doc test "Couldn't compile the test" on aarch64 + LTO

[pnkfelix]

Update 2021-12-27: More minimized MCVE at repo here (also linked from comment below.)

---

I tried this code (note that the crate name needs to match its usage in the doc test to reproduce the bug properly):

# Cargo.toml

[package]
name = "a64_doctestfail"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
tokio = { version = "0.2", features = ["full"] }

[profile.release]
lto = true

// lib.rs

pub fn bad() {
    let _ = tokio::runtime::Builder::new();
}

/// ```
/// use a64_doctestfail;
/// ```
pub struct X;

And then ran cargo --release --doc.

I expected to see this happen:

(Working behavior from 1.56.1, witnessed via nightly-2021-10-13)

% cargo +nightly-2021-10-13 test --release --doc
   Compiling a64_doctestfail v0.1.0 (/Users/pnkfelix/Dev/Rust/a64_doctestfail)
    Finished release [optimized] target(s) in 0.09s
   Doc-tests a64_doctestfail

running 1 test
test src/lib.rs - X (line 7) ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.19s

Instead, this happened:

(Broken behavior from 1.57.0, witnessed via nightly-2021-10-14)

% cargo +nightly-2021-10-14 test --release --doc
    Finished release [optimized] target(s) in 0.01s
   Doc-tests a64_doctestfail

running 1 test
test src/lib.rs - X (line 7) ... FAILED

failures:

---- src/lib.rs - X (line 7) stdout ----
Couldn't compile the test.

failures:
    src/lib.rs - X (line 7)

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.68s

error: test failed, to rerun pass '--doc'

Meta

rustc +nightly-2021-10-14 --version --verbose:
rustc 1.57.0-nightly (dfc5add 2021-10-13)
binary: rustc
commit-hash: dfc5add
commit-date: 2021-10-13
host: aarch64-apple-darwin
release: 1.57.0-nightly
LLVM version: 13.0.0

`rustc +stable --version --verbose`:

rustc 1.57.0 (f1edd04 2021-11-29)
binary: rustc
commit-hash: f1edd04
commit-date: 2021-11-29
host: aarch64-apple-darwin
release: 1.57.0
LLVM version: 13.0.0

[pnkfelix]

(by the way, the reason that I didn't include cargo-bisect-rustc output above is that the tool was not working for me today for this target; some kind of failure in its attempts to download the pre-built binaries per-commit. I haven't looked in more detail into it; for all I know, maybe we don't keep per-commit builds for aarch64...)

[pnkfelix]

Here are the relevant commits over the period where this bug was injected:

% git log --author=bors d7c97a02d..dfc5add91 --format=oneline:

• dfc5add Auto merge of Remove textual span from diagnostic string #89555 - oli-obk:nll_member_constraint_diag, r=estebank
• eeb16a2 Auto merge of Deduplicate regions ids before merging them #89822 - tmiasko:overflap-duplicates, r=cjgillot
• 81117ff Auto merge of Rollup of 12 pull requests #89847 - JohnTitor:rollup-xfymeo4, r=JohnTitor
• 5728bd6 Auto merge of Include rmeta candidates in "multiple matching crates" error #89587 - camelid:all-candidates, r=petrochenkov
• a16f686 Auto merge of Update cargo #89802 - ehuss:update-cargo, r=ehuss
• ef4b306 Auto merge of inline next() on &mut Iterator impl #89774 - the8472:inline-mut-iter-next, r=m-ou-se

[pnkfelix]

(it would be good to further minimize this. if possible I'd like to rewrite it as a normal test rather than a rustdoc one, if possible.)

[jyn514]

@pnkfelix can you run cargo test --release --doc -- --nocapture and post the output? I think rustdoc is hiding the actual compile error.

[jyn514]

It seems strange this only happens with doctests - does it also happen if you add a second crate that depends on a64_doctestfail (without a doctest)?

[hkratz]

Bisecting worked fine for me. The regression pinpointed to the cargo update #89802

bisected with cargo-bisect-rustc v0.6.1

searched toolchains d7c97a0 through dfc5add

---

Regression in a16f686

---

searched nightlies: from nightly-2021-10-13 to nightly-2021-12-08
regressed nightly: nightly-2021-10-14
searched commit range: d7c97a0...dfc5add
regressed commit: a16f686

Host triple: aarch64-apple-darwin
Reproduce with:

cargo bisect-rustc 2021-10-13 -- test --release --doc -- --nocapture

[pnkfelix]

@pnkfelix can you run cargo test --release --doc -- --nocapture and post the output? I think rustdoc is hiding the actual compile error.

Adding --nocapture alone does not seem to help here. I think you are right that rustdoc is hiding the actual compiler error; but I also think that we need to go deeper to dig out the root cause. (I do wish that --nocapture did resolve it, or some better still, some set of --verbose flags...)

[pnkfelix]

I figured out how to use rustdoc --test-builder to feed in a wrapper script around rustc that also emits the arguments that were passed into rustc. That should hopefully be enough for me to narrow this down to a bug that doesn't rely on rustdoc for us to replicate it.