Rollup merge of #156901 - Zalathar:needs-asm-ret, r=folkertdev

compiletest: Simplify `//@ needs-asm-mnemonic: ret` to just `//@ needs-asm-ret`

- Simplification of rust-lang/rust#155692.
---

The `needs-asm-mnemonic` directive was very general, but in practice was only being used for `ret`. There are very few other mnemonics that it could plausibly be useful for (e.g. `nop`), because any instruction that requires arguments is probably going to be non-portable.

This PR replaces `needs-asm-mnemonic` with a simpler `needs-asm-ret` directive that uses the same machinery as other simple needs directives.

If we happend to need more mnemonics in the future, we can just add more simple directives as appropriate (e.g. `needs-asm-nop`).

Author: JonathanBrouwer

src/tests/compiletest.md

@@ -346,17 +346,6 @@ See also the [codegen tests](#codegen-tests) for a similar set of tests.
 If you need to work with `#![no_std]` cross-compiling tests, consult the
 [`minicore` test auxiliary](./minicore.md) chapter.
 
-#### Conditional assembly tests based on instruction support
-
-Tests that depend on specific assembly instructions being available can use the
-`//@ needs-asm-mnemonic: <MNEMONIC>` directive.
-This will skip the test if the target backend does not support the specified instruction mnemonic.
-
-For example, a test that requires the `RET` instruction:
-```rust,ignore
-//@ needs-asm-mnemonic: RET
-```
-
 [`tests/assembly-llvm`]: https://github.com/rust-lang/rust/tree/HEAD/tests/assembly-llvm
 
 

src/tests/directives.md

@@ -164,9 +164,10 @@ The following directives will check rustc build settings and target settings:
   For tests that cross-compile to explicit targets
   via `--target`, use `needs-llvm-components` instead to ensure the appropriate
   backend is available.
-- `needs-asm-mnemonic: <MNEMONIC>` — ignores if the target backend does not
-  support the specified assembly mnemonic (e.g., `RET`, `NOP`).
-  Only supported with the LLVM backend.
+- `needs-asm-ret` - ignores if the target does not have a `ret` instruction
+  in its assembly syntax. Most target architectures have this instruction,
+  making it handy for portable inline-assembly tests, but some architectures
+  (e.g. 32-bit ARM) do not have it.
 - `needs-profiler-runtime` — ignores the test if the profiler runtime was not
   enabled for the target (`build.profiler = true` in `bootstrap.toml`)
 - `needs-sanitizer-support` — ignores if the sanitizer support was not enabled