1. Rewrite `if let` into `match` to return earl and avoid indenting giant block 
2. Assign `spans_updated` only once

`taken` is actually used afterwards

…type.

Add some tests for macros in extern blocks, remove duplicate tests

 - Refactors the Emscripten target spec to share code with other wasm
   targets.
 - Replaces the incorrect wasm32 C call ABI with the old asmjs
   version, which is correct for both wasm32 and JS.
 - Updates the varargs ABI used by Emscripten and deletes the old one.
 - Removes the obsolete wasm32-experimental-emscripten target.
 - Temporarily makes Emscripten targets use panic=abort by default
   because supporting unwinding will require an LLVM patch.

This commit silences the unreachable code lint when it originates from
within a await desugaring.

Signed-off-by: David Wood <david@davidtw.co>

Make function return early if source map is not present

Also avoid excessive cloning

Reborrowing doesn't work for loops

Closure typing obligations flow in both direcitons to properly infer
types. Because of this, we will get 2 type errors whenever there's
an unfulfilled obligation. To avoid this, we deduplicate them in the
`InferCtxt`.

…, r=alexcrichton

Upgrade Emscripten targets to use upstream LLVM backend

 - Refactors the Emscripten target spec to share code with other wasm
   targets.
 - Replaces the incorrect wasm32 C call ABI with the old asmjs
   version, which is correct for both wasm32 and JS.
 - Updates the varargs ABI used by Emscripten and deletes the old one.
 - Removes the obsolete wasm32-experimental-emscripten target.
 - Temporarily makes Emscripten targets use panic=abort by default
   because supporting unwinding will require an LLVM patch.

syntax: Support modern attribute syntax in the `meta` matcher

Where "modern" means rust-lang#57367:
```
PATH
PATH `(` TOKEN_STREAM `)`
PATH `[` TOKEN_STREAM `]`
PATH `{` TOKEN_STREAM `}`
```

Unfortunately, `meta` wasn't future-proofed using the `FOLLOW` token set like other matchers (rust-lang#34011), so code like `$meta:meta {` or `$meta:meta [` may break, and we need a crater run to find out how often this happens in practice.

Closes rust-lang#49629 (by fully supporting `meta` rather than removing it.)

Stabilize macros in some more positions

- Fn-like macros and attribute macros in `extern` blocks
- Fn-like procedural macros in type positions
- ~Attribute macros on inline modules~ (moved to rust-lang#64273)

Stabilization report: rust-lang#63931 (comment).

Closes rust-lang#49476
cc rust-lang#54727

…tebank

syntax: recover trailing `|` in or-patterns

Fixes rust-lang#64879.

For example (this also shows that we are sensitive to the typo `||`):

```
error: a trailing `|` is not allowed in an or-pattern
  --> $DIR/remove-leading-vert.rs:33:11
   |
LL |         A || => {}
   |         - ^^ help: remove the `||`
   |         |
   |         while parsing this or-pattern starting here
   |
   = note: alternatives in or-patterns are separated with `|`, not `||`
```

r? @estebank

A small amount of tidying-up factored out from PR rust-lang#64648

As requested by @Mark-Simulacrum, I put this in a separate commit to make it easier to review. (As far as I can tell, no violations of the policy here, and they are simply in a separate PR because they're not directly related to the import of that PR.)

r? @Mark-Simulacrum

Add tests for some issues

Closes rust-lang#50571
Closes rust-lang#58022
Closes rust-lang#58344

…it, r=petrochenkov

Silence unreachable code lint from await desugaring

Fixes rust-lang#61798.

This PR silences the unreachable code lint when it originates from within an await desugaring.

…__code-clarity, r=estebank

Improve code clarity

No commit except 55b5428 address performance, just making the existing code more clear.

r? @estebank

Deduplicate closure type errors

Closure typing obligations flow in both direcitons to properly infer
types. Because of this, we will get 2 type errors whenever there's
an unfulfilled obligation. To avoid this, we deduplicate them in the
`InferCtxt`.