[scss][less] Accept keyframe selectors in nested rule set declarations

[Rusty Raven (kakiuwang-ui)]

Fixes microsoft/vscode#227452

Problem

The SCSS parser reports false errors (css-rcurlyexpected, css-ruleorselectorexpected) for mixins that contain keyframe selectors, even though this is valid Sass that compiles fine:

@mixin slideX($from, $to) {
  0% { transform: translateX($from); }
  100% { transform: translateX($to); }
}

@keyframes slide-left { @include slideX(0, -100%); }

The same class of bug also reproduces one nesting level deeper (@mixin a { @if $b { 0% { } } }), inside @include content blocks wrapped in control statements, and in LESS parametric mixins (.m() { 0% { } } — cf. microsoft/vscode#28091, which fixed only detached rulesets in 2017).

Fix

Rather than adding another per-construct special case (there were already two: _parseMixinReferenceBodyStatement for scss @include bodies and _parseDetachedRuleSetBody for less detached rulesets, both added for vscode#28091), accept keyframe selectors in the shared nested fallback: the non-at-keyword branch of the scss/less _parseRuleSetDeclaration now tries _tryParseKeyframeSelector:

• after _tryParseRuleset, so plain nested rulesets like e1 { } keep their RuleSet node type (the existing SCSS - Selector Printing › nested selector test guards this), and
• before _parseDeclaration, which would otherwise consume from, 50% and emit a colon-expected error.

Because every nested construct (@mixin bodies, @include content blocks, @if/@each/@for bodies, less mixins and detached rulesets) funnels through _parseRuleSetDeclaration, this one change covers them all, and the two existing special-case body parsers are removed (net −9 lines in the parsers). _tryParseKeyframeSelector fully backtracks, so declarations, variable declarations and error recovery are unaffected.

Plain CSS is unchanged: .a { 0% { } } in a .css file still errors, since the base parser's _parseRuleSetDeclaration is untouched.

This matches dart-sass/less semantics, which parse rule bodies permissively and validate selectors at evaluation time (a mixin declaration cannot know its inclusion site).

Tests

Added parser tests for scss (0%/100% in @mixin, mixed from, 50% lists, keyframe selectors under @if/@else inside a mixin) and less (parametric mixins with 0%/100% and mixed lists). Full suite passes: 710 pass, 0 fail.

[Rusty Raven (kakiuwang-ui)]

@microsoft-github-policy-service agree