Wrong compiler error for assignment in `if` conditonal

[anshin]

I'm running a recent nightly (rustc 0.12.0-pre-nightly (0d3bd7720 2014-08-27 21:31:13 +0000)) on x86_64 GNU/Linux. I get this error:

assign.rs:6:18: 6:19 error: expected `:`, found `!`
assign.rs:6             print!("{}", i);
                             ^

with the following program:

fn main() {
    let n = 11u;

    for mut i in range(1u, 10) {
        if i = n {
            print!("{}", i);
        }
    }
}

I don't know whether assignments within if conditions are supposed to be allowed, but the compiler should probably recognize that case and report something more useful. I also have no idea what it wants with the colon.

[ftxqxd]

Assignments in if conditions are not allowed, as the condition has to be a bool but assignments always return (). They are supposed to be allowed syntactically, though. Here you’re seeing the result of an ambiguity in the parser—when the parser reads i = n { ... }, it parses it as an assignment to a struct initialiser:

if i = (n {
    ...
})

instead of an assignment to a variable:

if i = (n) {
    ...
}

(Note that that latter example can be used to force the parser to parse it as expected.) I was under the impression that #14803 was supposed to fix this, making the parser resolve it as the latter instead of the former.

[CameronNemo]

One workaround to get the effect is this:

if { i = n; i } {
    print!("{}", i);
}

[bkoropoff]

Taking a look at this one

[bkoropoff]

It looks like propagating the struct literal restriction through to the RHS expression of an assignment yields a much better error message:

Code

fn main() {
    let x = 1u;
    if x = x {
        println!("{}", x);
    }
}

Before

test.rs:4:16: 4:17 error: expected `:`, found `!`
test.rs:4         println!("{}", x);
                         ^

After

test.rs:3:8: 3:13 error: mismatched types: expected `bool`, found `()` (expected bool, found ())
test.rs:3     if x = x {
                 ^~~~~

I'll see about getting a PR out for this.

[brson]

cc @nick29581

[steveklabnik]

This now gives

hello.rs:5:28: 5:33 error: mismatched types:
 expected `bool`,
    found `()`
(expected bool,
    found ()) [E0308]
hello.rs:5                         if i = n {
                                      ^~~~~

which is much, much better.