Add a MIR pass manager (Taylor's Version)
The final draft of #91386 and #77665.
While the compile-time constraints in #91386 are cool, I decided on a more minimal approach for now. I want to explore phase constraints and maybe relative-ordering constraints in the future, though. This should preserve existing behavior **exactly** (please let me know if it doesn't) while making the following changes to the way we organize things today:
- Each `MirPhase` now corresponds to a single MIR pass. `run_passes` is not responsible for listing the correct MIR phase.
- `run_passes` no longer silently skips passes if the declared MIR phase is greater than or equal to the body's. This has bitten me multiple times. If you want this behavior, you can always branch on `body.phase` yourself.
- If your pass is solely to emit errors, you can use the `MirLint` interface instead, which gets a shared reference to `Body` instead of a mutable one. By differentiating the two, I hope to make it clearer in the short term where lints belong in the pipeline. In the long term perhaps we could enforce this at compile-time?
- MIR is no longer dumped for passes that aren't enabled, or for lints.
I tried to check that `-Zvalidate` still works correctly, since the MIR phase is now updated as soon as the associated pass is done, instead of at the end of all the passes in `run_passes`. However, it looks like `-Zvalidate` is broken with current nightlies anyways 😢 (it spits out a bunch of errors).
cc `@oli-obk` `@wesleywiser`
r? rust-lang/wg-mir-opt
Mark places as initialized when mutably borrowed
Fixes the example in #90752, but does not handle some corner cases involving raw pointers and unsafe. See [this comment](https://github.com/rust-lang/rust/issues/90752#issuecomment-965822895) for more information, or the second test.
Although I talked about both `MaybeUninitializedPlaces` and `MaybeInitializedPlaces` in #90752, this PR only changes the latter. That's because "maybe uninitialized" is the conservative choice, and marking them as definitely initialized (`!maybe_uninitialized`) when a mutable borrow is created could lead to problems if `addr_of_mut` to an uninitialized local is allowed. Additionally, places cannot become uninitialized via a mutable reference, so if a place is definitely initialized, taking a mutable reference to it should not change that.
I think it's correct to ignore interior mutability as nbdd0121 suggests below. Their analysis doesn't work inside of `core::cell`, which *does* have access to `UnsafeCell`'s field, but that won't be an issue unless we explicitly instantiate one with an `enum` within that module.
r? `@wesleywiser`
This performs a substitution of code following the pattern:
let <id> = if let <pat> = ... { identity } else { ... : ! };
To simplify it to:
let <pat> = ... { identity } else { ... : ! };
By adopting the let_else feature.
Document behavior of `MaybeLiveLocals` regarding enums and field-senstivity
This arose from a [discussion on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/189540-t-compiler.2Fwg-mir-opt/topic/MaybeLiveLocals.20and.20Discriminants) where a new contributor attempted to implement a dead-store elimination pass using this analysis. They ran into a nasty hack around `SetDiscriminant` the effect of which is to lets handle assignments of literals to enum-typed locals (e.g. `x = Some(4)`) correctly. This took me a while to figure out.
Document this oddity, so the next person will have an easier time, and add a test to enshrine the current behavior.
r? ``@tmiasko``
Stabilize `const_panic`
Closes#51999
FCP completed in #89006
```@rustbot``` label +A-const-eval +A-const-fn +T-lang
cc ```@oli-obk``` for review (not `r?`'ing as not on lang team)
