Allow multi-statement macros

[sfackler]

Macros can now expand to multiple items, but not multiple statements. This is a bit sad, especially since a macro that can expand in a module can't expand in a function. This will probably require a bit of restructuring in the parser, since there doesn't seem to be a good way to parse a series of expressions. parser::parse_block_tail is almost right, but would need to be pulled apart a bit.

[sfackler]

Macros expanding to statements seem kind of broken right now even without this:

#[feature(macro_rules)]

macro_rules! foo (
    () => (
        struct Foo;
    )
)

fn main() {
    foo!()
}

test.rs:5:2: 5:8 error: found `struct` in ident position
test.rs:5       struct Foo;
                ^~~~~~
test.rs:5:9: 5:12 error: macro expansion ignores token `Foo` and any following
test.rs:5       struct Foo;
                       ^~~
error: aborting due to 2 previous errors

[emberian]

cc @jbclements @paulstansifer

[thehydroimpulse]

@cmr @sfackler Any news on this issue?

[sfackler]

I haven't been working on it.

[treeman]

Still an issue

$ rustc -v
rustc 0.12.0-pre (2e5aea65c 2014-08-23 01:40:48 +0000)

#![feature(macro_rules)]

macro_rules! foo (
    () => (
        struct Foo;
    )
)

fn main() {
    foo!()
}

issue-10681.rs:5:9: 5:15 error: found `struct` in ident position
issue-10681.rs:5         struct Foo;
                         ^~~~~~
issue-10681.rs:5:16: 5:19 error: macro expansion ignores token `Foo` and any following
issue-10681.rs:5         struct Foo;
                                ^~~
error: aborting due to 2 previous errors

Would this require an RFC?

[emberian]

@treeman no, I don't believe so.

[alexcrichton]

@sfackler I feel like this issue is still open, but the original example you have compiles today. Do you have an updated example for this?

[sfackler]

@alexcrichton

macro_rules! foo (
    () => (
        struct Foo;
        struct Bar;
    )
);

fn main() {
    foo!();
}

[durka]

The situation is improved by #24155 but there are still hygiene issues (reported). Here is an example: http://is.gd/sGjll7

[marjakm]

Another example: http://is.gd/oZTig7
Also broken if I give in an ident for vector: http://is.gd/rg1Hh1
The workaround in the durkas example doesn't work in this case because of the lifetime. I couldn't find a way to work around this issue.

[durka]

triage: multi statement macros are still allowed, but broken, because hygiene runs separately on each statement, so any bindings introduced are not accessible, even within the macro expansion.

cc @chris-chambers