📎 [Internal/DX] Inconsistent Windows path handling in tsconfig test suite

[anarefolio]

Description

Description

While exploring the codebase on Windows, I noticed some internal
inconsistency regarding how paths are handled in the tsconfig.json
test suite.

Currently, the suite relies on Unix-style absolute paths (e.g.,
/test/tsconfig.json). On Windows, these are not recognized as
absolute, leading to "brittle" tests that require manual workarounds
to pass.

Technical Debt Identified

I found several places where we are currently "patching" this behavior
rather than solving it systemically:

1. tsconfig_json.rs:89 — a debug assertion is disabled on Windows
because of the path style:

#[cfg(not(target_os = "windows"))]
debug_assert!(path.is_absolute());

2. manifest_spec_tests.rs — backslashes are manually replaced in
snapshot output:

snapshot_result.push_str(&format!("{tsconfig:#?}").replace("\\\\", "/"));

Concrete Example

I wrote a small test to confirm the behavior on Windows:

let (tsconfig, _) = TsConfigJson::parse(
    Utf8Path::new("/test/tsconfig.json"),
    json, // { "compilerOptions": { "baseUrl": "src" } }
);

Which fails with:
thread '...test_base_url_with_fake_unix_path_on_windows' panicked:
assertion failed: base_url resolved to '\test\src' instead of '/test/src'
left: "\test\src"
right: "/test/src"

Why This Matters

While real users are not affected (production code handles Windows paths
correctly), this infrastructure debt makes the test suite harder to
maintain. It creates a risk of masking real Windows-specific regressions
because we are accustomed to "working around" path issues in tests.

Since other parts of Biome already use MemoryFileSystem for OS-agnostic
testing, it might be worth aligning these tests with that pattern in
the future.

Just flagging this for the team's awareness! 😊

Environment

• OS: Windows 11, MINGW64
• Biome version: 5.0.0 (main branch)
• Rust: 1.95.0