closes rust-lang#79667

functions that does allocations

…thewjasper

Implement if-let match guards

Implements rust-lang/rfcs#2294 (tracking issue: rust-lang#51114).

I probably should do a few more things before this can be merged:
- [x] Add tests (added basic tests, more advanced tests could be done in the future?)
- [x] Add lint for exhaustive if-let guard (comparable to normal if-let statements)
- [x] Fix clippy

However since this is a nightly feature maybe it's fine to land this and do those steps in follow-up PRs.

Thanks a lot `@matthewjasper` ❤️ for helping me with lowering to MIR! Would you be interested in reviewing this?
r? `@ghost` for now

Take into account negative impls in "trait item not found" suggestions

This removes the suggestion to implement a trait for a type when that type already has a negative implementation for the trait, and replaces it with a note to point out that the trait is explicitely unimplemented, as suggested by `@scottmcm.`

Helps with rust-lang#79683.

r? `@scottmcm` do you want to review this?

Remove memoization leftovers from constant evaluation machine

Closes rust-lang#79667

Allow `since="TBD"` for rustc_deprecated

Closes rust-lang#78381.

This PR only affects `#[rustc_deprecated]`, not `#[deprecated]`, so there is no effect on any stable language feature.

Likewise this PR only implements `since="TBD"`, it does not actually tag any library functions with it, so there is no effect on any stable API.

Overview of changes:

* `rustc_middle/stability.rs`:
    * change `deprecation_in_effect` function to return `false` when `since="TBD"`
    * tidy up the compiler output when a deprecated item has `since="TBD"`
* `rustc_passes/stability.rs`:
    * allow `since="TBD"` to pass the sanity check for stable_version < deprecated_version
    * refactor the "invalid stability version" and "invalid deprecation version" error into separate errors
* rustdoc: make `since="TBD"` message on a deprecated item's page match the command-line deprecation output
* tests:
    * test rustdoc output
    * test that the `deprecated_in_future` lint fires when `since="TBD"`
    * test the new "invalid deprecation version" error message

Fix issue rust-lang#78496

EarlyOtherwiseBranch finds MIR structures like:

```
bb0: {
  ...
  _2 = discriminant(X)
  ...
  switchInt(_2) -> [1_isize: bb1, otherwise: bb3]
}
bb1: {
  ...
  _3 = discriminant(Y)
  ...
  switchInt(_3) -> [1_isize: bb2, otherwise: bb3]
}
bb2: {...}
bb3: {...}
```

And transforms them into something like:

```
bb0: {
  ...
  _2 = discriminant(X)
  _3 = discriminant(Y)
  _4 = Eq(_2, _3)
  switchInt(_4) -> [true: bb4, otherwise: bb3]
}
bb2: {...} // unchanged
bb3: {...} // unchanged
bb4: {
  switchInt(_2) -> [1_isize: bb2, otherwise: bb3]
}
```

But that is not always a safe thing to do -- sometimes the early `otherwise` branch is necessary so the later block could assume the value of `discriminant(X)`.

I am not totally sure what's the best way to detect that, but fixing rust-lang#78496 should be easy -- we just check if `X` is a sub-expression of `Y`. A more precise test might be to check if `Y` contains a `Downcast(1)` of `X`, but I think this might be good enough.

Fix rust-lang#78496

…ikomatsakis

Move binder for dyn to each list item

This essentially changes `ty::Binder<&'tcx List<ExistentialTraitRef>>` to `&'tcx List<ty::Binder<ExistentialTraitRef>>`.

This is a first step in moving the `dyn Trait` representation closer to Chalk, which we've talked about in ``````@rust-lang/wg-traits.``````

r? ``````@nikomatsakis``````

…on-panic, r=dtolnay

Fix overflow when converting ZST Vec to VecDeque

```rust
let v = vec![(); 100];
let queue = VecDeque::from(v);
println!("{:?}", queue);
```
This code will currently panic with a capacity overflow.
This PR resolves this issue and makes the code run fine.

Resolves rust-lang#78532

…ulacrum

BTreeMap: more expressive local variables in merge

r? ``````@Mark-Simulacrum``````

…ulacrum

BTreeSet: simplify implementation of pop_first/pop_last

…and stop it interfering in rust-lang#79245.
r? ``````@Mark-Simulacrum``````

…Mark-Simulacrum

expand-yaml-anchors: Make the output directory separator-insensitive

Fixes rust-lang#75709

Optimization for bool's PartialOrd impl

Fix rust-lang#80034.