css: fold min(), max() and clamp() over plain numbers

[robobun]

#39577 (the clamp() bound fix and the partial_cmp_args helper) has landed; this PR is the number case on top of it, rebased onto main.

Blocked on #38553: this PR makes min()/max() fold inside relative colors too, and on main the relative color parser resolves channel keywords on the wrong scale (#38553). rgb(from c min(r, 20) g b), which is passed through unchanged today, would print the wrong color until #38553 is in. With #38553 applied on top of this branch the probes below are correct. Merge #38553 first.

Problem

• The CSS minifier (bun build on a .css file, Bun.build) folds min(), max() and clamp() over dimensions but never over plain numbers. width: min(3px, 2px) prints width:2px, while opacity: min(.5, .2) prints opacity:min(.5,.2), and the same for line-height, flex-grow, font-weight, color channels, and every other <number> position. Same on bun 1.4.0 and main.
• This is not only output size. A property parser that wants a <number> rejects the unfolded function, and what happens next depends on the property:
  • flex: min(1, 2) 1 0 (grow 1, shrink 1, basis 0) prints flex:1 0 min(1,2): Flex::parse (src/css/properties/flex.rs:144-152) fails to read the function as grow, takes it as the <length-percentage> basis, and reads the following numbers as grow and shrink. Browsers reject the output. flex: min(1, 2) 2 prints flex:2 min(1,2), which swaps grow and shrink.
  • @media (aspect-ratio: min(1, 2) / 1) is a build error (error: Invalid media query), because a media feature has no unparsed fallback.
  • rgb(from #336699 r g b / min(alpha, .5)) prints rgb(51 102 153/min(alpha,.5)), which browsers reject (the from is gone, the keyword is not): the alpha falls into the path in src/css/properties/custom.rs that keeps an alpha it cannot parse as tokens.
  • opacity, line-height and the others keep the declaration as the tokens it was written as.
• Cause: Calc::reduce_args (the min()/max() simplifier) and the clamp() arm in src/css/values/calc.rs only compare arguments that are Calc::Value. A bare number inside a math function always parses as Calc::Number (Calc::parse_value), even for Calc<CSSNumber>, so nothing is ever compared. round(), rem(), mod(), hypot(), abs() and sign() already fold numbers, because apply_op/apply_map handle the Number case. Upstream fixed the same two sites in Reduce min(), max() and clamp() with number arguments parcel-bundler/lightningcss#1131 (folds in lightningcss 1.33.0, not in the 1.30.2 this repo has in node_modules).

Fix

• partial_cmp_args (added in css: keep the lower bound when simplifying clamp() #39577) gets the (Calc::Number, Calc::Number) arm, and reduce_args goes through it, so min(), max() and clamp() order two numbers the same way they order two values. A number and a value are never compared, so max(1.2, 1.5, 1em, 2em) becomes max(1.5,2em) and min(.5, 20%) or clamp(1px, 2, 3px) are left alone. NaN compares with nothing, so a NaN argument is never removed, as before. This is the reduction lightningcss#1131 performs.
• Correct because the value of a parse-time number is exact, so two numbers decide an argument the same way two px values already do, and an argument is only dropped when another argument of the same kind is known to be at least as good.
• A single remaining number is returned exactly as a one-argument min(.5) is today, so every consumer already handles the shape: CSSNumber, color channels, Flex and Ratio take it, Percentage/Angle/Time reject it and the declaration falls back as it does now, Length/DimensionPercentage store it and print the number, which is what width: calc(2) already prints for that (invalid) input.
• reduce_args itself now drains the vector instead of the iter_mut plus mem::replace(.., Calc::Number(420.0)) placeholders. The value path is unchanged, including the winner taking the loser's position (min(3px, 10%, 2px) still prints min(2px,10%)), which the tests pin.
• opacity: calc(1 - min(.5, .2)) prints 0 on main. That comes from Percentage::from_calc returning NaN for a calc it cannot reduce (fixed in css: parse calc(<channel> * <percentage>) in relative colors #38513); this PR only stops the input from reaching it, since the min() now folds before the subtraction. It is in the tests as a fold inside calc(), not claimed as a fix of that arm.
• Relative colors: the alpha keyword is a number on the same scale as a literal on main and in css: resolve relative color channel keywords as numbers in the function's range #38553, so min(alpha, .5), max(alpha, .5) and clamp(.1, alpha, .5) fold to the right color with this PR alone and are tested. The other channels are resolved on the wrong scale on main (css: resolve relative color channel keywords as numbers in the function's range #38553), which is why this PR is blocked on it, see the top; they are not tested here, css: resolve relative color channel keywords as numbers in the function's range #38553 carries them. Same-basis expressions such as min(r, g) print the same bytes before and after.
• Tests: test/js/bun/css/css.test.ts. New min() and max() simplification block (32 rows: <number> properties, three arguments with the winner in each position, equal arguments, infinity, a product argument, nested max(min()), min() inside calc(), rgb() in both syntaxes, hsl(), alpha, font-weight, the font shorthand, aspect-ratio, the three flex shorthand forms, the aspect-ratio media feature, relative alpha, mixed number/dimension and number/percentage arguments, NaN, and the existing dimension reduction), and 13 number rows added to the clamp() simplification block from css: keep the lower bound when simplifying clamp() #39577 (center in range, below, above and equal to each bound, flex, rgb(), relative alpha, inside calc(), a number/dimension mix, a number/percentage mix, NaN). Against a build of main (which has css: keep the lower bound when simplifying clamp() #39577), 36 of these 45 rows fail and the 9 pins pass; with this PR all 69 rows of the two blocks pass.
• Also run: css.test.ts, color.test.ts and test/bundler/css/ (2415 pass), cargo clippy -p bun_css clean.
• The alpha path in custom.rs mentioned above still prints the invalid form for an alpha this PR cannot fold, for example rgb(from #336699 r g b / min(alpha, var(--x))) prints rgb(51 102 153/min(alpha,var(--x))) before and after. That path needs to keep the whole relative color as written when the alpha uses a keyword; it is a separate fix and not attempted here.

