Semicolons are not required on some statements in macros

[jseyfried]

For example, this compiles:

macro_rules! m { () => {
    let x = 1
    let y = 2
    println!("{}", x)
    println!("{}", y)
} }

fn main() {
    m!();
}

[durka]

Who knew that Rust had optional semicolon syntax built in‽ That must mean it's web scale! This is the best feature discovery since fn foo(*)!

[alexcrichton]

Nominating as this seems like it may actually be likely to cause fallout when fixed

[jseyfried]

I fixed this in #34660.

[dashed]

I'm unsure if this is related, but I'm getting these errors on nightly for compiling chomp library:

<chomp macros>:5:59: 5:60 error: expected expression, found `.`
<chomp macros>:5 __parse_internal ! { @ EXPR ( $ input ; ) $ ( $ exp ) * } . bind (
                                                                           ^
<chomp macros>:9:59: 9:60 error: expected expression, found `.`
<chomp macros>:9 __parse_internal ! { @ EXPR ( $ input ; ) $ ( $ exp ) * } . bind (
                                                                           ^
<chomp macros>:5:59: 5:60 error: expected expression, found `.`
<chomp macros>:5 __parse_internal ! { @ EXPR ( $ input ; ) $ ( $ exp ) * } . bind (
                                                                           ^
<chomp macros>:9:59: 9:60 error: expected expression, found `.`
<chomp macros>:9 __parse_internal ! { @ EXPR ( $ input ; ) $ ( $ exp ) * } . bind (
                                                                           ^
<chomp macros>:9:59: 9:60 error: expected expression, found `.`
<chomp macros>:9 __parse_internal ! { @ EXPR ( $ input ; ) $ ( $ exp ) * } . bind (

Build fail example: https://travis-ci.org/dashed/chomp/jobs/142891865

---

This is my attempted fix for this: dashed/chomp@9c62b9f

which passes on nightly: https://travis-ci.org/dashed/chomp/builds/142892066

[jseyfried]

@dashed, yeah -- that is related.
After #34660 lands, you still won't be able to use a braced macro invocation at the start of an expression in a statement position, just like you can't do this:

fn main() {
    if true { 0 } else { 1 } + 2; // This is parsed as if there were a semicolon after the `}`
}

However, after #34660 lands, you will be able to used a non-braced macro invocation at the start of an expression in a statement position (currently you can't do this in macro-expanded code but you can do it in unexpanded code). That is, after #34660, changing the __parse_internal! invocation to use parenthesis or brackets instead of braces would fix it.

[pnkfelix]

We are treating this as a bug that PR #34660 will fix. There are still some crater regressions to address.

[nrc]

triage: p-high

[pnkfelix]

P-high