rustdoc doctest UI suite shouldn't need to manually normalize $DIR

Summary

I wrote the following UI test:

//@ check-pass
//@ edition: 2018
//@ compile-flags: --test --test-args=--test-threads=1 --merge-doctests=yes -Z unstable-options
//@ normalize-stdout: "tests/rustdoc-ui" -> "$$DIR"
//@ normalize-stdout: "finished in \d+\.\d+s" -> "finished in $$TIME"
//@ normalize-stdout: "ran in \d+\.\d+s" -> "ran in $$TIME"
//@ normalize-stdout: "compilation took \d+\.\d+s" -> "compilation took $$TIME"
//@ normalize-stdout: ".rs:\d+:\d+" -> ".rs:$$LINE:$$COL"

// FIXME: compiletest doesn't support `// RAW` for doctests because the progress messages aren't
// emitted as JSON. Instead the .stderr file tests that this contains a
// "merged compilation took ..." message.

/// ```
/// let x = 12;
/// ```
///
/// These two doctests should be force-merged, even though this uses edition 2018.
///
/// ```
/// fn main() {
///     println!("owo");
/// }
/// ```
pub struct Foo;

In particular, note //@ normalize-stdout: "tests/rustdoc-ui" -> "$$DIR". That was cargo-culted from other tests in rustdoc.

This worked ok locally and generated a .stderr file with

test $DIR/doctest/force-merge.rs - Foo (line 14) ... ok
test $DIR/doctest/force-merge.rs - Foo (line 20) ... ok

It failed in CI because of normalization differences:

-	test $DIR/doctest/force-merge.rs - Foo (line 14) ... ok
-	test $DIR/doctest/force-merge.rs - Foo (line 20) ... ok
+	test $DIR/force-merge.rs - Foo (line 14) ... ok
+	test $DIR/force-merge.rs - Foo (line 20) ... ok

	Note: some mismatched output was normalized before being compared
-	test /checkout/tests/rustdoc-ui/doctest/force-merge.rs - Foo (line 14) ... ok
-	test /checkout/tests/rustdoc-ui/doctest/force-merge.rs - Foo (line 20) ... ok
+	test $DIR/force-merge.rs - Foo (line 14) ... ok
+	test $DIR/force-merge.rs - Foo (line 20) ... ok

That second note is really weird! Those are absolute paths. I don't see absolute paths when I run rustdoc locally, and I looked at the rustdoc invocation locally and CI and didn't notice any interesting differences. The only thing I can think of is something related to remap-path-prefix, but that job didn't set remap-path-prefix.

I think what happened is that the absolute path got normalized by compiletest, not by the test directive: https://github.com/rust-lang/rust/blob/ab196d7a7de1170ac2b376fb343f633d29d75685/src/tools/compiletest/src/runtest.rs#L2557-L2558

Command used

./x t --stage 1 tests/rustdoc-ui/doctest/force-merge.rs

Expected behaviour

Compiletest normalizes the path to the UI directory automatically.

Actual behaviour

Compiletest normalizes the path in CI, but not locally.

Bootstrap configuration (bootstrap.toml)

profile = "compiler"
change-id = 102579

[rust]
lld = true
deny-warnings = false

download-rustc = true
debug-assertions = false
verbose-tests = true

[build]
description = "\u001b[0;95mlove you love you love you\u001b[0m"

Operating system

macOS Sequoia 15.5 (24F74)

HEAD

ab196d7a7de1170ac2b376fb343f633d29d75685

Additional context

Extracted from https://github.com/rust-lang/rust/pull/149565#issuecomment-3608564732.

@rustbot label T-rustdoc A-testsuite A-doctests

Run ./x t --stage 1 tests/rustdoc-ui/doctest/force-merge.rs and inspect src/tools/compiletest/src/runtest.rs around lines 2557-2558. Compare local and CI normalization of the paths shown in the issue, then verify the test's .stderr output no longer needs the manual tests/rustdoc-ui-to-$DIR directive and is consistent in both environments.