Background

• Calc<V> is the parsed form of a math expression for a value type V (Length, Percentage, Angle, CSSNumber, ...). Calc::Value holds a V; Calc::Number holds a bare number, which exists for every V because numbers are also the multipliers of products. For V = CSSNumber the number a property wants therefore arrives as Calc::Number, not Calc::Value.
• reduce_args keeps one argument out of each group of mutually comparable arguments, so min(1px, 1em, 2px) becomes min(1px, 1em): px compares with px, nothing compares with em at parse time. If one argument is left, the function disappears.
• Unparsed fallback: when a property's value parser rejects what it was given, bun keeps the declaration as the original tokens and only minifies whitespace. Shorthands and media features do not have it: a shorthand tries its other slots (the flex case above) and a media feature fails the stylesheet.

Probes: relative colors with this branch alone, and with #38553 applied on top

input                                                 main                         this branch      this branch + #38553
rgb(from #336699 calc(r + 10) g b)                    #f69 (wrong)                 #f69             #3d6699
rgb(from #336699 round(r, 10) g b)                    #069 (wrong)                 #069             #326699
rgb(from #336699 min(r, g) g b)                       #369                         #369             #369
rgb(from #336699 max(r, g) g b)                       #669                         #669             #669
rgb(from #336699 min(r, 20) g b)                      rgb(from #369 min(r,20)g b)  #369 (wrong)     #146699
rgb(from #336699 max(r, 200) g b)                     rgb(from #369 max(r,200)g b) #f69 (wrong)     #c86699
rgb(from #336699 r g b / min(alpha, .5))              rgb(51 102 153/min(alpha,.5)) (invalid)  #33669980   #33669980
rgb(from rgb(200 100 50) min(r, alpha) g b)           -                            -                #016432 (value #38553 asserts)
lab(from lab(50% 20 30) min(l, a) a b)                -                            -                lab(20% 20 30) (value #38553 asserts)
hsl(from #336699 min(h, 100) s l)                     left as written              #593             #593

Probes: other inputs whose output changes (bun build --minify, main -> this branch)

flex:min(1,2) 1 0                        flex:1 0 min(1,2)            -> flex:1 1 0
flex:min(1,2) 2                          flex:2 min(1,2)              -> flex:1 2
@media (aspect-ratio:min(1,2)/1){...}    error: Invalid media query   -> @media (aspect-ratio:1){...}
font:12px/min(1.2,1.5) serif             unchanged                    -> font:12px/1.2 serif
opacity:max(.5,sign(-1))                 unchanged                    -> opacity:.5
opacity:calc(1 - min(.5,.2))             opacity:0                    -> opacity:.8
opacity:calc(1 - 2 * min(.5,.2))         opacity:0                    -> opacity:.6
opacity:clamp(0,.5,.2)                   unchanged                    -> opacity:.2

unchanged on purpose:
opacity:min(.5,var(--x),.2)   opacity:min(NaN,.5)   opacity:clamp(0,NaN,1)   width:min(3px,2)
z-index:min(3,2) (<integer> does not parse math functions; lightningcss leaves it too)

---

[review] gate passed · iteration 1 · 2 files touched

fails on main (without fix)

ASAN without fix: 36 failed, 67 skipped
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/mechgate.xml" test/js/bun/css/css.test.ts
bun test v1.4.0 (8326d1bd3)

test/js/bun/css/css.test.ts:
Output .flexrow {
  flex-direction: row;
}

.flexcol {
  flex-direction: column;
}

.hello {
  flex-wrap: wrap;
}

.world {
  flex-wrap: nowrap;
}

(pass) css tests > .flexrow {
	flex-direction: row;
}

.flexcol {
	flex-direction: column;
}

.hello {
	flex-wrap: wrap;
}

.world {
	flex-wrap: nowrap;
} [3.70ms]
Output .foo {
  content: "+";
}

(pass) css tests > .foo {
  content: "\2b";
} [1.16ms]
Output div {
  --foo: 1 1 1 #0101011a, 2 2 2 #02020233;
  --bar: 1 1 1 #01010116, 2 2 2 #02020233;
}

