rustc: Fix a number of stability lint holes

There are a number of holes that the stability lint did not previously cover,
including:

* Types
* Bounds on type parameters on functions and impls
* Where clauses
* Imports
* Patterns (structs and enums)

These holes have all been fixed by overriding the `visit_path` function on the
AST visitor instead of a few specialized cases. This change also necessitated a
few stability changes:

* The `collections::fmt` module is now stable (it was already supposed to be).
* The `thread_local::imp::Key` type is now stable (it was already supposed to
  be).
* The `std::rt::{begin_unwind, begin_unwind_fmt}` functions are now stable.
  These are required via the `panic!` macro.
* The `std::old_io::stdio::{println, println_args}` functions are now stable.
  These are required by the `print!` and `println!` macros.
* The `ops::{FnOnce, FnMut, Fn}` traits are now `#[stable]`. This is required to
  make bounds with these traits stable. Note that manual implementations of
  these traits are still gated by default, this stability only allows bounds
  such as `F: FnOnce()`.

Additionally, the compiler now has special logic to ignore its own generated
`__test` module for the `--test` harness in terms of stability.

Closes #8962
Closes #16360
Closes #20327

[breaking-change]

1 files changed, 2 insertions, 0 deletions

diff --git a/src/libcore/fmt/mod.rs b/src/libcore/fmt/mod.rs
index 0357b723b3c..f940300a269 100644
--- a/src/libcore/fmt/mod.rs
+++ b/src/libcore/fmt/mod.rs
@@ -268,6 +268,7 @@ pub trait Debug {
     fn fmt(&self, &mut Formatter) -> Result;
 }
 
+#[allow(deprecated)]
 impl<T: Show + ?Sized> Debug for T {
     #[allow(deprecated)]
     fn fmt(&self, f: &mut Formatter) -> Result { Show::fmt(self, f) }
@@ -295,6 +296,7 @@ pub trait Display {
     fn fmt(&self, &mut Formatter) -> Result;
 }
 
+#[allow(deprecated)]
 impl<T: String + ?Sized> Display for T {
     #[allow(deprecated)]
     fn fmt(&self, f: &mut Formatter) -> Result { String::fmt(self, f) }