Keywords are not handled as keywords when immediately followed by ::

[huonw]

fn main() {
    struct::foo();
    mut::baz();
}

<anon>:2:5: 2:16 error: failed to resolve. Use of undeclared module `struct`
<anon>:2     struct::foo();
             ^~~~~~~~~~~
<anon>:2:5: 2:16 error: unresolved name `struct::foo`.
<anon>:2     struct::foo();
             ^~~~~~~~~~~
<anon>:3:5: 3:13 error: failed to resolve. Use of undeclared module `mut`
<anon>:3     mut::baz();
             ^~~~~~~~
<anon>:3:5: 3:13 error: unresolved name `mut::baz`.
<anon>:3     mut::baz();
             ^~~~~~~~

Due to the falses:

• rust/src/libsyntax/parse/parser.rs

  Line 579 in e6a3dab

  token::IDENT(sid, false) if kw.to_name() == sid.name => {

• rust/src/libsyntax/parse/token.rs

  Line 704 in e6a3dab

  token::IDENT(sid, false) => { kw.to_name() == sid.name }

• rust/src/libsyntax/parse/token.rs

  Line 711 in e6a3dab

  token::IDENT(sid, false) => {

(etc.)

Adding a space, e.g. struct ::foo() causes errors like expected ident, found ::``, while a normal module path can freely have spaces.

[huonw]

Ah, this allows

fn qux() {
    println!("hi");
}
fn main() {
    self::qux();
}

to work. (Is still very weird though.)

[huonw]

Nominating for something (doesn't seem backcompat-lang though, but I could be wrong).

[pnkfelix]

Assigning P-backcompat-lang, 1.0 (team members pointed out e.g. that macros could read in such a path and rewrite it to something that is valid).