functions that does allocations

This will make rustdoc behave properly when -Dwarnings is given

…r=jyn514

Declare a new lint to properly deny warnings in rustdoc

This declares a new lint: `INVALID_RUST_CODEBLOCK` that is used by `rustdoc` to properly follow `-D warnings` instead of unconditionally emitting a warning.

## Todo List

- [x] Declare lint.
- [x] Document lint (file: `src/doc/rustdoc/src/lints.md`).
- [x] Use lint in `rustdoc` (file: `src/librustdoc/passes/check_code_block_syntax.rs`, maybe others).
- [x] Write tests.
  - [x] Note: one for the behaviour of the new lint when the error is in a dependency, not the crate being tested (rust-lang#79816 (comment))
- [x] Refactor things.

## Questions

- How/where are lints tested ?
- Where are lints for `rustdoc` tested ?

Fix rust-lang#79792.

``@rustbot`` label T-rustdoc A-lint

Remove memoization leftovers from constant evaluation machine

Closes rust-lang#79667

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.

…=matthewjasper

Remove unused `TyEncoder::tcx` required method

Unsure if this is helpful or not...

r? `@ghost` cc `@matthewjasper` `@jackh726`