fix: preserve Less negative values

[magic-akari]

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Summary

Fixes Less formatting for negative values that appear after another value.

Malva previously formatted:

margin: 0 -@card-spacing;

as:

margin: 0 - @card-spacing;

In Less, those are not equivalent. The first is a value list containing 0 and -@card-spacing; the second is binary subtraction.

This PR preserves unary negative values such as:

margin: 0 -@card-spacing;
margin: 0 -(@card-spacing);

while still formatting actual binary subtraction normally:

margin: 0 - @card-spacing;
margin: 0 - (@card-spacing);

Format Flow

flowchart TD
  A["Less input: margin: 2 -@x * 2"] --> B["Raffia AST: LessBinaryOperation"]
  B --> C{"Is spaced unary negative?"}

  C -->|"minus operator, left ends before -, and - touches right"| D["Find the start atom of the right expression"]
  C -->|"otherwise"| E["Print operator with normal spacing"]

  D --> F["If right is nested LessBinaryOperation, follow its left operand"]
  F --> G{"Start atom can be a Less negative value?"}

  G -->|"yes: @var, @@var, $prop, (...), or @var[lookup]"| H["Print operator without suffix space"]
  G -->|"no"| E

  H --> I["Output: margin: 2 -@x * 2"]
  E --> J["Output: binary subtraction formatting"]
  I --> K["Less value-list semantics preserved"]

Loading

Tests

Added Less snapshot coverage for:

• margin: 2 -@card-spacing;
• margin: 2 - @card-spacing;
• margin: 2-@card-spacing;
• margin: 2 -(@card-spacing);
• margin: 2 - (@card-spacing);

Also updated the existing parenthesized negative-value snapshot to preserve Less semantics.

Verified with:

cargo fmt --all --check
RUST_MIN_STACK=33554432 cargo test

• Fixes Less negative values after another value are formatted as binary subtraction #61.

[magic-akari]

Follow-up Fix

After the previous commit, I found one more edge case: Less negative values can be followed by a higher-precedence operation, such as multiplication.

For example:

margin: 2 -@card-spacing * 2;
margin: 2 -(@card-spacing) * 2;

The previous fix handled cases where the right-hand side of the outer LessBinaryOperation was directly a Less negative-value atom, such as @var or (...).

That missed cases where Raffia parses the right-hand side as another LessBinaryOperation. For 2 -@card-spacing * 2, the outer operation is effectively:

left: 2
op: -
right:
  left: @card-spacing
  op: *
  right: 2

So the right-hand side is not directly @card-spacing; it is a multiplication expression whose start atom is @card-spacing.

Side note: this feels like we are fitting the gap between Raffia's parser and Less syntax.

The new fix models this more accurately. Instead of asking whether the entire right-hand side is a Less negative value, it asks whether the right-hand side starts with a value that can be negated in Less. If the right-hand side is a nested LessBinaryOperation, it recursively checks its left operand until it reaches the starting atom.

This preserves cases like:

margin: 2 -@card-spacing * 2;
margin: 2 -@card-spacing / 2;
margin: 2 -(@card-spacing + 1px) * 2;

while still formatting real binary subtraction normally:

margin: 2 - @card-spacing * 2;
margin: 2 - (@card-spacing) * 2;

[g-plane]

This should be a parser bug. I'll fix this in parser.