…n of unions

This clarifies that we're adding an "entry point", not just adding an "entry"
of some kind.

This allows us to use real field names instead of tuple element numbers.

Renaming `previous_drops` to `existing_drops_map` clarifies that "previous" was
unrelated to drop order.

The `sig_dfl` variant of the attribute is the most likely variant to be
stabilized first, and thus to become the "most allowed" variant of the
attribute. Because of this, it is the most appropriate variant to use in
syntax tests, because even if the most allowed variant is used, the
compiler shall still emit errors if it e.g. is used in the wrong places.

…r=oli-obk

Allow codegen backends to opt-out of parallel codegen

This makes it a bit easier to write cursed codegen backends.

…li-obk,cjgillot

Fix type resolution of associated const equality bounds (take 2)

Instead of trying to re-resolve the type of assoc const bindings inside the `type_of` query impl in an incomplete manner, transfer the already (correctly) resolved type from `add_predicates_for_ast_type_binding` to `type_of`/`anon_type_of` through query feeding.

---

Together with rust-lang#118668 (merged) and rust-lang#121258, this supersedes rust-lang#118360.
Fixes rust-lang#118040.

r? `@ghost`

… r=oli-obk

Add tests (and a bit of cleanup) for interior mut handling in promotion and const-checking

Basically these are the parts of rust-lang#121786 that can be salvaged.

r? `@oli-obk`

Clarity improvements to `DropTree`

These changes are based on some points of confusion I had when initially trying to understand this code.

The only “functional” change is an additional assertion in `<ExitScopes as DropTreeBuilder>::link_entry_point`, checking that the dummy terminator is `TerminatorKind::UnwindResume` as expected.

Improve diagnostics for parenthesized type arguments

Fixes rust-lang#120892

r? fmease

…=davidtwco

Remove the unused `field_remapping` field from `TypeLowering`

The `field_remapping` field of `TypeLowering` has  been unused since rust-lang#121665. This PR removes it, then replaces the `TypeLowering` struct with its only remaining member `&'ll Type`.

interpret: do not call machine read hooks during validation

Fixes rust-lang/miri#3347

r? `@oli-obk`

…r=nnethercote

Store backtrace for `must_produce_diag`

This makes it significantly easier to debug a `must_produce_diag` ICE, since we have no other way to know where the heck the bug originates from.

Backtrace rendering kinda sucks right now since we're just printing it in the panic message; happy to apply some suggestions to make it prettier or reuse other bug printing machinery, but also don't want to iterate too much on the rendering since this really is just for debug purposes.

r? nnethercote

… r=lcnr

Revision-related tweaks for next-solver tests

1. Add `ignore-compare-mode-next-solver` to any test that already has explicit `current next` revisions, since the test failures when testing with `--compare-mode=next-solver` will be false positives.
2. Explicitly add revisions to a handful of tests where we expect behavior to diverge.

r? lcnr

…idtwco

unix_sigpipe: Replace `inherit` with `sig_dfl` in syntax tests

The `sig_dfl` variant of the attribute is the most likely variant to be stabilized first, and thus to become the "most allowed" variant of the attribute. Because of this, it is the most appropriate variant to use in syntax tests, because even if the most allowed variant is used, the compiler shall still emit errors if it e.g. is used in the wrong places.

r? `@davidtwco` who already [approved ](rust-lang#120832 (review)) this commit in rust-lang#120832.

It would be nice to land the last preparatory commit of that PR before we begin to [rename ](rust-lang#120832 (comment)) things which will of course create a lot of code conflicts.

bootstrap readme: fix, improve, update