Always parse macro invocations with () or [] as expressions

[ftxqxd]

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.

Resolves #364.

I think #208 also does this as a by-product of resolving another ambiguity with attributes.

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Status: Active
RFC document: https://github.com/rust-lang/rfcs/blob/master/text/0378-expr-macros.md

[pnkfelix]

I can shepherd this. Assigning to self.

[reem]

+1. I would like this change because it would make expression macros much more usable, especially those like the one in https://github.com/reem/rust-enforce.

[nrc]

+1 I hope the semicolon rule for macros as statements (as opposed to as items) falls out naturally from the syntax for expressions and statements.

[brson]

Curious what @pcwalton and @nikomatsakis think. I don't have strong opinion but it seems to be more consistent with non-macro stmt/expr parsing.

ISTR that macro!().method_call() doesn't parse correctly. Presumably that would be fixed by this proposal? If so it would be nice to call it out explicitly in the RFC.

This RFC is not using the template format for Rust RFCs.

[ftxqxd]

@brson The macro!().method_call() problem (which this RFC resolves) is already mentioned in the RFC.

[nikomatsakis]

I am generally in favor of this. As @P1start noted, we had similar rules as part of the (now withdrawn) #208 RFC. One thing to clarify: these same restrictions would apply in all contexts, right? That is, also at the top-level:

declare_item!() // <-- error, semi colon required

fn foo() {
    declare_item!();
}

If so, I am in favor because it improves the overall consistency of the language. @foo() as an item declaration without a semicolon always looks wrong to me, it feels like items/statements should end in {} or ;.

[ftxqxd]

Yes, these rules apply to item-position macros, too. The RFC states that it will ‘require all item-position macros to be invoked with either curly braces, {}, or be followed by a semicolon (for consistency)’.

[nikomatsakis]

@P1start sounds good to me, I missed that somehow.

[pnkfelix]

Accepted (squashed and merged as 4738e17 )

discussion, tracking