Offer to close TODO: parse rustflags

[MusicalNinjaDad]

in

trybuild/src/rustflags.rs

Line 5 in 69689b3

pub(crate) fn toml(extra_rustflags: &[&'static str]) -> toml::Value {

you have

trybuild/src/rustflags.rs

Lines 13 to 16 in 69689b3

	if let Some(flags) = env::var_os("RUSTFLAGS") {
	// TODO: could parse this properly and allowlist or blocklist certain
	// flags. This is good enough to at least support cargo-llvm-cov.
	if flags.to_string_lossy().contains("-C instrument-coverage") {

I now have a (very specific) use case where additional rustflags are required.

Suggestion & PR offer

I would be happy to offer to draft a PR (manually, not via AI) to offer something like the following API (reading from the env would clearly lead to flaky tests):

#[test]
fn specific_rustflags() {
    let t = trybuild::TestCases::with_rustflags(vec!["-Zallow-features=", "--cfg", "some_cfg"]);
    t.compile_fail("tests/some_cfg/*.rs");
}

Background

My case is the following - I'm sure there will be others who need rustflags for similar or very different reasons:

• I have a created wrapper around proc_macro::Diagnostic for proc_macro2.
• I provide a consistent API for all usage, regardless of which channel the final compilation occurs on.
• I also use additional experimental features (from the try_trait_v2 family) where available.
• The API is consistent, regardless of which channel is used for the final compilation. I have fall-back to (acceptable, non-breaking & clearly documented) reduced functionality where certain features are not available.
• I would like to consistently test (not just manually) the outcomes in the following situation:
  • current stable & beta: no diagnostics & no try_trait_v2 ✅ (I can run CI on various toolchains & cfg-gate the path passed to trybuild)
  • current nightly: diagnostics & try_trait_v2 ✅ (I can run CI on various toolchains & cfg-gate the path passed to trybuild)
  • possible future 1: diagnostics but no try_trait_v2 ❌ (can only test manually, I have no way to construct this situation in trybuild)
  • possible future 2: try_trait_v2 but no diagnostics ❌ (can only test manually, I have no way to construct this situation in trybuild)

[MusicalNinjaDad]

alternative approach that would also support cases from #283 for which CARGO_ENCODED_RUSTFLAGS does not work / is not suitable