unnecessary path disambiguator slips past deny(warnings)

[tspiteri]

#[deny(warnings)]
pub fn foo() {
    type _V = Vec::<u32>;
}

Although there is a deny(warnings) attribute, this code compiles with a warning only.

   Compiling warn v0.1.0 (/tmp/warn)
warning: unnecessary path disambiguator
 --> src/lib.rs:3:18
  |
3 |     type _V = Vec::<u32>;
  |                  ^^ try removing `::`

    Finished dev [unoptimized + debuginfo] target(s) in 0.23s

This behavior is present in all versions I tested including the latest nightly 2019-01-31 and 1.21.0 when relaxed path syntax was introduced.

[zackmdavis]

This happens because the warning is issued unconditionally, rather than under the purview of a lint:

rust/src/libsyntax/parse/parser.rs

Lines 2266 to 2267 in c9a8687

	self.diagnostic().struct_span_warn(self.prev_span, "unnecessary path disambiguator")
	.span_label(self.prev_span, "try removing `::`").emit();

Reservations were expressed at the time. (@estebank do we have a policy anywhere on unsilenceable warnings? It seems kind of bad for it to be a thing the compiler mostly doesn't do, except sometimes very rarely when it didn't feel "worthy" of a top-level lint.)

[petrochenkov]

All hard-coded warnings should generally be subsumed by lints or eliminated.
For this specific warning #57565 does the job.