There are 3 kinds of path styles:
Module, generic parameters disallowed i.e.
Result::Err. These styles of paths are used in:
- Macro invocations
- Module reference (
Type, double-colons prohibited before generic parameters i.e.
Result<T, U>::Err. These styles of paths are used in:
- Type and trait references
- Macros 1.0
Expression, double-colons required before generic parameters i.e.
(In a qualified path
<P as Trait>::Q, if the whole path uses style X, the inner path P is also parsed using style X.)
The oddity is
$x:path is parsed where a type is expected (i.e.
Vec<i32>::new is parsed like an inner type called
new inside the struct
Vec<i32>). After a path is parsed the style information is thrown out, so it can be used like an expression
$x(). This causes the strange syntax mismatch here.
I'm not sure if this is something Macros 2.0 (
#![feature(decl_macro)]) is going to improve or worsen here, as I heard that
$x there will eventually just mean the token stream
Vec<i32>::new without any special meaning, so it will become a syntax error.