Use ecx for const-prop local storage

r? @oli-obk

Remove `ast::Guard`

With the introduction of `ast::ExprKind::Let` in https://github.com/rust-lang/rust/pull/60861, the `ast::Guard` structure is now redundant in terms of representing [`if let` guards](https://github.com/rust-lang/rust/issues/51114) in AST since it can be represented by `ExprKind::Let` syntactically. Therefore, we remove `ast::Guard` here.

However, we keep `hir::Guard` because the semantic representation is a different matter and this story is more unclear right now (might involve `goto 'arm` in HIR or something...).

r? @petrochenkov

Run rustfmt on some libsyntax files

As part of #62008, run rustfmt on:
- src/libsyntax/ext/tt/macro_rules.rs
- src/libsyntax/ext/tt/quoted.rs

There is no semantic change. To fix potential merge conflicts, simply choose the other side then run rustfmt and fix any tidy check (like line length).

Speed up tidy

master:
  Time (mean ± σ):      3.478 s ±  0.033 s    [User: 3.298 s, System: 0.178 s]
  Range (min … max):    3.425 s …  3.525 s    10 runs

This PR:
  Time (mean ± σ):      1.098 s ±  0.006 s    [User: 783.7 ms, System: 310.2 ms]
  Range (min … max):    1.092 s …  1.113 s    10 runs

Alleviates https://github.com/rust-lang/rust/issues/59884. For the most part each commit stands on its own. Timings are on warm filesystem cache.

r? @EddyB

compiletest: Introduce `// {check,build,run}-pass` pass modes

Pass UI tests now have three modes
```
// check-pass
// build-pass
// run-pass
```
mirroring equivalent well-known `cargo` commands.

`// check-pass` will compile the test skipping codegen (which is expensive and isn't supposed to fail in most cases).
`// build-pass` will compile and link the test without running it.
`// run-pass` will compile, link and run the test.
Tests without a "pass" annotation are still considered "fail" tests.

Most UI tests would probably want to switch to `check-pass`.
Tests validating codegen would probably want to run the generated code as well and use `run-pass`.
`build-pass` should probably be rare (linking tests?).

https://github.com/rust-lang/rust/pull/61755 will provide a way to run the tests with any mode, e.g. bump `check-pass` tests to `run-pass` to satisfy especially suspicious people, and be able to make sure that codegen doesn't breaks in some entirely unexpected way.
Tests marked with any mode are expected to pass with any other mode, if that's not the case for some legitimate reason, then the test should be made a "fail" test rather than a "pass" test.
Perhaps some secondary CI can verify this invariant, but that's not super urgent.

`// compile-pass` still works and is equivalent to `build-pass`.
Why is `// compile-pass` bad - 1) it gives an impression that the test is only compiled, but not linked, 2) it doesn't mirror a cargo command.
It can be removed some time in the future in a separate PR.

cc https://github.com/rust-lang/rust/issues/61712

It's more efficient than the fs::read_dir API

[let_chains, 2/6] Introduce `Let(..)` in AST, remove IfLet + WhileLet and parse let chains

Here we remove `ast::ExprKind::{IfLet, WhileLet}` and introduce `ast::ExprKind::Let`.
Moreover, we also:
+ connect the parsing logic for let chains
+ introduce the feature gate
+ rewire HIR lowering a bit.

However, this does not connect the new syntax to semantics in HIR.
That will be the subject of a subsequent PR.

Per https://github.com/rust-lang/rust/issues/53667#issuecomment-471583239.
Next step after https://github.com/rust-lang/rust/pull/59288.

cc @Manishearth re. Clippy.

r? @oli-obk

Rollup of 5 pull requests

Successful merges:

 - #62047 (Trigger `unused_attribute` lint on `#[cfg_attr($pred,)]`)
 - #62049 (Fix one missing `dyn`.)
 - #62051 (Lint empty `#[derive()]` as unused attribute.)
 - #62057 (Deny explicit_outlives_requirements in the compiler)
 - #62068 (Fix meta-variable binding errors in macros)

Failed merges:

r? @ghost

Fix meta-variable binding errors in macros

The errors are either:
- The meta-variable used in the right-hand side is not bound (or defined) in the
  left-hand side.
- The meta-variable used in the right-hand side does not repeat with the same
  kleene operator as its binder in the left-hand side. Either it does not repeat
  enough, or it uses a different operator somewhere.

This change should have no semantic impact.

Found by https://github.com/rust-lang/rust/pull/62008

Deny explicit_outlives_requirements in the compiler

Lint empty `#[derive()]` as unused attribute.

Closes https://github.com/rust-lang/rust/issues/54651.

cc https://github.com/rust-lang/rust/issues/55112

r? @petrochenkov

Fix one missing `dyn`.

It's in the documentation of `Unsize`.

Trigger `unused_attribute` lint on `#[cfg_attr($pred,)]`

Lint on `#[cfg_attr($pred,)]` as decided in https://github.com/rust-lang/rust/issues/54881#issuecomment-441442173.

Closes https://github.com/rust-lang/rust/issues/54881.

r? @estebank

The errors are either:
- The meta-variable used in the right-hand side is not bound (or defined) in the
  left-hand side.
- The meta-variable used in the right-hand side does not repeat with the same
  kleene operator as its binder in the left-hand side. Either it does not repeat
  enough, or it uses a different operator somewhere.

This change should have no semantic impact.

Co-Authored-By: NVadim Petrochenkov <vadim.petrochenkov@gmail.com>