rustdoc: doctest-xcompile passes when it shouldn't

[bugadani]

In the following example, both doctests pass, even though they are identical except for the annotation (one of them is no_run, the other compile_fail).

Command to run: cargo test --doc -Zdoctest-xcompile -Zbuild-std=core,panic_abort --target riscv32imac-unknown-none-elf

[package]
name = "rustdoc-issue"
version = "0.1.0"
edition = "2024"

[dependencies]
esp-hal = { version = "1.0.0-beta.0", features = ["esp32c6", "unstable"] }

#![no_std]

#[macro_export]
macro_rules! pre {
    () => {
        r#"
        # #![no_std]
        # #![no_main]
        #
        # use esp_hal as _;
        #
        # #[panic_handler]
        # fn panic(_ : &core::panic::PanicInfo) -> ! {
        #     loop {}
        # }

        # fn example() {
        "#
    };
}

/// ```rust,compile_fail
#[doc = crate::pre!()]
/// let mut x = 5;
/// x += 2;
/// }
/// ```
fn _foo() {}

/// ```rust,no_run
#[doc = crate::pre!()]
/// let mut x = 5;
/// x += 2;
/// }
/// ```
fn _foo2() {}

Since the no_run test case compiles, I would expect the compile_fail example to fail. I can only trigger this behaviour if I reference esp_hal - I'm not sure if other third party dependencies would trigger it. Without the external dependency, the annotation works as expected.

rustc --version
rustc 1.88.0-nightly (092a284ba 2025-04-13)

[ehuss]

I believe the issue here is that no_run does not actually compile or link the test. It does the equivalent of cargo check, which works for that example. The compile_fail one does the full build/linking, but the linking fails, and thus it passes.

(This is mostly unrelated to cross-compiling, any post-analysis error will cause this.)

There is more discussion of this in #49514.

[bugadani]

Ah great, so what I really need is a check_fail annotation :/