)]` or a manual implementation
+error: type does not implement `Debug`; consider adding `#[derive(Debug)]` or a manual implementation
   --> $DIR/missing_debug_impls.rs:20:1
    |
 LL | pub struct Foo;