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Tolerate malformed comments in shebang lookahead #2331

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traviscross merged 1 commit into from
Aug 18, 2026
Merged

Tolerate malformed comments in shebang lookahead #2331

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19 changes: 16 additions & 3 deletions src/shebang.md
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Expand Up @@ -17,18 +17,31 @@ A *[shebang]* is an optional line that is typically used in Unix-like systems to
r[shebang.syntax]
```grammar,lexer
@root SHEBANG ->
`#!` !((WHITESPACE | LINE_COMMENT | BLOCK_COMMENT)* `[`)
`#!` !((WHITESPACE | LINE_COMMENT | SHEBANG_BLOCK_COMMENT)* `[`)
~LF* (LF | EOF)

SHEBANG_BLOCK_COMMENT ->
`/*` !(`!` | `*` ![`*` `/`])
( SHEBANG_NESTED_BLOCK_COMMENT | (!(`*/` | `/*`) CHAR) )*
`*/`

SHEBANG_NESTED_BLOCK_COMMENT ->
`/*`
( SHEBANG_NESTED_BLOCK_COMMENT | (!(`*/` | `/*`) CHAR) )*
`*/`
```

r[shebang.syntax-description]
The shebang starts with the characters `#!` and extends through the first `U+000A` (LF) or through EOF if no LF is present. If the `#!` characters are followed by `[` (ignoring any intervening [comments] or [whitespace]), the line is not considered a shebang (to avoid ambiguity with an [inner attribute]).
The shebang starts with the characters `#!` and extends through the first `U+000A` (LF) or through EOF if no LF is present. If the `#!` characters are followed by `[` (ignoring any intervening [whitespace] or [non-doc comments]), the line is not considered a shebang (to avoid ambiguity with an [inner attribute]).

> [!NOTE]
> Doc comments are not ignored when determining whether `[` follows the `#!` characters. For example, `#! /*! */ [allow(unused)]` at the start of a file is a shebang, not an inner attribute, because `/*! */` is a doc comment. Likewise, text following `#!` that resembles an unterminated block comment, as in `#!/*`, does not cause an error; the line is a shebang.

r[shebang.position]
The shebang may appear immediately at the start of the file or after the optional [byte order mark].

[byte order mark]: https://en.wikipedia.org/wiki/Byte_order_mark#UTF-8
[comments]: comments.md
[inner attribute]: attributes.md
[non-doc comments]: comments.normal
[shebang]: https://en.wikipedia.org/wiki/Shebang_(Unix)
[whitespace]: whitespace.md
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