ch as u32 < 0x80 syntax misinterpreted as a type parameter

[SimonSapin]

Code like this:

    pub fn push(&mut self, ch: char) {
        if ch as u32 < 0x80 {
            // ...

Gives a parse error:

error: expected type, found `0x80`
        if ch as u32 < 0x80 {
                       ^~~~

Adding parentheses to disambiguate works around the issue:

    pub fn push(&mut self, ch: char) {
        if (ch as u32) < 0x80 {
            // ...

I understand that u32 is being parsed as a type and so < is assumed to be the start of type parameters. But maybe when that fails, the parser should backtrack and try to parse < as an operator?

[bombless]

I think this is a little similar as #22644 .
I imagine if we change the parser's front-end to parse this ideally, of course it is potentially a breaking-change, but I doubt if it would actually break a single line of existing code.

[huonw]

There's been more discussion on #22644, so I'm closing this as a dupe of that.