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.

2 files changed, 8 insertions, 8 deletions

diff --git a/src/test/ui/interior-mutability/interior-mutability.rs b/src/test/ui/interior-mutability/interior-mutability.rs
index ddc882cccf3..c704acc22af 100644
--- a/src/test/ui/interior-mutability/interior-mutability.rs
+++ b/src/test/ui/interior-mutability/interior-mutability.rs
@@ -3,5 +3,5 @@ use std::panic::catch_unwind;
 fn main() {
     let mut x = Cell::new(22);
     catch_unwind(|| { x.set(23); });
-    //~^ ERROR the type `std::cell::UnsafeCell<i32>` may contain interior mutability and a
+    //~^ ERROR the type `UnsafeCell<i32>` may contain interior mutability and a
 }
diff --git a/src/test/ui/interior-mutability/interior-mutability.stderr b/src/test/ui/interior-mutability/interior-mutability.stderr
index a25acf10a1a..3e19746cc5c 100644
--- a/src/test/ui/interior-mutability/interior-mutability.stderr
+++ b/src/test/ui/interior-mutability/interior-mutability.stderr
@@ -1,18 +1,18 @@
-error[E0277]: the type `std::cell::UnsafeCell<i32>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
+error[E0277]: the type `UnsafeCell<i32>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
   --> $DIR/interior-mutability.rs:5:5
    |
 LL |     catch_unwind(|| { x.set(23); });
-   |     ^^^^^^^^^^^^ `std::cell::UnsafeCell<i32>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
+   |     ^^^^^^^^^^^^ `UnsafeCell<i32>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
    | 
   ::: $SRC_DIR/std/src/panic.rs:LL:COL
    |
 LL | pub fn catch_unwind<F: FnOnce() -> R + UnwindSafe, R>(f: F) -> Result<R> {
-   |                                        ---------- required by this bound in `std::panic::catch_unwind`
+   |                                        ---------- required by this bound in `catch_unwind`
    |
-   = help: within `std::cell::Cell<i32>`, the trait `std::panic::RefUnwindSafe` is not implemented for `std::cell::UnsafeCell<i32>`
-   = note: required because it appears within the type `std::cell::Cell<i32>`
-   = note: required because of the requirements on the impl of `std::panic::UnwindSafe` for `&std::cell::Cell<i32>`
-   = note: required because it appears within the type `[closure@$DIR/interior-mutability.rs:5:18: 5:35 x:&std::cell::Cell<i32>]`
+   = help: within `Cell<i32>`, the trait `RefUnwindSafe` is not implemented for `UnsafeCell<i32>`
+   = note: required because it appears within the type `Cell<i32>`
+   = note: required because of the requirements on the impl of `UnwindSafe` for `&Cell<i32>`
+   = note: required because it appears within the type `[closure@$DIR/interior-mutability.rs:5:18: 5:35 x:&Cell<i32>]`
 
 error: aborting due to previous error