We're not quite ready to ship parallel compilers by default, but the alt
builders are not used too much (in theory), so we believe that shipping
a possibly-broken compiler there is not too problematic.

…ible.

… new API.

This changes the default parallelism for parallel compilers to one,
instead of the previous default, which was "num cpus". This is likely
not an optimal default long-term, but it is a good default for testing
whether parallel compilers are not a significant regression over a
sequential compiler.

Notably, this in theory makes a parallel-enabled compiler behave
exactly like a sequential compiler with respect to the jobserver.

Add long error explanation for E0495

Part of rust-lang#61137.

…xcrichton

Make all alt builders produce parallel-enabled compilers

We're not quite ready to ship parallel compilers by default, but the alt
builders are not used too much (in theory), so we believe that shipping
a possibly-broken compiler there is not too problematic.

r? @nikomatsakis

…i-refactor, r=wesleywiser

SelfProfiler API refactoring and part one of event review

This PR refactors the `SelfProfiler` a little bit so that most profiling methods are RAII-based. The codegen backend code already had something similar, this refactoring pulls this functionality up into `SelfProfiler` itself, for general use.

The second commit of this PR is a review and update of the existing events we are already recording. Names have been made more consistent. CGU names have been removed from event names. They will be added back in when function parameter recording is implemented.

There is still some work to be done for adding new events, especially around trait resolution and the incremental system.

r? @wesleywiser

…i-obk

[const-prop] Handle remaining MIR Rvalue cases

r? @oli-obk

syntax: cleanup param, method, and misc parsing

Do some misc cleanup of the parser:
- Method and parameter parsing is refactored.
- A parser for `const | mut` is introduced that rust-lang#64588 can reuse.
- Some other misc parsing.

Next up in a different PR:
- ~Implementing rust-lang#64252 -- maybe some other time...
- Heavily restructuring up `item.rs` which is a mess (hopefully, no promises ^^).

r? @petrochenkov