Fixes: rust-lang#79569

Fixes: rust-lang#79566
Fixes: rust-lang#79565

For the first issue (rust-lang#79569), I got hit a `debug_assert!()` before
encountering the reported error message (because I have `debug = true`
enabled in my config.toml).

The assertion showed me that some `SwitchInt`s can have more than one
target pointing to the same `BasicBlock`.

I had thought that was invalid, but since it seems to be possible, I'm
allowing this now.

I added a new test for this.

----

In the last two cases above, both tests (intentionally) fail to compile,
but the `InstrumentCoverage` pass is invoked anyway.

The MIR starts with an `Unreachable` `BasicBlock`, which I hadn't
encountered before. (I had assumed the `InstrumentCoverage` pass
would only be invoked with MIRs from successful compilations.)

I don't have test infrastructure set up to test coverage on files that
fail to compile, so I didn't add a new test.

This was an adventure; see https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/'higher.20ranked.20subtype.20error'

Adds checks for:

* `no_core` attribute
* explicitly-enabled `legacy` symbol mangling
* mir_opt_level > 1 (which enables inlining)

I removed code from the `Inline` MIR pass that forcibly disabled
inlining if `-Zinstrument-coverage` was set. The default `mir_opt_level`
does not enable inlining anyway. But if the level is explicitly set and
is greater than 1, I issue a warning.

The new warnings show up in tests, which is much better for diagnosing
potential option conflicts in these cases.

This ensures consistent handling of default values for options that are
None if not specified on the command line.

…Nemo157

Show hidden elements by default when JS is disabled

Fixes  rust-lang#79301.

A lot of things are hidden by default which shouldn't when JS is disabled. This PR fixes it.

Before:

![Screenshot from 2020-11-24 14-10-16](https://user-images.githubusercontent.com/3050060/100099361-a16d5580-2e5f-11eb-891b-a4c005aeb1d0.png)

After:

![after](https://user-images.githubusercontent.com/3050060/100099382-a6caa000-2e5f-11eb-8190-14f330aff9a2.png)

r? `@jyn514`

…t-when-collapsing, r=jyn514

Hide associated constants too when collapsing implementation

Fixes rust-lang#71849.

r? `@jyn514`

…2.0, r=tmandry

Fixes reported bugs in Rust Coverage

Fixes: rust-lang#79569

Fixes: rust-lang#79566
Fixes: rust-lang#79565

For the first issue (rust-lang#79569), I got hit a `debug_assert!()` before
encountering the reported error message (because I have `debug = true`
enabled in my config.toml).

The assertion showed me that some `SwitchInt`s can have more than one
target pointing to the same `BasicBlock`.

I had thought that was invalid, but since it seems to be possible, I'm
allowing this now.

I added a new test for this.

----

In the last two cases above, both tests (intentionally) fail to compile,
but the `InstrumentCoverage` pass is invoked anyway.

The MIR starts with an `Unreachable` `BasicBlock`, which I hadn't
encountered before. (I had assumed the `InstrumentCoverage` pass
would only be invoked with MIRs from successful compilations.)

I don't have test infrastructure set up to test coverage on files that
fail to compile, so I didn't add a new test.

r? `@tmandry`
FYI: `@wesleywiser`

Fix `cargo-binutils` link

Use imports instead of rewriting the type signature of `RustcOptGroup::stable`

This was an adventure; see https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/'higher.20ranked.20subtype.20error'

r? `@GuillaumeGomez`

…crum

Replace some `println!` with `tidy_error!` to simplify