Format string parser does not fail on invalid input

[strega-nil]

This code:

struct Foo;

impl Display for Foo {
  fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
    write!(f, "{}", f.fill() as u32)
  }
}

fn main() {
  println!("{:\t}", Foo)
}

Compiles, and prints 32. According to the std::fmt docs,

[[fill]align][sign]['#']['0'][width]['.' precision][type]

is the syntax of format specifications; however, \t does not fulfill any of these elements. I assumed that it might accidentally be implementing the syntax

[fill][align][sign]['#']['0'][width]['.' precision][type]

however, if that were true, then this should print 9 (and not 32). I haven't looked into the parsing code, but will after filing this issue.

[sfackler]

This is just because the fmt parser ignores whitespace I believe: https://github.com/rust-lang/rust/blob/master/src/libfmt_macros/lib.rs#L397-L406.

[strega-nil]

It turns out, it's implementing the syntax

[[fill]align][sign]['#']['0'][width]['.' precision][type]ws*

due to a bug in this commit from 7 years ago: a447c3c

@sfackler whitespace is only ignored before the final }, and this doesn't do anything, so I would say it's at least a docs bug; it should also probably warn, as this should never be intentional.