Improve runtest revision redundant cfg check

[Hoverbear]

While attempting to ingest #148034 via ferrocene/ferrocene#2172 we noticed a test failure, as we add some compile_flags to tests.

We saw a failure looking like this:

Testing stage1 with compiletest suite=mir-opt mode=mir-opt (x86_64-unknown-linux-gnu)

running 2 tests
2026-02-20T20:21:28.846102Z ERROR compiletest::runtest: redundant cfg argument `copy` is already created by the revision

[mir-opt] tests/mir-opt/pre-codegen/copy_and_clone.rs#COPY ... F
.

While my Rust checkout passed:

Testing stage1 with compiletest suite=mir-opt mode=mir-opt (x86_64-unknown-linux-gnu)

running 2 tests
..

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 381 filtered out; finished in 107.10ms

This caused me to add some debugging statements.

Ferrocene:

2026-02-20T21:05:39.808427Z ERROR compiletest::runtest: "copy": does ["--edition=2015", "--cfg=copy"] contain `"--cfg=copy"` or `["--cfg", "copy"]`? true
2026-02-20T21:05:39.808427Z ERROR compiletest::runtest: "clone": does ["--edition=2015"] contain `"--cfg=clone"` or `["--cfg", "clone"]`? false
2026-02-20T21:05:39.808435Z ERROR compiletest::runtest: redundant cfg argument `copy` is already created by the revision

Rust:

2026-02-20T21:04:18.493158Z ERROR compiletest::runtest: "copy": does ["--cfg=copy"] contain `"--cfg=copy"` or `["--cfg", "copy"]`? false
2026-02-20T21:04:18.493158Z ERROR compiletest::runtest: "clone": does [] contain `"--cfg=clone"` or `["--cfg", "clone"]`? false

I noticed while reviewing the related functionality that there is a call to .windows(2) in the relevant check for redundant cfgs:

rust/src/tools/compiletest/src/runtest.rs

Lines 507 to 511 in 0376d43

	if self
	.props
	.compile_flags
	.windows(2)
	.any(|args| args == cfg_arg || args[0] == arg || args[1] == arg)

Noting:

rust/library/core/src/slice/mod.rs

Lines 1064 to 1066 in 0376d43

	/// Returns an iterator over all contiguous windows of length
	/// `size`. The windows overlap. If the slice is shorter than
	/// `size`, the iterator returns no values.

Because of this, the revision check was getting an empty iterator when self.props.compile_flags was length 0 or 1.

This fix adjusts the check to handle such cases.

I went ahead and fixed the relevant test (4b3cd9b) that was impacted by this. I do not suspect there are others, at least within the scope that Ferrocene tests, as we have not previously seen this failure.

[rustbot]

Some changes occurred in src/tools/compiletest

cc @jieyouxu

[rustbot]

r? @clubby789

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[clubby789]

A quick comment about the reason for the two iterations might be useful to avoid needing to remember/lookup the exact semantics of windows(), up to you. LGTM otherwise, r=me if green

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[Hoverbear]

@clubby789 I'm not someone who can merge so someone else, like you, needs to do the honors!

[jieyouxu]

Thanks
@bors r=clubby789,jieyouxu rollup

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[rust-bors]

📌 Commit 3c9ca9b has been approved by clubby789,jieyouxu

It is now in the queue for this repository.