(pass) css tests > custom property cases > div {
        --foo: 1 1 1 rgba(1, 1, 1, 0.1), 2 2 2 rgba(2, 2, 2, 0.2);
        --bar: 1 1 1 #01010116, 2 2 2 #02020233;
      } [1.08ms]
Output :root {
  --my-color: red;
  --my-bg: white;
  --my-font-size: 16px;
}

.element {
  color: var(--my-color);
  background-color: var(--my-bg);
  font-size: var(--my-font-size);
}

(pass) css tests > custom property cases > :root {
        --my-color: red;
        --my-bg: w
... (truncated)

release without fix: 36 failed, 67 skipped
bun test v1.4.0-canary.1 (7e15aeb3f)

test/js/bun/css/css.test.ts:
Output .flexrow {
  flex-direction: row;
}

.flexcol {
  flex-direction: column;
}

.hello {
  flex-wrap: wrap;
}

.world {
  flex-wrap: nowrap;
}

(pass) css tests > .flexrow {
	flex-direction: row;
}

.flexcol {
	flex-direction: column;
}

.hello {
	flex-wrap: wrap;
}

.world {
	flex-wrap: nowrap;
} [0.12ms]
Output .foo {
  content: "+";
}

(pass) css tests > .foo {
  content: "\2b";
} [0.02ms]
Output div {
  --foo: 1 1 1 #0101011a, 2 2 2 #02020233;
  --bar: 1 1 1 #01010116, 2 2 2 #02020233;
}

(pass) css tests > custom property cases > div {
        --foo: 1 1 1 rgba(1, 1, 1, 0.1), 2 2 2 rgba(2, 2, 2, 0.2);
        --bar: 1 1 1 #01010116, 2 2 2 #02020233;
      } [0.05ms]
Output :root {
  --my-color: red;
  --my-bg: white;
  --my-font-size: 16px;
}

.element {
  color: var(--my-color);
  background-color: var(--my-bg);
  font-size: var(--my-font-size);
}

(pass) css tests > custom property cases > :root {
        --my-color: red;
        --my-bg: white;
        --my-font-size: 16px;
      }
      .element {
        color: var(--my-color);
        background-color: var(--my-bg);
        font-size: 
... (truncated)

passes on PR (with fix)

ASAN with fix: 67 skipped
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/mechgate.xml" test/js/bun/css/css.test.ts
bun test v1.4.0 (8326d1bd3)

test/js/bun/css/css.test.ts:
Output .flexrow {
  flex-direction: row;
}

.flexcol {
  flex-direction: column;
}

.hello {
  flex-wrap: wrap;
}

.world {
  flex-wrap: nowrap;
}

(pass) css tests > .flexrow {
	flex-direction: row;
}

.flexcol {
	flex-direction: column;
}

.hello {
	flex-wrap: wrap;
}

.world {
	flex-wrap: nowrap;
} [20.75ms]
Output .foo {
  content: "+";
}

(pass) css tests > .foo {
  content: "\2b";
} [1.76ms]
Output div {
  --foo: 1 1 1 #0101011a, 2 2 2 #02020233;
  --bar: 1 1 1 #01010116, 2 2 2 #02020233;
}

(pass) css tests > custom property cases > div {
        --foo: 1 1 1 rgba(1, 1, 1, 0.1), 2 2 2 rgba(2, 2, 2, 0.2);
        --bar: 1 1 1 #01010116, 2 2 2 #02020233;
      } [1.77ms]
Output :root {
  --my-color: red;
  --my-bg: white;
  --my-font-size: 16px;
}

.element {
  color: var(--my-color);
  background-color: var(--my-bg);
  font-size: var(--my-font-size);
}

(pass) css tests > custom property cases > :root {
        --my-color: red;
        --my-bg: 
... (truncated)

