Rollup merge of rust-lang#139967 - jieyouxu:auxiliary, r=wesleywiser

Introduce and use specialized `//@ ignore-auxiliary` for test support files instead of using `//@ ignore-test`

### Summary

Add a semantically meaningful directive for ignoring test *auxiliary* files. This is for auxiliary files that *participate* in actual tests but should not be built by `compiletest` (i.e. these files are involved through `mod xxx;` or `include!()` or `#[path = "xxx"]`, etc.).

### Motivation

A specialized directive like `//@ ignore-auxiliary` makes it way easier to audit disabled tests via `//@ ignore-test`.
  - These support files cannot use the canonical `auxiliary/` dir because they participate in module resolution or are included, or their relative paths can be important for test intention otherwise.

Follow-up to:
- rust-lang#139705
- rust-lang#139783
- rust-lang#139740

See also discussions in:

- [#t-compiler > Directive name for non-test aux files?](https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Directive.20name.20for.20non-test.20aux.20files.3F/with/512773817)
- [#t-compiler > Handling disabled &rust-lang#96;//@ ignore-test&rust-lang#96; tests](https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Handling.20disabled.20.60.2F.2F.40.20ignore-test.60.20tests/with/512005974)
- [#t-compiler/meetings > &rust-lang#91;steering&rust-lang#93; 2025-04-11 Dealing with disabled tests](https://rust-lang.zulipchat.com/#narrow/channel/238009-t-compiler.2Fmeetings/topic/.5Bsteering.5D.202025-04-11.20Dealing.20with.20disabled.20tests/with/511717981)

### Remarks on remaining unconditionally disabled tests under `tests/`

After this PR, against commit 79a272c, only **14** remaining test files are disabled through `//@ ignore-test`:

<details>
<summary>Remaining `//@ ignore-test` files under `tests/`</summary>

```
tests/debuginfo/drop-locations.rs
4://@ ignore-test (broken, see rust-lang#128971)

tests/rustdoc/macro-document-private-duplicate.rs
1://@ ignore-test (fails spuriously, see issue rust-lang#89228)

tests/rustdoc/inline_cross/assoc-const-equality.rs
3://@ ignore-test (FIXME: rust-lang#125092)

tests/ui/match/issue-27021.rs
7://@ ignore-test (rust-lang#54987)

tests/ui/match/issue-26996.rs
7://@ ignore-test (rust-lang#54987)

tests/ui/issues/issue-49298.rs
9://@ ignore-test (rust-lang#54987)

tests/ui/issues/issue-59756.rs
2://@ ignore-test (rustfix needs multiple suggestions)

tests/ui/precondition-checks/write.rs
5://@ ignore-test (unimplemented)

tests/ui/precondition-checks/read.rs
5://@ ignore-test (unimplemented)

tests/ui/precondition-checks/write_bytes.rs
5://@ ignore-test (unimplemented)

tests/ui/explicit-tail-calls/drop-order.rs
2://@ ignore-test: tail calls are not implemented in rustc_codegen_ssa yet, so this causes 🧊

tests/ui/panics/panic-short-backtrace-windows-x86_64.rs
3://@ ignore-test (rust-lang#92000)

tests/ui/json/json-bom-plus-crlf-multifile-aux.rs
3://@ ignore-test Not a test. Used by other tests

tests/ui/traits/next-solver/object-soundness-requires-generalization.rs
2://@ ignore-test (see rust-lang#114196)
```
</details>

Of these, most are either **unimplemented**, or **spurious**, or **known-broken**. The outstanding one is `tests/ui/json/json-bom-plus-crlf-multifile-aux.rs` which I did not want to touch in *this* PR -- that aux file has load-bearing BOM and carriage returns and byte offset matters. I think those test files that require special encoding / BOM probably are better off as `run-make` tests. See rust-lang#139968 for that aux file.

### Review advice

- Best reviewed commit-by-commit.
- The directive name diverged from the most voted `//@ auxiliary` because I think that's easy to confuse with `//@ aux-{crate,dir}`.

r? compiler

Author: matthiaskrgr

src/doc/rustc-dev-guide/src/tests/best-practices.md

@@ -44,6 +44,7 @@ const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "ignore-arm-unknown-linux-gnueabihf",
     "ignore-arm-unknown-linux-musleabi",
     "ignore-arm-unknown-linux-musleabihf",
+    "ignore-auxiliary",
     "ignore-avr",
     "ignore-beta",
     "ignore-cdb",

src/doc/rustc-dev-guide/src/tests/directives.md

@@ -100,6 +100,10 @@ fn parse_cfg_name_directive<'a>(
         name: "test",
         message: "always"
     }
+    condition! {
+        name: "auxiliary",
+        message: "used by another main test file"
+    }
     condition! {
         name: &config.target,
         allowed_names: &target_cfgs.all_targets,

src/tools/compiletest/src/directive-list.rs

@@ -940,3 +940,9 @@ fn test_supported_crate_types() {
         "//@ needs-crate-type: bin, cdylib, dylib, lib, proc-macro, rlib, staticlib"
     ));
 }
+
+#[test]
+fn test_ignore_auxiliary() {
+    let config = cfg().build();
+    assert!(check_ignore(&config, "//@ ignore-auxiliary"));
+}

src/tools/compiletest/src/header/cfg.rs

@@ -1,4 +1,4 @@
-//@ ignore-test: this is not a test
+//@ ignore-auxiliary (used by `./main.rs`)
 
 #[inline]
 pub fn some_aux_mod_function() -> i32 {

src/tools/compiletest/src/header/tests.rs

@@ -1,5 +1,7 @@
 //@ ignore-android
-//@ ignore-test: #128971
+
+// FIXME: stepping with "next" in a debugger skips past end-of-scope drops
+//@ ignore-test (broken, see #128971)
 
 #![allow(unused)]
 

tests/codegen/remap_path_prefix/aux_mod.rs

@@ -1,4 +1,4 @@
-//@ ignore-test (auxiliary, used by other tests)
+//@ ignore-auxiliary (used by `./move-error-snippets.rs`)
 
 macro_rules! aaa {
     ($c:ident) => {{

tests/debuginfo/drop-locations.rs

@@ -1,4 +1,4 @@
-//@ ignore-test (auxiliary, used by other tests)
+//@ ignore-auxiliary (used by `./two_files.rs`)
 
 trait Foo { }
 

tests/ui/borrowck/move-error-snippets-ext.rs

@@ -1,3 +1,3 @@
-//@ ignore-test (auxiliary, used by other tests)
+//@ ignore-auxiliary (used by `./inner-cfg-non-inline-mod.rs`)
 
 #![cfg_attr(all(), cfg(false))]