Add stability inheritance

This commit makes several changes to the stability index infrastructure:

* Stability levels are now inherited lexically, i.e., each item's
  stability level becomes the default for any nested items.

* The computed stability level for an item is stored as part of the
  metadata. When using an item from an external crate, this data is
  looked up and cached.

* The stability lint works from the computed stability level, rather
  than manual stability attribute annotations. However, the lint still
  checks only a limited set of item uses (e.g., it does not check every
  component of a path on import). This will be addressed in a later PR,
  as part of issue #8962.

* The stability lint only applies to items originating from external
  crates, since the stability index is intended as a promise to
  downstream crates.

* The "experimental" lint is now _allow_ by default. This is because
  almost all existing crates have been marked "experimental", pending
  library stabilization. With inheritance in place, this would generate
  a massive explosion of warnings for every Rust program.

  The lint should be changed back to deny-by-default after library
  stabilization is complete.

* The "deprecated" lint still warns by default.

The net result: we can begin tracking stability index for the standard
libraries as we stabilize, without impacting most clients.

Closes #13540.
Check pattern refutability the same way exhaustiveness is checked
Provide a witness pattern for non-exhaustive patterns

Fixed #4321
Add support for fixed size vectors in let/arg patterns

Fixes #7784
Add unreachability detection for missized patterns of fixed size vectors

Fixed #13482
Fix #14393

String patterns should have a single constructor of arity 0.
Address review comments
syntax: Parse GT tokens from `>=` and `>>=`

The parser already has special logic for parsing `>` tokens from `>>`, and this
commit extends the logic to the acquiring a `>` from the `>=` and `>>=` tokens
as well.

Closes #15043
auto merge of #15056 : alexcrichton/rust/issue-15043, r=kballard

The parser already has special logic for parsing `>` tokens from `>>`, and this
commit extends the logic to the acquiring a `>` from the `>=` and `>>=` tokens
as well.

Closes #15043
Ignore issue-14393 on Windows
librustc: Put `#[unsafe_destructor]` behind a feature gate.

Closes #8142.

This is not the semantics we want long-term. You can continue to use
`#[unsafe_destructor]`, but you'll need to add
`#![feature(unsafe_destructor)]` to the crate attributes.

[breaking-change]
auto merge of #14988 : pcwalton/rust/unsafe-destructor-feature-gate, r=alexcrichton

Closes #8142.

This is not the semantics we want long-term. You can continue to use
`#[unsafe_destructor]`, but you'll need to add
`#![feature(unsafe_destructor)]` to the crate attributes.

[breaking-change]

r? @alexcrichton
libsyntax: Stop parsing `+` with no bounds after it.

This will break code that looks like `Box<Trait+>`. Change that code to
`Box<Trait>` instead.

Closes #14925.

[breaking-change]
auto merge of #14731 : jakub-/rust/pattern-matching-refactor, r=alexcrichton

This PR is changing the error messages for non-exhaustive pattern matching to include a more accurate witness, i.e. a pattern that is not covered by any of the ones provided by the user. Example:

```rust
fn main() {
	match (true, (Some("foo"), [true, true]), Some(42u)) {
		(false, _, _) => (),
		(true, (None, [true, _]), None) => (),
		(true, (None, [false, _]), Some(1u)) => ()
	}
}
```

```sh
/tmp/witness.rs:2:2: 6:3 error: non-exhaustive patterns: (true, (core::option::Some(_), _), _) not covered
/tmp/witness.rs:2 	match (true, (Some("foo"), [true, true]), Some(42u)) {
/tmp/witness.rs:3 		(false, _, _) => (),
/tmp/witness.rs:4 		(true, (None, [true, _]), None) => (),
/tmp/witness.rs:5 		(true, (None, [false, _]), Some(1u)) => ()
/tmp/witness.rs:6 	}
```

As part of that, I refactored some of the relevant code and carried over the changes to fixed vectors from the previous PR.

I'm putting it out there for now but the tests will be red.
auto merge of #15029 : aturon/rust/stability-index, r=brson

This commit makes several changes to the stability index infrastructure:

* Stability levels are now inherited lexically, i.e., each item's
  stability level becomes the default for any nested items.

* The computed stability level for an item is stored as part of the
  metadata. When using an item from an external crate, this data is
  looked up and cached.

* The stability lint works from the computed stability level, rather
  than manual stability attribute annotations. However, the lint still
  checks only a limited set of item uses (e.g., it does not check every
  component of a path on import). This will be addressed in a later PR,
  as part of issue #8962.

* The stability lint only applies to items originating from external
  crates, since the stability index is intended as a promise to
  downstream crates.

* The "experimental" lint is now _allow_ by default. This is because
  almost all existing crates have been marked "experimental", pending
  library stabilization. With inheritance in place, this would generate
  a massive explosion of warnings for every Rust program.

  The lint should be changed back to deny-by-default after library
  stabilization is complete.

* The "deprecated" lint still warns by default.

The net result: we can begin tracking stability index for the standard
libraries as we stabilize, without impacting most clients.

Closes #13540.
replace reference to ~"string" with box "string"
rustdoc: show default type parameters in generics

fix #12291
auto merge of #15062 : pcwalton/rust/trailing-plus, r=brson

This will break code that looks like `Box<Trait+>`. Change that code to
`Box<Trait>` instead.

Closes #14925.

[breaking-change]

r? @brson
auto merge of #15074 : conradkleinespel/rust/master, r=pcwalton

Using something like:

```rust
box "string"
```

yields
```shell
"`~\"string\"` has been removed; use `\"string\".to_string()` instead"
```

Should the error message maybe say `box "string"` instead?
Correctly set return type attributes on foreign function declarations

The ArgType type gives us a generic way to specify an attribute for a
type to ensure ABI conformance for foreign functions. But the code that
actually sets the argument attributes in the function declaration
only sets the attribute for the return type when the type is indirect.

Since LLVMAddAttribute() doesn't allow to set attributes on the return
type, we have to use LLVMAddFunctionAttribute() instead.

This didn't cause problems yet, because currently only some indirect
types require attributes to be set.