Canonicalization: do not canonicalize units in arbitrary values

[RobinMalfait]

This PR improves the canonicalization when dealing with arbitrary values.

As part of the canonicalization process we compute a signature for a given utility. This way we can ensure that when we canonicalize a candidate into a simpler candidate that it's still equivalent if the signatures match.

One thing we do during signature computation is normalizing dimensions (value + unit) into the same unit to make comparisons easier.

For example:

.foo { margin-top: 20in; }
.bar { margin-top: 1920px; }

Will both get converted to 1920px and therefore foo and bar will have the same signature.

Up until this part, everything is fine. However, this normalization also leaks when we try to canonicalize arbitrary values. One of the things we do is try to move the - into the arbitrary value:

- -mt-[20in]
+ mt-[calc(20in_*_-1)]

This is obviously not cleaner, but we can perform some canonicalization of the arbitrary value. As part of that we do constant folding and the normalization of base units. That means that we would see this:

- -mt-[20in]
- mt-[calc(20in_*_-1)]
+ mt-[-1920px]

But this might be very confusing because it might not make sense where the 1920px even came from... the only thing we should have done here is constant fold that calc expression.

That's what this PR does, it only does the constant folding but without the unit normalization:

- -mt-[20in]
- mt-[calc(20in_*_-1)]
+ mt-[-20in]

Which is exactly what we want!

Fixes: tailwindlabs/tailwindcss-intellisense#1573

Test plan

1. Added a regression test based on the linked issue
2. All other tests still pass

[coderabbitai]

Caution

Review failed

The pull request is closed.

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• CHANGELOG.md

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Walkthrough

Adds regression tests covering arbitrary inset spacing values with inch units. Introduces a normalizeUnit parameter (default true) to constantFoldDeclaration and constantFoldDeclarationAst so unit canonicalization can be disabled; when false, only the existing zero-length (0<length>) handling runs and original unit strings are preserved. Updates canonicalize-candidates to call constant-folding with explicit parameters to control normalization.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: preventing unit canonicalization when processing arbitrary values in Tailwind CSS.
Description check	✅ Passed	The description clearly explains the problem, the solution, and includes a test plan demonstrating the changes made.
Linked Issues check	✅ Passed	The PR directly addresses issue #1573 by preventing unintuitive unit conversions (e.g., 0.04in to 3.84px) in arbitrary values while preserving constant folding functionality.
Out of Scope Changes check	✅ Passed	All changes are focused on fixing unit canonicalization in arbitrary values: tests, function signatures, and constant-folding logic with explicit normalizeUnit parameter.

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[coderabbitai]

🧹 Nitpick comments (1)

packages/tailwindcss/src/canonicalize-candidates.test.ts (1)

1214-1225: Broaden the regression a bit.

This locks down the in case, but the new behavior is generic across all convertible length units. Adding one more case for a different unit, such as cm or rem, would better protect the normalizeUnit=false path from regressions.

Suggested addition

       ['-mt-[0.04in]', 'mt-[-0.04in]'],
       ['mt-[-0.04in]', 'mt-[-0.04in]'],
       ['-mt-[-0.04in]', 'mt-[0.04in]'],
+      ['-mt-[0.1cm]', 'mt-[-0.1cm]'],

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@packages/tailwindcss/src/canonicalize-candidates.test.ts` around lines 1214 -
1225, Add a new test case to the existing test.each block (the table driving the
test with test.each and the call to expectCanonicalization) to cover a different
length unit (e.g., use 'cm' or 'rem') so the normalization behavior with
normalizeUnit=false is protected more generally; mirror the three-case pattern
used for 'in' (e.g., ['-mt-[0.04cm]', 'mt-[-0.04cm]'], ['mt-[-0.04cm]',
'mt-[-0.04cm]'], ['-mt-[-0.04cm]', 'mt-[0.04cm]']) and place it alongside the
existing 'in' cases in the same test.each array.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In `@packages/tailwindcss/src/canonicalize-candidates.test.ts`:
- Around line 1214-1225: Add a new test case to the existing test.each block
(the table driving the test with test.each and the call to
expectCanonicalization) to cover a different length unit (e.g., use 'cm' or
'rem') so the normalization behavior with normalizeUnit=false is protected more
generally; mirror the three-case pattern used for 'in' (e.g., ['-mt-[0.04cm]',
'mt-[-0.04cm]'], ['mt-[-0.04cm]', 'mt-[-0.04cm]'], ['-mt-[-0.04cm]',
'mt-[0.04cm]']) and place it alongside the existing 'in' cases in the same
test.each array.

---

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📒 Files selected for processing (3)

• packages/tailwindcss/src/canonicalize-candidates.test.ts
• packages/tailwindcss/src/canonicalize-candidates.ts
• packages/tailwindcss/src/constant-fold-declaration.ts