I've added the panic_col feature in PR #42938.
Now it's time to stabilize it!
Closes #42939.

Shorten names of some compiler generated artifacts.

This PR makes the compiler mangle codegen unit names by default. The name of every codegen unit name will now be a random string of 16 characters. It also makes the file extensions of some intermediate compiler products shorter. Hopefully, these changes will reduce the pressure on tools with path length restrictions like buildbot. The change should also solve problems with case-insensitive file system.

cc #47186 and #47222

r? @alexcrichton

Clean emitted diagnostics when `reset_err_count` is called.

When external tools like `rustfmt` calls to `reset_err_count` for handler
reusing, it will set the error count on the handler to 0, but since
https://github.com/rust-lang/rust/pull/47146 the handler will contain
status that will prevent the error count to be bumped if this handler is
reused.

This caused `rustfmt` idempotency tests to fail:
https://github.com/rust-lang-nursery/rustfmt/issues/2338

Fixes: https://github.com/rust-lang-nursery/rustfmt/issues/2338

Rollup of 10 pull requests

- Successful merges: #47210, #47233, #47246, #47254, #47256, #47258, #47259, #47263, #47270, #47272
- Failed merges: #47248

Add missing links

r? @QuietMisdreavus

(please wait for CI, I have a few doubts about the `Write` trait links...)

Don't look for niches inside generator types. Fixes #47253

r? @EddyB

rustdoc: Don't import macros from private imports

Fixes #47038

Add HashMap::remove_entry

Implements #46344

r? @dtolnay

rustc::ty: Rename struct_variant to non_enum_variant

r? @EddyB

Rename ReprExtern to ReprC

… and similarily rename a few other field and locals that mentioned "extern repr".

Replace empty array hack with repr(align)

As a side effect, this fixes the warning about repr(C, simd) that has been reported during x86_64 windows builds since #47111 (see also: #47103)

r? @alexcrichton

Make wasm obey backtrace feature, like other targets

E.g. https://github.com/rust-lang/rust/blob/6828cf90146c7fefc4ba4f16dffe75f763f2d910/src/libstd/sys/unix/mod.rs#L40-L41

Remove unused LLVM related code

Ticks a few more boxes on #46437

Rollup merge of #47210 - zackmdavis:the_3rd_of_2_hardest_problems_in_computer_science, r=QuietMisdreavus

fix the doc-comment-decoration-trimming edge-case rustdoc ICE

This `horizontal_trim` function strips the leading whitespace from
doc-comments that have a left-asterisk-margin:

```
  /**
   * You know what I mean—
   *
   * comments like this!
   */
```

The index of the column of asterisks is `i`, and if trimming is deemed
possible, we slice each line from `i+1` to the end of the line. But if, in
particular, `i` was 0 _and_ there was an empty line (as in the example
given in the reporting issue), we ended up panicking trying to slice an
empty string from 0+1 (== 1).

Let's tighten our check to say that we can't trim when `i` is even the same
as the length of the line, not just when it's greater. (Any such cases
would panic trying to slice `line` from `line.len()+1`.)

Resolves #47197.

When external tools like `rustfmt` calls to `reset_err_count` for handler
reusing, it will set the error count on the handler to 0, but since
https://github.com/rust-lang/rust/pull/47146 the handler will contain
status that will prevent the error count to be bumped if this handler is
reused.

This caused `rustfmt` idempotency tests to fail:
https://github.com/rust-lang-nursery/rustfmt/issues/2338

Fixes: https://github.com/rust-lang-nursery/rustfmt/issues/2338

Make double ended searchers use dependent fingers

(fixes #47175)

r? @burntsushi @alexcrichton

needs uplift to beta

Add help message for incorrect pattern syntax

When I was getting started with rust I often made the mistake of using `||` instead of `|` to match multiple patterns and spent a long time staring at my code wondering what was wrong.

for example:

```
fn main() {
    let x = 1;

    match x {
        1 || 2 => println!("1 or 2"),
        _ => println!("Something else"),
    }
}

```

If you compile this with current rustc you will see

```
error: expected one of `...`, `..=`, `..`, `=>`, `if`, or `|`, found `||`
 --> test.rs:5:11
  |
5 |         1 || 2 => println!("1 or 2"),
  |          -^^ unexpected token
  |          |
  |          expected one of `...`, `..=`, `..`, `=>`, `if`, or `|` here

error: aborting due to previous error
```

With my proposed change it will show:
```
error: unexpected token `||` after pattern
 --> test.rs:5:11
  |
5 |         1 || 2 => println!("1 or 2"),
  |           ^^
  |
  = help: did you mean to use `|` to specify multiple patterns instead?

error: aborting due to previous error
```

Make normalize_and_test_predicates into a query

From #44891.

