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.

…about the lint

…=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.

Implement NOOP_METHOD_CALL lint

Implements the beginnings of rust-lang/lang-team#67 - a lint for detecting noop method calls (e.g, calling `<&T as Clone>::clone()` when `T: !Clone`).

This PR does not fully realize the vision and has a few limitations that need to be addressed either before merging or in subsequent PRs:
* [ ] No UFCS support
* [ ] The warning message is pretty plain
* [ ] Doesn't work for `ToOwned`

The implementation uses [`Instance::resolve`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/instance/struct.Instance.html#method.resolve) which is normally later in the compiler. It seems that there are some invariants that this function relies on that we try our best to respect. For instance, it expects substitutions to have happened, which haven't yet performed, but we check first for `needs_subst` to ensure we're dealing with a monomorphic type.

Thank you to `@davidtwco,` `@Aaron1011,` and `@wesleywiser` for helping me at various points through out this PR ❤️.

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::Import

Per the title. Part of cleaning up doctree