Recover on `const X = 42;` and infer type + Error Stash API
Here we:
1. Introduce a notion of the "error stash".
This is a map in the `Handler` to which you can `err.stash(...)` away your diagnostics and then steal them in a later "phase" of the compiler (e.g. stash in parser, steal in typeck) to enrich them with more information that isn't available in the previous "phase".
I believe I've covered all the bases to make sure these diagnostics are actually emitted eventually even under `#[cfg(FALSE)]` but please check my logic.
2. Recover when parsing `[const | static mut?] $ident = $expr;` which has a missing type.
Use the "error stash" to stash away the error and later steal the error in typeck where we emit the error as `MachineApplicable` with the actual inferred type. This builds on https://github.com/rust-lang/rust/pull/62804.
cc https://github.com/rust-lang/rfcs/pull/2545
r? @estebank
Refactor librustc_errors::Handler API
This should be reviewed by-commit.
The last commit moves all fields into an inner struct behind a single lock; this is done to prevent possible deadlocks in a multi-threaded compiler, as well as inconsistent state observation.
Cleanup syntax::ext::build
I suspect most of this code could be inlined but I only removed the bits where the inlining didn't really hurt readability (i.e., method call -> function call) or the completely unused code.
Cleanup handling of hygiene for built-in macros
This makes most identifiers generated by built-in macros use def-site hygiene, not only the ones that previously used gensyms.
* `ExtCtxt::ident_of` now takes a `Span` and is preferred to `Ident::{from_str, from_str_and_span}`
* Remove `Span::with_legacy_ctxt`
* `assert` now uses call-site hygiene because it needs to resolve `panic` unhygienically.
* `concat_idents` now uses call-site hygiene because it wouldn't be very useful with def-site hygiene.
* everything else is moved to def-site hygiene
r? @petrochenkov