release with fix: 67 skipped
$ bun scripts/build.ts --profile=release
[configured] bun-profile → bun (stripped) in 755ms (unchanged)
ninja: Entering directory `/workspace/bun/build/release'
[1/22] cxx obj/unified/UnifiedSource-src_jsc_bindings_node-0.cpp.o
[2/22] cxx obj/src/jsc/bindings/BunObject.cpp.o
[3/22] cxx obj/src/jsc/bindings/BunProcess.cpp.o
[4/22] cxx obj/unified/UnifiedSource-src_jsc_bindings_webcore-3.cpp.o
[5/22] cxx obj/unified/UnifiedSource-src_jsc_modules-0.cpp.o
[6/22] cxx obj/src/jsc/bindings/bindings.cpp.o
[7/22] cxx obj/unified/UnifiedSource-src_jsc_bindings_webcore-2.cpp.o
[8/22] cxx obj/unified/UnifiedSource-src_jsc_bindings-3.cpp.o
[9/22] cxx obj/unified/UnifiedSource-src_jsc_bindings-4.cpp.o
[10/22] cxx obj/unified/UnifiedSource-src_jsc_bindings-1.cpp.o
[11/22] cxx obj/unified/UnifiedSource-src_jsc_bindings-0.cpp.o
[12/22] cxx obj/unified/UnifiedSource-src_jsc_bindings-2.cpp.o
[13/22] cxx obj/codegen/JSSink.cpp.o
[14/22] cxx obj/src/jsc/bindings/napi.cpp.o
[15/22] cxx obj/src/jsc/bindings/ZigGlobalObject.cpp.o
[16/22] cxx obj/codegen/ZigGeneratedClasses.cpp.o
[17/22] gen generated_host_exports.rs
generated_host_exports.rs: 92 exports (host=3, lazy=10, generic=79, rust
... (truncated)

diff hotspot

src/css/values/calc.rs      | 44 ++++++++++-------------------------
 test/js/bun/css/css.test.ts | 56 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+), 32 deletions(-)

gate history · 2 passed · 0 rejected · iteration 1

evidence per changed file

file                         reads  edits  tests
src/css/values/calc.rs          12     10      0
test/js/bun/css/css.test.ts      3      7      0

[robobun]

Status: reproduced on bun 1.4.0 and main with bun build --minify: opacity:min(.5,.2) is kept as written, flex:min(1,2) 1 0 prints flex:1 0 min(1,2), @media (aspect-ratio: min(1,2)/1) is a build error. The clamp() bound fix that this was stacked on landed as #39577; this PR is now one commit on main adding the number case. Against a build of main, 36 of the 45 rows this PR adds to test/js/bun/css/css.test.ts fail and all pass with the fix. The description notes the #38553 ordering for relative colors.

[coderabbitai]

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---

Walkthrough

The change updates CSS Calc reduction for min(), max(), and clamp(). It compares compatible arguments, preserves incomparable expressions, removes placeholder values, and adds minification tests for comparable and unresolved expressions.

Changes

CSS Calc Reduction

Layer / File(s)	Summary
Calc comparison and reduction src/css/values/calc.rs	partial_cmp_args compares compatible values or numbers. reduce_args replaces retained comparable arguments and preserves incomparable expressions.
Calc minification validation test/js/bun/css/css.test.ts	Tests cover numeric, nested, unit, color-channel, ratio, infinity, NaN, dimensional, and unresolved min(), max(), and clamp() expressions.

Possibly related PRs

• oven-sh/bun#39577: Both changes modify calc.rs comparison and reduction logic for CSS min(), max(), and clamp() simplification.

Suggested reviewers: jarred-sumner

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the primary change: folding CSS min(), max(), and clamp() expressions over plain numbers.
Description check	✅ Passed	The description explains the problem, implementation, limitations, testing, and verification results, although it does not use the template headings verbatim.

---

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[claude]

LGTM — small, well-scoped fix that extends reduce_args to compare Calc::Number arguments the same way it already compares Calc::Value arguments.

What was reviewed:

• Traced the rewritten loop against the old iter_mut + mem::replace(.., 420.0) body — the Calc::Value path is behavior-identical (winner still takes loser's position, pinned by the min(3px, 10%, 2px) test), and non-Value/non-Number args still fall through to reduced.push.
• Confirmed protocol::PartialCmp for f32 (partial_cmp_f32 in generics.rs) returns None for NaN and orders infinities, matching the NaN/infinity test expectations; equal args hit the Some(_) arm and drop the duplicate.
• Numbers and Values never compare (the _ => None arm), so min(.5, 20%) and max(1.5, 2em) keep both groups as asserted.

Extended reasoning...

Overview

The PR touches one function body, Calc::reduce_args in src/css/values/calc.rs, plus a new 6-line helper partial_cmp_args. The old body only compared Calc::Value arguments, so min()/max() over plain numbers (opacity, line-height, color channels) was never folded and the declaration fell back to the unparsed token stream. The new body drains the argument vector and compares each incoming arg against each kept arg via partial_cmp_args, which handles both (Value, Value) and (Number, Number) and returns None for any mixed pair. 22 new minify_test cases in test/js/bun/css/css.test.ts cover number properties, three-argument permutations, equal args, ±infinity, products, nesting, calc composition, color channels, mixed number/dimension, NaN, and pin the existing dimension reduction (winner takes loser's slot).

Security risks

None. This is a CSS minifier output-size optimization; the worst possible failure mode is a declaration falling back to its unparsed form or a suboptimal fold, neither of which is security-relevant.

