addresses:

clippy::or_fun_call
clippy::single_char_add_str
clippy::comparison_to_empty
clippy::or_fun_call

At the `if` branch `filter` (the `let` binding) is `None` iff `filter` (the parameter) was `None`.
We can branch on the parameter, move the binding into the `if`, and the complexity of handling
`Option<Option<_>` largely dissolves.

This silences the following clippy lints in ./x.py clippy:

many_single_char_names (there are a lot of warnings caused by stdarch)
collapsible_if (can reduce readability)
type_complexity
missing_safety_doc (there are almost 3K warnings issued)
too_many_arguments
needless_lifetimes (people want 'tcx lifetimes etc)
wrong_self_convention (warns about from_..(), to_..(), into_..().. fns that do or do not take self by reference.

…have correctly

…-Simulacrum

Mention Arc::make_mut and Rc::make_mut in the documentation of Cow

Following this discussion: https://users.rust-lang.org/t/should-the-cow-documentation-mention-arc/53341

_Rendered (the last paragraph is new):_

![Screenshot_20201228_171551](https://user-images.githubusercontent.com/3986214/103228135-5d72e200-4930-11eb-89e1-38b5c86b08c7.png)

`@rustbot` modify labels: T-doc, T-libs

…k-Simulacrum

bootstrap: clippy fixes

addresses:

clippy::or_fun_call
clippy::single_char_add_str
clippy::comparison_to_empty
clippy::or_fun_call

Add check for `[T;N]`/`usize` mismatch in astconv

Helps clarify the issue in rust-lang#80506
by adding a specific check for mismatches between [T;N] and usize.

r? `@lcnr`

…k-Simulacrum

Remove reverted change from relnotes

r? `@Mark-Simulacrum`

Builder: Warn if test file does not exist

Running `./x.py test` with a file that does not exists (but where the path belongs to a test suite) silently ignores the missing file and runs the whole test suite. This PR prints a warning to reduce the potential surprise factor.

Closes rust-lang#80621

Use Option::filter instead of open-coding it

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

Move variable into the only branch where it is relevant

At the `if` branch `filter` (the `let` binding) is `None` iff `filter` (the parameter) was `None`.
We can branch on the parameter, move the binding into the `if`, and the complexity of handling
`Option<Option<_>` largely dissolves.

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

Note: I have no idea how hot this code is. If this method frequently gets called with a `None` filter, there might be a small perf improvement.

Fixed documentation error for `std::hint::spin_loop`

Fixes rust-lang#80644.

Fix missing link for "fully qualified syntax"

This issue can currently be seen at https://doc.rust-lang.org/stable/std/rc/index.html#toggle-all-docs:~:text=%5B-,fully%20qualified%20syntax

It originates from rust-lang#76138, where the link was added to `library/alloc/src/sync.rs`, but not `library/alloc/src/rc.rs`.

… r=Mark-Simulacrum

./x.py clippy: allow the most noisy lints

This silences the following clippy lints in ./x.py clippy:

many_single_char_names (there are a lot of warnings caused by stdarch)
collapsible_if (can reduce readability)
type_complexity
missing_safety_doc (there are almost 3K warnings issued)
too_many_arguments
needless_lifetimes (people want 'tcx lifetimes etc)
wrong_self_convention (warns about from_..(), to_..(), into_..().. fns that do or do not take self by reference.

Just for clarification; this only changes the output of `x.py clippy` inside the rustc repo and does not change anything about clippy or how `cargo clippy` is run on peoples crates.

doc -- list edit for consistency

make sure that promoteds which fail to evaluate in dead const code behave correctly

rust-lang#80243 showed that we'll have to live with these kinds of failing promoteds for a while, so let's make sure we have a test that covers them.