Improve documentation when adding a new target
https://github.com/rust-lang/rust/pull/133631#issuecomment-2607877936 shows that it can be a bit difficult process-wise to add a new target.
I've added a bit of text to the docs, suggesting that users add the target defintion/spec first, and later work on `std` support.
I also found that we have two places where we document how to add a new target. I've linked these for now, but they should probably be merged somehow in the future.
`@rustbot` label A-docs
r? compiler
CC `@workingjubilee` who's worked a lot on target specs IIRC.
Simplify and consolidate the way we handle construct `OutlivesEnvironment` for lexical region resolution
This is best reviewed commit-by-commit. I tried to consolidate the API for lexical region resolution *first*, then change the API when it was finally behind a single surface.
r? lcnr or reassign
It seems one can't overwrite a cache entry:
```
Failed to save: Unable to reserve cache with key linkcheck--0.8.1, another job may be creating this cache. More details: Cache already exists. Scope: refs/heads/master, Key: linkcheck--0.8.1, Version: 33f8fd511bed9c91c40778bc5c27cb58425caa894ab50f9c5705d83cb78660e0
Warning: Cache save failed.
```
A proper solution is to use `restore-keys`:
https://github.com/actions/cache/blob/main/tips-and-workarounds.md#update-a-cache
Tidy Python improvements
Fixes display of Python formatting diffs in tidy, and refactors the code to make it simpler and more robust. Also documents Python formatting and linting in the Rustc dev guide.
Fixes: https://github.com/rust-lang/rust/issues/135942
r? `@onur-ozkan`
Implement `needs-subprocess` directive, and cleanup a bunch of tests to use `needs-{subprocess,threads}`
### Summary
Closes#128295.
- Implements `//@ needs-subprocess` directive in compiletest as requested in #128295. However, compiletest is a host tool, so we can't just try to spawn process because that spawns the process on *host*, not the *target*, under cross-compilation scenarios.
- The short-term solution is to add *Yet Another* list of allow-list targets.
- The long-term solution is to first check if a `$target` supports std, then try to run a binary to do run-time capability detection *on the target*. But that is tricky because you have to build-and-run a binary *for the target*.
- This PR picks the short-term solution, because the long-term solution is highly non-trivial, and it's already an improvement over individual `ignore-*`s all over the place.
- Opened an issue about the long-term solution in #135928.
- Documents `//@ needs-subprocess` in rustc-dev-guide.
- Replace `ignore-{wasm,wasm32,emscripten,sgx}` with `needs-{subprocess,threads}` where suitable in tests.
- Some drive-by test changes as I was trying to figure out if I could use `needs-{subprocess,threads}` and found some bits needlessly distracting.
Count of tests that use `ignore-{wasm,wasm32,emscripten,sgx}` before and after this PR:
| State | `ignore-sgx` | `ignore-wasm` | `ignore-emscripten` |
| - | - | - | - |
| Before this PR | 96 | 88 | 207 |
| After this PR | 36 | 38 | 61 |
<details>
<summary>Commands used to find out locally</summary>
```
--- before
[17:40] Joe:rust (fresh) | rg --no-ignore -l "ignore-sgx" tests | wc -l
96
[17:40] Joe:rust (fresh) | rg --no-ignore -l "ignore-wasm" tests | wc -l
88
[17:40] Joe:rust (fresh) | rg --no-ignore -l "ignore-emscripten" tests | wc -l
207
--- after
[17:39] Joe:rust (needs-subprocess-thread) | rg --no-ignore -l "ignore-sgx" tests | wc -l
36
[17:39] Joe:rust (needs-subprocess-thread) | rg --no-ignore -l "ignore-wasm" tests | wc -l
38
[17:39] Joe:rust (needs-subprocess-thread) | rg --no-ignore -l "ignore-emscripten" tests | wc -l
61
```
</details>
### Review advice
- Best reviewed commit-by-commit.
- Non-trivial test changes (not mechanically simple replacements) are split into individual commits to help with review. Their individual commit messages give some basic description of the changes.
- I *could* split some test changes out into another PR, but I found that I needed to change some tests to `needs-threads`, some to `needs-subprocess`, and some needed to use *both*, so they might conflict and become very annoying.
---
r? ``@ghost`` (need to run try jobs)
try-job: x86_64-msvc-1
try-job: i686-msvc-1
try-job: i686-mingw
try-job: x86_64-mingw-1
try-job: x86_64-apple-1
try-job: aarch64-apple
try-job: aarch64-gnu
try-job: test-various
try-job: armhf-gnu
