compiletest: implied needs-target-std directive for codegen mode tests by default

A `codegen-llvm` test (and other codegen test mode tests) will now by
default have an implied `//@ needs-target-std` directive, *unless* the
test explicitly has an `#![no_std]`/`#![no_core]` attribute which
disables this implied behavior.

- When a test has both `#![no_std]`/`#![no_core]` and `//@
  needs-target-std`, the explicit `//@ needs-target-std` directive will
  cause the test to be ignored for targets that do not support std
  still.

This is to make it easier to test out-of-tree targets / custom targets
(and targets not tested in r-l/r CI) without requiring target
maintainers to do a bunch of manual `//@ needs-target-std` busywork.

Co-authored-by: Edoardo Marangoni <ecmm@anche.no>

Author: jieyouxu

src/tests/compiletest.md

@@ -311,14 +311,20 @@ For example, `./x test tests/debuginfo -- --debugger gdb` will only test GDB com
 
 ### Codegen tests
 
-The tests in [`tests/codegen-llvm`] test LLVM code generation. They compile the test
-with the `--emit=llvm-ir` flag to emit LLVM IR. They then run the LLVM
+The tests in [`tests/codegen-llvm`] test LLVM code generation. They compile the
+test with the `--emit=llvm-ir` flag to emit LLVM IR. They then run the LLVM
 [FileCheck] tool. The test is annotated with various `// CHECK` comments to
 check the generated code. See the [FileCheck] documentation for a tutorial and
 more information.
 
 See also the [assembly tests](#assembly-tests) for a similar set of tests.
 
+By default, codegen tests will have `//@ needs-target-std` *implied* (that the
+target needs to support std), *unless* the `#![no_std]`/`#![no_core]` attribute
+was specified in the test source. You can override this behavior and explicitly
+write `//@ needs-target-std` to only run the test when target supports std, even
+if the test is `#![no_std]`/`#![no_core]`.
+
 If you need to work with `#![no_std]` cross-compiling tests, consult the
 [`minicore` test auxiliary](./minicore.md) chapter.
 

src/tests/directives.md

@@ -200,6 +200,8 @@ The following directives will check rustc build settings and target settings:
   on `wasm32-unknown-unknown` target because the target does not support the
   `proc-macro` crate type.
 - `needs-target-std` — ignores if target platform does not have std support.
+  - See also [`#![no_std]`/`#![no_core]` and implied `needs-target-std` for
+    codegen tests](./compiletest.md#codegen-tests).
 - `ignore-backends` — ignores the listed backends, separated by whitespace characters.
   Please note
   that this directive can be overriden with the `--bypass-ignore-backends=[BACKEND]` command line