Level of scrutiny

Low-to-medium. The diff is ~40 lines of a self-contained algorithm rewrite plus tests. I traced the new labeled-loop body against the old Option<Option<usize>> bookkeeping: for Calc::Value args the control flow is identical (first comparable kept arg either gets replaced or causes the incoming arg to be dropped; incomparable → push). For non-Value/non-Number args (Sum, Product, Function) both versions push unconditionally since partial_cmp_args returns None. The drain(..) rewrite removes the Calc::Number(420.0) placeholder trick, which is a strict simplification with no ownership hazard. protocol::PartialCmp for f32 in generics.rs returns None for NaN, so a NaN argument is never dropped (tests pin this).

Other factors

The PR description is unusually thorough: it explains the mechanism, cites the lightningcss reference, enumerates every consumer of the single-remaining-Calc::Number shape, and documents the interaction with three related open PRs (#32290 clamp, #38513 Percentage NaN, #38553 relative-color scale). The one behavioral wrinkle — relative-color min(r, 20) now folds through the same wrong-scale path that calc(r + 10) and round(r, 10) already hit on main — is a pre-existing bug in color.rs (#38553), not introduced here; this PR just makes min/max consistent with sibling math functions, and the description is transparent about it. The clamp() arm is deliberately left unchanged with a stated reason. Test coverage is comprehensive across the variant matrix and the author reports the full css and bundler suites pass.

[robobun]

^{Updated 1:35 AM PT - Aug 19th, 2026}

✅ @robobun, your commit 6870b5cdc5a280ef651a5cefb7c5540949909001 passed in Build #101149! 🎉

---

🧪   To try this PR locally:

bunx bun-pr 39515

That installs a local version of the PR into your bun-39515 executable, so you can run:

bun-39515 --bun

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@src/css/values/calc.rs`:
- Around line 1035-1039: Update partial_cmp_args for Calc::Number in the numeric
comparison path to preserve CSS signed-zero ordering, placing -0 below +0 while
keeping NaN values incomparable and retaining the existing ordering for other
non-NaN numbers. Add regression coverage for min(0, -1 * 0), max(-1 * 0, 0), and
clamp(0, -1 * 0, 1), verifying formatted results preserve -0.

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Fix all unresolved CodeRabbit comments on this PR:

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[claude]

I reviewed this PR and didn't find any bugs. Since it rewrites the clamp() reduction logic and is explicitly blocked on #38553 for merge ordering, a human look before landing is still worthwhile.

What was reviewed:

• Traced the new clamp() arm against max(min, min(center, max)) for all nine comparable/incomparable combinations of the three bounds — each produces the asserted output, and the old swapped-bound bug (min = None when center ≤ max) is gone.
• reduce_args rewrite: the drain/replace-in-place preserves the winner's position (test at css.test.ts:222 pins min(3px,10%,2px) → min(2px,10%)); non-Value/non-Number args (Sum, Product, Function) still fall through to reduced.push as before.
• CodeRabbit's signed-zero note: partial_cmp_f32 (generics.rs:1557) treats ±0 as Equal, but that is the pre-existing behavior for the Calc::Value path too, and -0 cannot be written as a CSS literal token — not a regression introduced here.

Extended reasoning...

Overview

The PR touches two sites in src/css/values/calc.rs: it adds a partial_cmp_args helper that orders two Calc arguments when both are Calc::Value or both are Calc::Number (returning None otherwise), rewrites reduce_args around it using drain(..) instead of the mem::replace(.., Calc::Number(420.0)) placeholder pattern, and restructures the clamp() arm of parse_with to apply the max bound first then the min bound. 53 new minify_test cases are added to test/js/bun/css/css.test.ts.

Security risks

None. This is parse-time constant folding in the CSS minifier over already-tokenized numeric values; no untrusted length arithmetic, allocation sizing, or filesystem/network access is involved.

Level of scrutiny

Medium. The change is small (~45 lines of logic) and heavily tested, but it affects the correctness of user-facing minified CSS output across every <number> property, and the PR itself states it must land after #38553 to avoid emitting wrong relative-color values. The old clamp() code had a real bug (dropping the wrong bound), so the rewrite deserves a careful read rather than a rubber stamp.

Other factors

• The PR is explicitly marked blocked on #38553; auto-approving would contradict the author's stated merge order.
• The comment-cop bot's three long-comment flags were addressed in 401800f (all threads resolved).
• CodeRabbit raised a signed-zero spec concern. I checked protocol::PartialCmp for f32 at src/css/generics.rs:1572 — it delegates to partial_cmp_f32, which uses <=/>= and so already treats ±0 as Equal for the pre-existing Calc::Value path (dimensions). The Number path added here uses the same function, so this is not a regression; and per the spec text CodeRabbit itself quoted, -0 cannot be written as a literal CSS token. I don't consider it a blocker but left the assessment visible for the human reviewer.
• Test coverage is thorough: the winner in each of three positions, equal args, ±infinity, NaN (kept), mixed number/dimension (kept), the #32290 clamp regression cases, and the position-preservation invariant for dimensions.

