[robobun]

Problem

• The CSS printer writes a stray space before a position keyword when no safe/unsafe keyword precedes it. .a{align-items:center} minifies to .a{align-items: center}. The same happens for align-self, align-content, place-items, place-self and place-content.
• The cause is gen_field_seq_to_css in src/css_derive/lib.rs:874. It generates the printer for #[derive(css::ToCss)] structs such as AlignItemsSelfPosition { overflow: Option<OverflowPosition>, value: SelfPosition } (src/css/properties/align.rs:392). It wrote the space between fields unconditionally, also when the leading Option field was None and printed nothing.

Fix

• Write the separator before a field only when an earlier field printed something. The generated body tracks this in a __needs_space flag. This matches the lightningcss ToCss derive.
• Only the three *Position structs in align.rs use this code path, so no other output changes.
• Verified: test/js/bun/css/css.test.ts (12 new minify_test cases, 7 fail on stock bun). Also ran all of test/js/bun/css/ and test/bundler/css/.
• Self-reviewed: 3 concerns raised, 0 needed a change here. A stricter compare in the cssTest helper, a missing try_parse in PlaceContent::parse, and the place-self auto collapse are separate changes (see Notes).

Background

• css_derive is the proc-macro crate behind #[derive(css::Parse, css::ToCss)] in src/css/. For a named-field struct, ToCss prints the fields in declaration order, separated by one space, and unwraps Option fields.
• The justify-* properties were not affected: their to_css is hand-written and skips the space when overflow is None.
• The existing cssTest cases compare with toEqualIgnoringWhitespace, so they did not catch the extra space. minify_test compares exactly.

Notes

• Repro on bun 1.4.3:

  printf '.a{align-items:center}\n.b{align-items:safe center}\n.d{place-self:center}' > d.css
  bun build --minify d.css
  # .a{align-items: center}.b{align-items:safe center}.d{place-self: center}
  

• The removed Zig DeriveToCss.__generateToCss had the same unconditional separator, so this predates the Rust port.
• Generated body for AlignItemsSelfPosition after the change:

  let mut __needs_space = false;
  if let Some(__v) = &self.overflow {
      if __needs_space { __dest.write_char(b' ')?; }
      __v.to_css(__dest)?;
      __needs_space = true;
  }
  if __needs_space { __dest.write_char(b' ')?; }
  self.value.to_css(__dest)?;
  __needs_space = true;

  The dead store after the last field raises no warning because rustc does not lint derive output.
• Review follow-ups, not in this PR:
  • cssTest/prefix_test in test/js/bun/css/util.ts compare with toEqualIgnoringWhitespace. A per-line trimmed exact compare would have caught this bug. It only becomes possible after this fix, because several existing flex expectations carry the stray space on stock bun.
  • PlaceContent::parse (align.rs:803) calls JustifyContent::parse without input.try_parse, unlike PlaceItems/PlaceSelf and upstream.
  • PlaceSelf::to_css treats JustifySelf::Auto as equal to any align value (align.rs:733), so place-self: center auto prints as center. Upstream has the same line.
• The failures in a local debug run of test/js/bun/css/ are the !isCI fuzz loops and the nesting-bound tests that hit their fixed 5 s and 10 s timeouts under the ASAN build. They pass with a longer timeout and do not touch this code.

---

[human-review] gate passed · iteration 0 · 2 files touched

fails on main (without fix)

