RFC 0378: expr-macros

lang (macros)

• Start Date: 2014-10-09
• RFC PR #: https://github.com/rust-lang/rfcs/pull/378
• Rust Issue #: https://github.com/rust-lang/rust/issues/18635

Summary

Parse macro invocations with parentheses or square brackets as expressions no matter the context, and require curly braces or a semicolon following the invocation to invoke a macro as a statement.

Motivation

Currently, macros that start a statement want to be a whole statement, and so expressions such as foo!().bar don’t parse if they start a statement. The reason for this is because sometimes one wants a macro that expands to an item or statement (for example, macro_rules!), and forcing the user to add a semicolon to the end is annoying and easy to forget for long, multi-line statements. However, the vast majority of macro invocations are not intended to expand to an item or statement, leading to frustrating parser errors.

Unfortunately, this is not as easy to resolve as simply checking for an infix operator after every statement-like macro invocation, because there exist operators that are both infix and prefix. For example, consider the following function:

fn frob(x: int) -> int {
    maybe_return!(x)
    // Provide a default value
    -1
}

Today, this parses successfully. However, if a rule were added to the parser that any macro invocation followed by an infix operator be parsed as a single expression, this would still parse successfully, but not in the way expected: it would be parsed as (maybe_return!(x)) - 1. This is an example of how it is impossible to resolve this ambiguity properly without breaking compatibility.

Detailed design

Treat all macro invocations with parentheses, (), or square brackets, [], as expressions, and never attempt to parse them as statements or items in a block context unless they are followed directly by a semicolon. Require all item-position macro invocations to be either invoked with curly braces, {}, or be followed by a semicolon (for consistency).

This distinction between parentheses and curly braces has precedent in Rust: tuple structs, which use parentheses, must be followed by a semicolon, while structs with fields do not need to be followed by a semicolon. Many constructs like match and if, which use curly braces, also do not require semicolons when they begin a statement.

Drawbacks

• This introduces a difference between different macro invocation delimiters, where previously there was no difference.
• This requires the use of semicolons in a few places where it was not necessary before.

Alternatives

• Require semicolons after all macro invocations that aren’t being used as expressions. This would have the downside of requiring semicolons after every macro_rules! declaration.

Unresolved questions

None.