[claude]

I reviewed this PR and didn't find any bugs — the partial_cmp_args helper and the restructured clamp() arm trace correctly against all 53 new test cases, and reduce_args preserves the existing value-path ordering while extending it to Calc::Number. Deferring to a human because the PR is explicitly blocked on #38553 (merge order matters: without it, folding min()/max() inside relative color channels would print wrong colors), and that dependency needs to be confirmed landed before this merges.

What was reviewed:

• Traced the new clamp() logic against each bound-ordering case (in-range, below min, above max, min > max, incomparable min, incomparable max, NaN, mixed number/dimension) — all match max(min, min(center, max)).
• Verified reduce_args still keeps the first-seen position for the winner (min(3px, 10%, 2px) → min(2px, 10%)) and that arr2 / MathFunction::Min remain live via other call sites.
• Checked the CodeRabbit signed-zero concern — the number arm uses the same partial_cmp_f32 as the existing dimension path, so no new divergence.

Extended reasoning...

Overview

This PR touches two files: src/css/values/calc.rs (~60 lines net change in Calc::parse_with's Clamp arm and reduce_args, plus a new 8-line partial_cmp_args helper) and test/js/bun/css/css.test.ts (53 new minify_test cases across two describe blocks). The change makes the CSS minifier fold min(), max(), and clamp() over plain numbers the same way it already folds them over dimensions, and fixes a port bug where clamp() dropped the wrong bound (min = None instead of max = None) when the center was not above the maximum.

Security risks

None. This is CSS minifier output simplification with no I/O, no untrusted-length arithmetic, and no memory-safety surface (all owned Vec/Box moves via drain(..), replacing the old mem::replace placeholder pattern).

Level of scrutiny

Medium. The logic is compact and well-explained, matches the upstream lightningcss fix (parcel-bundler/lightningcss#1131) the author cites, and comes with comprehensive tests (53 cases covering argument position, ties, infinity, NaN, nested functions, products, mixed number/dimension, colors, the flex shorthand mis-parse, media features, and regression pins for the existing dimension path). I traced every clamp branch by hand and the max(min, min(center, max)) decomposition is applied correctly, including the min-wins-over-smaller-max case. The reduce_args rewrite is behavior-preserving for Calc::Value and strictly additive for Calc::Number.

Other factors

The author explicitly states the PR is blocked on #38553: this change makes min()/max() fold through the relative-color <number> pass in color.rs, and on main that pass resolves channel keywords on the wrong scale, so rgb(from #336699 min(r, 20) g b) would print an incorrect color if this lands first. The tests here deliberately avoid non-alpha channel keywords for that reason. A human should confirm #38553 has landed (or confirm the merge order) before approving. The author also flags a small textual conflict with #39501 in the same block. All prior review threads (comment-cop on comment length, CodeRabbit on signed-zero ordering) are resolved.

[claude]

I reviewed this PR and didn't find any bugs. The change is small and well-tested, but the description states it's blocked on #38553 (merging this first would make rgb(from c min(r, 20) g b) fold to a wrong color instead of passing through), so a human should confirm that dependency is satisfied before merge.

What was reviewed:

• reduce_args refactor: verified the drain-based loop preserves the old value-path semantics (first-comparable wins, winner takes loser's position) — pinned by the min(3px, 10%, 2px) test.
• partial_cmp_args Number arm: NaN returns None (never removed), Number×Value returns None (mixed args left alone) — both covered by tests.
• No memory concerns: drain(..) moves each arg out, *kept = arg drops the replaced value, and the final *args = reduced reassigns an already-empty vec.

Extended reasoning...

Overview

The PR adds a (Calc::Number, Calc::Number) arm to partial_cmp_args in src/css/values/calc.rs so min(), max(), and clamp() fold over plain numbers the same way they already fold over dimensioned values. It also refactors reduce_args from an iter_mut() + mem::replace(.., Calc::Number(420.0)) placeholder loop to a cleaner drain(..) loop that goes through partial_cmp_args. 45 new test rows in test/js/bun/css/css.test.ts cover number folding across opacity, line-height, flex, color channels, font, aspect-ratio, media features, relative-color alpha, mixed number/dimension args, NaN, and pins for the existing dimension behavior.

Security risks

None. This is CSS minification of parse-time-known numeric arguments; no untrusted input reaches allocation sizes, syscalls, or JS.

Level of scrutiny

Medium. The Rust change is ~15 net lines in a single function pair. I traced the refactored loop against the old one: for the existing Calc::Value path the control flow is identical (first comparable in reduced decides; Some(order) replaces in place, Some(other) drops the new arg, None pushes), and the position-preservation the tests pin (min(3px, 10%, 2px) → min(2px,10%)) is maintained by *kept = arg. The new Number arm is a single f32::partial_cmp via the existing protocol trait, so NaN yields None and cross-kind (Number vs Value) also yields None, matching the tests. This mirrors upstream lightningcss#1131.

