For larger applications it's important that users set `RUST_MIN_STACK`
at the start of their program because `min_stack` caches the value.
Not doing so can lead to their `env::set_var` call surprisingly not having any effect.

this test was added for rust 0.4 and doesn't test anything specific.
The repro originally relied on extern functions which are now just
ordinary methods. It's also a run pass test even though `main` has
been commented out.

This is necessary for the subsequent commits.

…macro

…s such

Re-phrase in a way that handles input values being either 0 or
negative.

…rename usages of bound_impl_subject to impl_subject

This patch updates the error reporting when Safe Transmute is not
possible between 2 types by including the reason.

Also, fix some small bugs that occur when computing the `Answer` for
transmutability.

Clarify that RUST_MIN_STACK may be internally cached

For larger applications it's important that users set `RUST_MIN_STACK` at the start of their program because [`min_stack`](https://github.com/rust-lang/rust/blob/7d3e03666a93bd2b0f78b3933f9305832af771a5/library/std/src/sys_common/thread.rs) caches the value. Not doing so can lead to their `env::set_var` call surprisingly not having any effect.

In my own testing `RUST_MIN_STACK` had no effect until I moved it to the top of `main()`. Hopefully this clarification in the docs will help others going forward.

…-errors, r=compiler-errors

Improve safe transmute error reporting

This patch updates the error reporting when Safe Transmute is not possible between 2 types by including the reason.

Also, fix some small bugs that occur when computing the `Answer` for transmutability.

Remove obsolete test case

This test case was supposed to cover issue rust-lang#31109 at some point.
It never did anything, as the issue was still open at the time of its creation.
When the issue was resolved, the `issue31109` test case was created,
making the existence of this test pointless.

don't uniquify regions when canonicalizing

uniquifying causes a bunch of issues, most notably it causes `AliasEq(<?x as Trait<'a>>::Assoc, <?x as Trait<'a>>::Assoc)` to result in ambiguity because both `normalizes-to` paths result in ambiguity and substs equate should trivially succeed but doesn't because we uniquified `'a` to two different regions.

I originally added uniquification to make it easier to deal with requirement 6 from the dev-guide: https://rustc-dev-guide.rust-lang.org/solve/trait-solving.html#requirements

> ### 6. Trait solving must be (free) lifetime agnostic
>
> Trait solving during codegen should have the same result as during typeck. As we erase
> all free regions during codegen we must not rely on them during typeck. A noteworthy example
> is special behavior for `'static`.

cc rust-lang/rustc-dev-guide#1671

Relying on regions being identical may cause ICE during MIR typeck, but even without this PR we can end up relying on that as type inference vars can resolve to types which contain an identical region. Let's land this and deal with any ICE that crop up as we go. Will look at this issue again before stabilization.

r? ```@compiler-errors```

…r=lcnr

Assemble `Unpin` candidates specially for generators in new solver

Fixes compiler-errors/next-solver-hir-issues#16

r? ``@lcnr``

…h726

Remove all but one of the spans in `BoundRegionKind::BrAnon`

There are only three places where `BoundRegionKind::BrAnon` uses `Some(span)` instead of `None`. Two of them are easy to remove, which this PR does.

r? ```@jackh726```

…ve, r=notriddle

rustdoc: Correctly handle built-in compiler proc-macros as proc-macro and not macro

Part of rust-lang#110111.

There were actually one issue split in two parts:
 * Compiler built-in proc-macro were incorrectly considered as macros and not proc-macros.
 * Re-exports of compiler built-in proc-macros were considering them as macros.

Both issues can be fixed by looking at the `MacroKind` variant instead of just relying on information extracted later on.

r? ``@fmease``

…ses, r=thomcc

Cover edge cases for {f32, f64}.hypot() docs

Fixes rust-lang#88944

The Euclidean distance is a more general way to express what these functions do, and covers the edge cases of zero and negative inputs.

Does not cover the case of non-normal input values (as the [POSIX docs](https://pubs.opengroup.org/onlinepubs/9699919799.2008edition/) do), but the docs for the rest of the functions in these modules do not address this, I assumed it was not desired.

…ct, r=compiler-errors

Switch to `EarlyBinder` for `impl_subject` query

Part of the work to finish rust-lang#105779.

Several queries `X` have a `bound_X` variant that wraps the output in `EarlyBinder`. This adds `EarlyBinder` to the return type of the `impl_subject` query and removes `bound_impl_subject`.

r? ```@lcnr```