I'm not real solid on how `dep_graph` stuff works, but if a node is going to have a key (again, not sure how important that is), then the key needs to be `Copy`. So since `normalize_and_test_predicates` only had one out-of-module use, I changed that call site to use a new function, `substitute_normalize_and_test_predicates` which is the query and enables having the arguments be `Copy`. Hopefully this makes sense.

r? @nikomatsakis
and/or @michaelwoerister

It is also intended for use with unions.

Provide suggestion when trying to use method on numeric literal

New output:

```
error[E0688]: can't call method `powi` on ambiguous numeric type `{float}`
  --> $DIR/method-on-ambiguous-numeric-type.rs:12:17
   |
12 |     let x = 2.0.powi(2);
   |                 ^^^^
help: you must specify a concrete type for this numeric value, like `f32`
   |
12 |     let x = 2.0_f32.powi(2);
   |             ^^^^^^^
```

Previous output:

```
error[E0599]: no method named `powi` found for type `{float}` in the current scope
  --> src/main.rs:12:17
   |
12 |     let x = 2.0.powi(2);
   |                 ^^^^
   |
   = help: items from traits can only be used if the trait is in scope
help: the following trait is implemented but not in scope, perhaps add a `use` for it:
   |
11 | use core::num::Float;
   |
```

Fix #40985.

Implements #46344

Rename ReprExtern to ReprC, and similarily rename a few other fields and locals that mentioned "extern repr"

As a side effect, this fixes the warning about repr(C, simd) that has been reported during x86_64 windows builds since #47111 (see also: #47103)

Try to fix a perf regression by updating log

Upgrade `log` to `0.4` in multiple crates ~and `cargo`~.
Cc: #47154.

Upgrade `log` to `0.4` in multiple crates.

rustdoc: Add missing src links for generic impls on trait pages

`implementor2item` would return `None` for generic impls so instead this clones the entire `clean::Item` into the `implementors` map which simplifies some code.

Auto merge of #47177 - michaelwoerister:erase-invalid-spans-during-metadata-encoding, r=alexcrichton

Map invalid Spans to DUMMY_SP during crate metadata encoding.

This mirrors what we do for stabilizing the incr. comp. cache and is necessary for reproducible builds. For the incr. comp. cache, we *have* to do this because the encoding there cannot represent broken Spans. Metadata encoding has to be in sync with that as not to get unexpected interactions when compiling incrementally.

This should help with fixing issue https://github.com/rust-lang/rust/issues/47066.

r? @alexcrichton

ci: use a shared script to build musl

The dist-x86_64-musl, dist-various-1 and dist-i586-gnu-i686-musl builders had different scripts to build musl. This PR creates an unified script, which makes it easier to add new musl targets and update musl and libunwind (used in the musl targets).

The libunwind is update from 3.7 to 3.9 for dist-x86_64-musl and dist-i586-gnu-i686-musl (dist-various-1 already used 3.9 version).

Refs #46437 as it also removes LLVMRustWriteDebugLocToString()

Refs #46437

The same effect can be achieved using -Cllvm-args=-debug

Refs #46437 as it removes LLVMRustSetDebug()

Support `extern` in paths

Implement the primary alternative to https://github.com/rust-lang/rust/pull/46613 + https://github.com/rust-lang/rust/pull/45771, achieving the same effect without requiring changes to other imports.
Both need to be experimentally evaluated before making further progress.

The PR also adds docs for all these related features into the unstable book.

cc https://github.com/rust-lang/rust/issues/44660
r? @nikomatsakis

Rollup of 7 pull requests

- Successful merges: #46947, #47170, #47190, #47205, #47217, #47220, #47230
- Failed merges: #47233