Other factors

The reason I'm deferring rather than approving is the explicit merge-order dependency: the description states this PR is blocked on #38553 because folding now applies inside relative colors, and on main the non-alpha channel keywords are resolved on the wrong scale. Merging this before #38553 would regress rgb(from #336699 min(r, 20) g b) from pass-through (browser handles it) to a wrong folded color. That's a merge-sequencing decision a maintainer needs to confirm, not something an automated approval should green-light. All prior inline threads (comment-cop paragraph comments, coderabbit signed-zero) are resolved.

[claude]

LGTM — small, focused change that adds a (Number, Number) arm to partial_cmp_args and refactors reduce_args to drain instead of placeholder-swap.

What was reviewed:

• Traced the reduce_args refactor against the old code: value path is behavior-identical (winner-takes-loser's-position, Equal drops the duplicate), non-Value/non-Number args still incomparable and pushed as before.
• Confirmed partial_cmp_f32 returns None for NaN, so NaN args are never removed (matches the pinned tests).
• Checked the signed-zero concern raised earlier — the number arm uses the same partial_cmp_f32 as the existing dimension path, so behavior is consistent; serializer normalizes zero.
• 45 new test rows cover the fold across <number> properties, mixed number/dimension args, three-arg winner positions, nesting, NaN, and pin the existing dimension reduction order.

Extended reasoning...

Overview

This PR adds plain-number folding to the CSS minifier's min()/max()/clamp() simplification in src/css/values/calc.rs. Two code changes: (1) a new (Calc::Number, Calc::Number) arm in partial_cmp_args so bare numbers compare the same way typed values already do, and (2) a refactor of reduce_args from iter_mut + mem::replace(.., Calc::Number(420.0)) placeholders to a drain(..) + labeled-continue loop that goes through partial_cmp_args. 45 new test rows in test/js/bun/css/css.test.ts cover the number fold across opacity/line-height/flex/font/color/aspect-ratio/media, mixed number+dimension args, NaN, and pin the existing dimension reduction (winner-takes-loser's-position).

Security risks

None. This is a pure CSS minification optimization operating on parsed f32 values. No untrusted-size arithmetic, no allocation driven by input magnitudes, no path/URL handling.

Level of scrutiny

Low-to-medium. The change is small (~30 lines net in calc.rs), matches the upstream lightningcss fix (parcel-bundler/lightningcss#1131), and the refactor is mechanically verifiable. I traced the new reduce_args loop against the old one for each arm: for Calc::Value args the sequence of compare/replace/push decisions is identical; for Sum/Product/Function args partial_cmp_args returns None for every kept entry so they are pushed unchanged as before; Calc::Number args now compare with other numbers, which is the intended new behavior. The Some(_) arm covers Equal the same way the old found = Some(None) path did (drop the duplicate). partial_cmp_f32 at src/css/generics.rs:1557 returns None for NaN, so NaN arguments are never removed — matching the pinned min(NaN, .5) and clamp(0, NaN, 1) tests.

Other factors

The base PR #39577 (which introduced partial_cmp_args and the clamp() fix) is merged. The description notes a merge-order dependency on #38553 for non-alpha relative-color channels, but this PR's tests deliberately only exercise the alpha channel (which the description confirms works with this PR alone); the non-alpha cases are carried by #38553's own tests. All prior review threads (comment-cop long-comment flags, CodeRabbit's signed-zero concern) are resolved — the signed-zero reasoning is sound: the new number arm uses the same partial_cmp_f32 as the existing dimension path, and the serializer normalizes zero, so no observable divergence is introduced. No CODEOWNERS coverage for src/css/. Test coverage is thorough: 36 of the 45 new rows fail on the base branch and pass with the fix, and the 9 pins guard existing dimension behavior.

[claude]

LGTM — small, well-scoped addition that mirrors lightningcss#1131.

What was reviewed:

• The new (Calc::Number, Calc::Number) arm in partial_cmp_args routes through the same partial_cmp_f32 as the existing value arm, so NaN returns None and is never dropped (tested).
• The reduce_args rewrite is behavior-preserving for the value path: winner still takes the loser's slot (min(3px, 10%, 2px) → min(2px,10%) pinned), and non-Value/non-Number args still fall through to reduced.push.
• 45 new test rows cover <number> properties, three-arg positions, equal args, infinity, nested/product args, mixed number/dimension and number/percentage (left alone), NaN, flex shorthand, media features, relative-color alpha, and regression pins for the existing dimension reduction.
• The signed-zero concern raised earlier was checked and does not apply: the number arm matches the existing dimension arm's ordering and the serializer normalizes both to 0.

Extended reasoning...

