Target feature unavailable only in doctest

[DaniPopes]

Using std::arch::is_<arch>_feature_detected!("feature") + #[target_feature = "feature"] doesn't work in doctests:

/// ```
/// let result = miri_doctest::add(2, 2);
/// assert_eq!(result, 4);
/// ```
pub fn add(left: u64, right: u64) -> u64 {
    if !std::arch::is_x86_feature_detected!("ssse3") {
        panic!("SSSE3 is not supported");
    }
    unsafe { add_with_ssse3(left, right) }
}

#[target_feature(enable = "ssse3")]
pub unsafe fn add_with_ssse3(left: u64, right: u64) -> u64 {
    left + right
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn it_works() {
        let result = add(2, 2);
        assert_eq!(result, 4);
    }
}

The #[test] it_works works fine, but the add doctest doesn't; looks like the is_feature_detected call returns true but Miri still errors out:

$ RUSTFLAGS=-Ctarget-feature=+ssse3 cargo miri test --target x86_64-unknown-linux-gnu
   Compiling miri-doctest v0.1.0 (/tmp/miri-doctest)
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.08s
     Running unittests src/lib.rs (target/miri/x86_64-unknown-linux-gnu/debug/deps/miri_doctest-0d5da2e0254807af)

running 1 test
test tests::it_works ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.17s

   Doc-tests miri_doctest

running 1 test
test src/lib.rs - add (line 1) ... FAILED

failures:

---- src/lib.rs - add (line 1) stdout ----
Test executable failed (exit status: 1).

stderr:
error: Undefined Behavior: calling a function that requires unavailable target features: ssse3
 --> /tmp/miri-doctest/src/lib.rs:9:14
  |
9 |     unsafe { add_with_ssse3(left, right) }
  |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^ calling a function that requires unavailable target features: ssse3
  |
  = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
  = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
  = note: BACKTRACE:
  = note: inside `miri_doctest::add` at /tmp/miri-doctest/src/lib.rs:9:14: 9:41
note: inside `main::_doctest_main_src_lib_rs_1_0`
 --> src/lib.rs:3:14
  |
5 | let result = miri_doctest::add(2, 2);
  |              ^^^^^^^^^^^^^^^^^^^^^^^
note: inside `main`
 --> src/lib.rs:5:3
  |
7 | } _doctest_main_src_lib_rs_1_0() }
  |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to 1 previous error




failures:
    src/lib.rs - add (line 1)

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.08s

error: doctest failed, to rerun pass `--doc`

[saethlin]

This is a rustdoc footgun, not a Miri issue. RUSTFLAGS are not passed to rustdoc, you need to set RUSTDOCFLAGS as well. For example: https://github.com/saethlin/crater-at-home/blob/e8145d70bb1a3243d882887b15910a45f4940dec/docker/run.sh#L37

I do not know why rustdoc behaves like this, but I suspect there was a good reason for it at some point. If you figure out the reason, don't hesitate to give a shout on this issue, it's probably buried in the rust-lang/rust issue tracker.

[RalfJung]

Also see #3869, of which this is a duplicate.