To check if a method's receiver type is object safe, we create a new receiver type by substituting in a bogus type parameter (let's call it `U`) for `Self`, and checking that the unmodified receiver type implements `DispatchFromDyn<receiver type with Self = U>`. It would be better to use `dyn Trait` directly, and the only reason we don't is because it triggers another check that `Trait` is object safe, resulting in a query cycle. Once the feature `object_safe_for_dispatch` (tracking issue rust-lang#43561) is stabilized, this will no longer be the case, and we'll be able to use `dyn Trait` as the unsized `Self` type. I've updated the comments in object_safety.rs accordingly.

We also sever syntax's dependency on rustc_target as a result.
This should slightly improve pipe-lining.

Moreover, some cleanup is done in related code.

Add lint and tests for unnecessary parens around types

This is my first contribution to the Rust project, so I apologize if I'm not doing things the right way.

The PR fixes rust-lang#64169. It adds a lint and tests for unnecessary parentheses around types. I've run `tidy` and `rustfmt` — I'm not totally sure it worked right, though — and I've tried to follow the instructions linked in the readme.

I tried to think through all the variants of `ast::TyKind` to find exceptions to this lint, and I could only find the one mentioned in the original issue, which concerns types with `dyn`. I'm not a Rust expert, thought, so I may well be missing something.

There's also a problem with getting this to build. The new lint catches several things in the, e.g., `core`. Because `x.py` seems to build with an equivalent of `-Werror`, what would have been warnings cause the build to break. I got it to build and the tests to pass with `--warnings warn` on my `x.py build` and `x.py test` commands.

Update comments re type parameter hack in object safety

To check if a method's receiver type is object safe, we create a new receiver type by substituting in a bogus type parameter (let's call it `U`) for `Self`, and checking that the unmodified receiver type implements `DispatchFromDyn<receiver type with Self = U>`. It would be better to use `dyn Trait` directly, and the only reason we don't is because it triggers another check that `Trait` is object safe, resulting in a query cycle. Once the feature `object_safe_for_dispatch` (tracking issue rust-lang#43561) is stabilized, this will no longer be the case, and we'll be able to use `dyn Trait` as the unsized `Self` type. I've updated the comments in object_safety.rs accordingly.

cc @Centril @nikomatsakis @bovinebuddha

…chenkov

syntax: ABI-oblivious grammar

This PR has the following effects:

1. `extern $lit` is now legal where `$lit:literal` and `$lit` is substituted for a string literal.

2. `extern "abi_that_does_not_exist"` is now *syntactically* legal whereas before, the set of ABI strings was hard-coded into the grammar of the language. With this PR, the set of ABIs are instead validated and translated during lowering. That seems more appropriate.

3. `ast::FloatTy` is now distinct from `rustc_target::abi::FloatTy`. The former is used substantially more and the translation between them is only necessary in a single place.

4. As a result of 2-3, libsyntax no longer depends on librustc_target, which should improve pipe-lining somewhat.

cc @rust-lang/lang -- the points 1-2 slightly change the definition of the language but in a way which seems consistent with our general principles (in particular wrt. the discussions of turning things into semantic errors). I expect this to be uncontroversial but it's worth letting y'all know. :)

r? @varkor

…davidtwco

Use structured suggestion for unnecessary bounds in type aliases

…twco

Fix an incorrect docstring for Immediate in librustc_mir/interpret.

I suspect `Immediate` was once called `Value`?