Fixes rust-lang#80233

We already have logic in `evaluate_predicates` that tries to add
unimplemented predicates to our `ParamEnv`. Trying to add a predicate
that already holds can lead to errors later on, since projection
will prefer trait candidates from the `ParamEnv` to predicates from an
impl.

…=estebank

Don't try to add nested predicate to Rustdoc auto-trait `ParamEnv`

Fixes rust-lang#80233

We already have logic in `evaluate_predicates` that tries to add
unimplemented predicates to our `ParamEnv`. Trying to add a predicate
that already holds can lead to errors later on, since projection
will prefer trait candidates from the `ParamEnv` to predicates from an
impl.

Remove unreachable panics from VecDeque::{front/back}[_mut]

`VecDeque`'s `front`, `front_mut`, `back` and `back_mut` methods are implemented in terms of the index operator, which causes these functions to contain [unreachable panic calls](https://rust.godbolt.org/z/MTnq1o).

This PR reimplements these methods in terms of `get[_mut]` instead.

Use Option::map_or instead of `.map(..).unwrap_or(..)`

``@rustbot`` modify labels +C-cleanup +T-compiler

…ndry

Don't ICE when computing a layout of a generator tainted by errors

Fixes rust-lang#80998.

… r=jyn514

Remove doctree::Variant

This was easy, probably was missed when whatever used it was removed