…ref`

The makes the heading / documentation distinction clearer.

Add rustc lint, warning when iterating over hashmaps

r? rust-lang/wg-incr-comp

…ietroalbini

Skip documentation for tier 2 targets on dist-x86_64-apple-darwin

I don't have an easy way to test this locally, but I believe it should work. Based on one log result should shave ~14 minutes off the dist-x86_64-apple builder (doesn't help with aarch64 dist or x86_64 test builder, so not actually decreasing total CI time most likely).

r? ```@pietroalbini```

…mez,camelid

Fix alignment of method headings for scannability

We sometimes use indentation to indicate something is a heading: The section that comes after is indented by 24px relative to the heading. However, the relationship between the "Implementations" section heading, the `impl` headings it contains, and the `pub fn` subheadings within each impl, is awkward. It goes **Implementations**, 15px indent, `impl`, 5px indent, `pub fn`, 4px indent, docblock.

I line up `impl` and `pub fn` with the `Implementations` heading, give `impl` a larger font size to indicate it is higher in the hierarchy, and indent the docblock a full 24px relative to their parent, matching the indents we use elsewhere to distinguish section headings. By letting the `pub fn` stick out to the left of the docblock, I think this makes methods significantly more scannable.

Related to rust-lang#59829

r? `````@camelid`````

[Old](https://doc.rust-lang.org/nightly/std/string/struct.String.html#implementations):

[![image](https://user-images.githubusercontent.com/220205/138380233-9c63a0f2-0f80-40a3-ab3d-a1ee9fb7c5d8.png)](https://doc.rust-lang.org/nightly/std/string/struct.String.html#implementations)

[New](https://jacob.hoffman-andrews.com/rust/outdent-methods/std/string/struct.String.html#implementations):

[![image](https://user-images.githubusercontent.com/220205/138392479-b45fce3f-bf43-42e0-81ee-c4bb9ac35cda.png)](https://jacob.hoffman-andrews.com/rust/outdent-methods/std/string/struct.String.html#implementations)

…_ref_mut, r=oli-obk

Mark `{array, slice}::{from_ref, from_mut}` as const fn

This PR marks the following APIs as `const`:
```rust
// core::array
pub const fn from_ref<T>(s: &T) -> &[T; 1];
pub const fn from_mut<T>(s: &mut T) -> &mut [T; 1];

// core::slice
pub const fn from_ref<T>(s: &T) -> &[T];
pub const fn from_mut<T>(s: &mut T) -> &mut [T];
```

Note that `from_ref` methods require `const_raw_ptr_deref` feature (which seems totally fine, since it's being stabilized, see rust-lang#89551), `from_mut` methods require `const_mut_refs` (which seems fine too since this PR marks `from_mut` functions as const unstable).

r? ````@oli-obk````

Fix ICE when forgetting to `Box` a parameter to a `Self::func` call

Closes rust-lang#90213 .

Assuming we can get the `DefId` of the receiver causes an ICE if the receiver is `Self`. We can just avoid doing this though.

…rflow-checks, r=Mark-Simulacrum

Temporarily turn overflow checks off for rustc-rayon-core

The rustc fork of Rayon has deadlock detection code which intermittently causes overflows in the CI (see rust-lang#90227). So, as a workaround, we unconditionally turn overflow checks off for this crate only.

This workaround should be removed once rust-lang#90227 is fixed.

r? `@Mark-Simulacrum`

cc `@matthiaskrgr`