Overview

This PR adds one match arm to Calc::partial_cmp_args so two Calc::Number arguments can be ordered, and rewrites reduce_args to drain the argument vector and route through partial_cmp_args instead of the old iter_mut + mem::replace(.., Calc::Number(420.0)) placeholder dance. The net effect is that min(), max() and clamp() now fold over plain numbers the same way they already fold over dimensions, matching parcel-bundler/lightningcss#1131. 45 test rows are added to test/js/bun/css/css.test.ts.

Security risks

None. This is a pure CSS-to-CSS transformation in the minifier; no untrusted-input parsing surface is added, no allocations sized from user data, no runtime/JS-heap interaction.

Level of scrutiny

Low-to-medium. The change is confined to the CSS minifier's constant-folding path, touches ~30 lines of Rust, and follows the upstream reference implementation this code was ported from. The refactored reduce_args is strictly simpler than what it replaces and is pinned by regression tests for the existing dimension behavior.

Other factors

• I traced the old vs. new reduce_args control flow: for the pre-existing Calc::Value path, the new labeled loop with *kept = arg; continue 'args / continue 'args / fall-through-to-push is exactly the old found = Some(Some(idx)) / Some(None) / None behavior. Non-Value, non-Number variants (Sum, Product, Function) get None from partial_cmp_args for every kept and are pushed, as before.
• protocol::PartialCmp for f32 is partial_cmp_f32 (src/css/generics.rs:1557), which returns None for NaN — verified this matches the tested min(NaN, .5) → unchanged behavior and is the same helper the dimension arms use.
• All prior review feedback is resolved: the comment-cop long-comment flags were cut to one-liners in 401800f, and the coderabbit signed-zero finding was investigated and withdrawn (the serializer prints all zeros as 0, and the number arm intentionally matches the existing dimension-arm ordering).
• The author's merge-order note (#39577 → #38553 → this) is a coordination concern about a pre-existing relative-color channel-scale bug that this PR does not introduce; the tests here deliberately cover only the alpha channel, which is already on the correct scale.

[Jarred-Sumner]

@robobun rebase

[coderabbitai]

Note

GitHub couldn't provide a complete incremental comparison for this pull request, so CodeRabbit is performing a full review instead. This review may take a little longer.

[robobun]

Rebased onto main (#39577 is in there now). The branch is the one commit 6870b5c with the number arm, the reduce_args rewrite and the 45 rows; the two test blocks pass locally (69 of 69) on the new base. The description still notes the #38553 ordering for relative colors.

[claude]

I reviewed this and didn't find any bugs — the reduce_args rewrite is behavior-preserving for the value path and the new (Number, Number) arm is sound. Deferring to a human on merge order: the description flags this as blocked on #38553, and the probe table shows that landing this first turns rgb(from #336699 min(r, 20) g b) from pass-through into a wrong fold.

What was reviewed:

• reduce_args: the drain(..) rewrite preserves winner-takes-loser's-position and the incomparable-arg push; non-Value/non-Number args still fall through to reduced.push.
• partial_cmp_args number arm goes through partial_cmp_f32, so NaN returns None and mixed Number/Value returns None — matches the pinned tests.
• Confirmed the signed-zero concern raised earlier is out of scope (same partial_cmp_f32 as the existing dimension path).

Extended reasoning...

Overview

Two files. src/css/values/calc.rs adds a (Calc::Number, Calc::Number) arm to partial_cmp_args and rewrites reduce_args to drain the input vec instead of the mem::replace(.., Calc::Number(420.0)) placeholder dance. test/js/bun/css/css.test.ts gains 45 minify_test rows across a new min()/max() block and the existing clamp() block.

Security risks

None. Pure CSS constant-folding over parsed f32 values; no I/O, no allocation size derived from untrusted counts, no user-reachable panics introduced.

Level of scrutiny

Medium. The Rust change is small (~30 net lines) and mirrors upstream lightningcss#1131. I traced the reduce_args rewrite against the old control flow: found = None → push, Some(Some(idx)) → replace at idx, Some(None) → drop; the new labeled-loop form maps to the same three outcomes via partial_cmp_args, and non-Value/non-Number args (Sum/Product/Function) still get None from every comparison and are pushed, as before. partial_cmp_f32 returns None for NaN, so NaN args are never dropped. The tests pin winner-position (min(3px, 10%, 2px) → min(2px,10%)), mixed Number/Value (max(1.2, 1.5, 1em, 2em) → max(1.5,2em)), and NaN.

Other factors

The PR description explicitly says "Blocked on #38553... Merge #38553 first" and provides a probe table showing that with this branch alone, rgb(from #336699 min(r, 20) g b) folds to #369 (wrong) instead of being passed through as on main. That is a user-visible regression if merged out of order. #38553 does not appear in the recent git history on this checkout. Jarred is already engaged (requested the rebase), so this is a merge-order call for a maintainer, not a code defect. The earlier CodeRabbit signed-zero finding was investigated and correctly withdrawn — the number arm uses the same partial_cmp_f32 as the existing dimension path, so any signed-zero change belongs there, not here.