What is this warning?

[nagisa]

src/librustc_mir/pretty.rs:20:36: 20:37 warning: whoops, no =?
src/librustc_mir/pretty.rs:20                                   w: &mut W) -> io::Result<()> {
                                                                 ^

[jonas-schievink]

https://github.com/rust-lang/rust/blame/master/src/libsyntax/parse/parser.rs#L4339

cc @nrc

[nagisa]

The code which caused this was similar to

pub fn write_mir_pretty<'a, W, I>(iter: &Iterator<Item=(&'a NodeId, &'a Mir<'a>), w: &mut W) -> io::Result<()>

note the missing > in iterator parameter list.

The warning with signature on one line:

src/librustc_mir/pretty.rs:19:84: 19:85 warning: whoops, no =?
src/librustc_mir/pretty.rs:19 pub fn write_mir_pretty<'a, W, I>(iter: &Iterator<Item=(&'a NodeId, &'a Mir<'a>), w: &mut W) -> io::Result<()>
                                                                                                                 ^

[jonas-schievink]

This also causes the warning:

fn f<T:Iterator<Item=(), w>() {}

<anon>:1:27: 1:28 warning: whoops, no =?
<anon>:1 fn f<T:Iterator<Item=(), w>() {}
                                   ^
<anon>:1:27: 1:28 error: expected type, found `>`
<anon>:1 fn f<T:Iterator<Item=(), w>() {}
                                   ^

[nagisa]

@durka this is a parser and it has no/should not have notion of any such things.

I propose “error: expected =, found ...”, personally.

[nrc]

I'm a bit surprised that we bother to warn at all. Either it's an error and we should give an error (with a better message). Or we should not emit anything and rely on the error being caught later on (perhaps with a more informative error message).

In either case, we should explain why there is an error here - because you can't have unbound type variables after bound ones.