ASAN without fix: BUILD FAILED (no junit output)
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/js/bun/css/css.test.ts
ninja: Entering directory `/workspace/bun/build/debug'
[1/162] gen generated_host_exports.rs
generated_host_exports.rs: 122 exports (host=5, lazy=10, generic=107, rust=0); 242 extern-C blocks audited
[2/162] gen cpp.rs (cppbind)
[2/162] cargo bun_runtime → libbun_runtime.a
FAILED: rust-target/x86_64-unknown-linux-gnu/debug/libbun_runtime.a 
/workspace/bun/build/release/bun /workspace/bun/scripts/build/stream.ts rust --console --cwd=/workspace/bun --env=CARGO_TERM_COLOR=always --env=BUN_CODEGEN_DIR=/workspace/bun/build/debug/codegen --env=CC=/usr/lib/llvm-21/bin/clang --env=CXX=/usr/lib/llvm-21/bin/clang++ --env=AR=/usr/lib/llvm-21/bin/llvm-ar --env=CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_LINKER=/usr/lib/llvm-21/bin/clang++ --env=CARGO_HOME=/root/.cargo --env=RUSTUP_HOME=/root/.rustup --env=RUSTUP_TOOLCHAIN=nightly-2026-07-20 --env=CARGO_PROFILE_RELEASE_LTO=off --env=CARGO_PROFILE_RELEASE_CODEGEN_UNITS=16 --env=CARGO_PROFILE_RELEASE_DEBUG_ASSERTIONS=true --env=CARGO_ENCODED_RUSTFLAGS='-Crelocation-model=static
... (truncated)

release without fix: 7 failed, 67 skipped
bun test v1.4.3-canary.1 (f42e98025)

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;
} [0.17ms]
Output .foo {
  content: "+";
}

(pass) css tests > .foo {
  content: "\2b";
} [0.04ms]
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/pr_gate.xml" test/js/bun/css/css.test.ts
bun test v1.4.3 (f42e98025)

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;
} [3.48ms]
Output .foo {
  content: "+";
}

(pass) css tests > .foo {
  content: "\2b";
} [1.35ms]
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;
      } [1.20ms]
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: wh
... (truncated)

release with fix: 67 skipped
$ bun scripts/build.ts --profile=release
[configured] bun-profile → bun (stripped) in 762ms (unchanged)
ninja: Entering directory `/workspace/bun/build/release'
[1/123] gen generated_host_exports.rs
generated_host_exports.rs: 122 exports (host=5, lazy=10, generic=107, rust=0); 242 extern-C blocks audited
[2/123] gen cpp.rs (cppbind)
[2/123] cargo bun_runtime → libbun_runtime.a
�[1m�[92m   Compiling�[0m bun_css_derive v0.0.0 (/workspace/bun/src/css_derive)
�[1m�[92m   Compiling�[0m bun_core v0.0.0 (/workspace/bun/src/bun_core)
�[1m�[92m   Compiling�[0m bun_errno v0.0.0 (/workspace/bun/src/errno)
�[1m�[92m   Compiling�[0m bun_ptr v0.0.0 (/workspace/bun/src/ptr)
�[1m�[92m   Compiling�[0m bun_boringssl_sys v0.0.0 (/workspace/bun/src/boringssl_sys)
�[1m�[92m   Compiling�[0m bun_safety v0.0.0 (/workspace/bun/src/safety)
�[1m�[92m   Compiling�[0m bun_base64 v0.0.0 (/workspace/bun/src/base64)
�[1m�[92m   Compiling�[0m bun_cares_sys v0.0.0 (/workspace/bun/src/cares_sys)
�[1m�[92m   Compiling�[0m bun_zlib_sys v0.0.0 (/workspace/bun/src/zlib_sys)
�[1m�[92m   Compiling�[0m bun_zstd v0.0.0 (/workspace/bun/src/zstd)
�[1m�[92m   Compiling�[0m bun_picohttp v0.0.0 (/workspace/bu
... (truncated)

diff hotspot

src/css_derive/lib.rs       | 46 ++++++++++++++++++++++++---------------------
 test/js/bun/css/css.test.ts | 14 ++++++++++++++
 2 files changed, 39 insertions(+), 21 deletions(-)

gate history · 1 passed · 0 rejected · iteration 0

evidence per changed file

file                         reads  edits  tests
src/css_derive/lib.rs            5      7      6
test/js/bun/css/css.test.ts      3      1      6

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📥 Commits

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📒 Files selected for processing (2)

• src/css_derive/lib.rs
• test/js/bun/css/css.test.ts

---

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[robobun]

Status

Closed. The fix for this report landed on main in #47025 (7942008).

Reproduced on bun 1.4.3:

printf '.a{align-items:center}\n.b{align-items:safe center}\n.d{place-self:center}' > d.css
bun build --minify d.css
# .a{align-items: center}.b{align-items:safe center}.d{place-self: center}

[claude]

LGTM — the __needs_space flag correctly gates the separator on whether a prior field actually printed, and None optionals no longer contribute a stray space.

What was reviewed:

• Traced both call sites of gen_field_seq_to_css (struct branch and NamedFields enum arm); for all-non-Option field sequences the generated output is byte-identical to before, so the only behavior change is the intended one.
• Confirmed the three #[css(generate_to_css)] structs in align.rs are the only field-sequence derive consumers with a leading Option, matching the PR's scope claim.
• New minify_test cases use exact-string comparison (not toEqualIgnoringWhitespace) and cover both present/absent <overflow-position> across all six affected properties.

Extended reasoning...

Overview

This PR fixes gen_field_seq_to_css in the css_derive proc-macro crate (src/css_derive/lib.rs). The old codegen emitted an inter-field space unconditionally based on positional index, so a leading Option<_> field that was None still contributed a separator, producing .a{align-items: center} instead of .a{align-items:center} in minified output. The new codegen threads a runtime __needs_space boolean: each field writes a space only if the flag is set, then sets it after printing. None optionals print nothing and don't set the flag. The iterator bound is relaxed from ExactSizeIterator + Clone to plain Iterator since length/index are no longer needed. Doc comments on the helper and the struct branch are updated to describe the new contract. Twelve minify_test cases are appended to the existing test/js/bun/css/css.test.ts alongside other alignment tests.

Security risks

None. This is a proc-macro that generates CSS serialization code at compile time; it manipulates syn/quote token streams and takes no untrusted runtime input. The generated code writes literal space bytes and delegates to existing to_css impls — no parsing, allocation sizing, or path handling is involved.

Level of scrutiny

Low-to-moderate. The diff is ~30 lines of proc-macro codegen with a mechanically verifiable behavior: for field sequences containing no Option, the new output is identical to the old (space before every field except the first). The only divergence is when an Option field is None, where the old code emitted a spurious space and the new code correctly omits it — this can only fix output, never regress it. I confirmed both call sites (named.named.iter() for structs at line 957 and fields.named.iter() for enum NamedFields variants at line 981) still type-check against the relaxed Iterator bound, and grepped for generate_to_css to verify the three *Position structs in align.rs are the only leading-Option consumers, matching the PR's scope claim.

Other factors

Tests follow repo conventions: added to the existing css.test.ts next to related flex/alignment coverage, reuse the minify_test helper (which does exact string comparison, unlike cssTest's toEqualIgnoringWhitespace that masked this bug), and cover the variant matrix — both present and absent <overflow-position> across all six affected properties (align-content/self/items, place-content/self/items). The inline comment about rustc not linting derive output for the trailing dead store is load-bearing (answers "why no #[allow]?") and passes REVIEW.md's "would the next reader spend tool calls figuring this out" test. No CODEOWNERS entries cover src/css_derive/ or test/js/bun/css/. No prior reviews or outstanding objections in the timeline. Hunt exit reason was dry_streak.

[robobun]

^{Updated 9:02 AM PT - Sep 8th, 2026}

❌ @robobun, your commit 10be7f9 has 2 failures in Build #112948 (All Failures):

• test/js/bun/s3/s3.test.ts - code 1 on 🪟 2019 x64
• test/js/node/test/parallel/test-crypto-dh-leak.js - code 1 on 🐧 13 x64-asan

---

🧪   To try this PR locally:

bunx bun-pr 41993

That installs a local version of the PR into your bun-41993 executable, so you can run:

bun-41993 --bun

[Jarred-Sumner]

Landed in #47025 (7942008).

Print align-items:center without a stray space when no safe/unsafe keyword is given. 3,317 real stylesheets

[robobun]

Confirmed on main at 7942008. A debug build of that commit prints the align and place cases without the stray space:

.a{align-items:center}.b{align-items:safe center}.d{place-self:center}.e{align-self:center}.f{place-items:center start}

The non-minified output has no double space either. test/js/bun/css/css.test.ts passes on that build (2018 pass, 0 fail).

The change to gen_field_seq_to_css covers all three structs in align.rs that derive ToCss with an optional field. Nothing is left for this PR.