To explain this, since it took me a minute:

Rust has a syntax called "turbofish" for disambiguating ambiguous generic calls. (Coined by Anna Harren on twitter, the first reply is an illustration from Karen Rustad Tölva, the inventor of Ferris [1]).

Although Vec is generic over any item type, you can normally just write

    let foo = Vec::new();

If you're doing something complicated where the compiler can't infer the type, you instead use the turbofish syntax

    let foo = Vec::<u32>::new();

Some people hate this syntax. One of the better arguments I've seen against it comes (I think) from ManishEarth, who points out that it's very hard for the compiler to detect that incorrect attempts at writing a turbofish are in fact attempts at the turbofish and offer help.

That test case (minus some excellent poetry) is

    let (oh, woe, is, me) = ("the", "Turbofish", "remains", "undefeated");
    let _: (bool, bool) = (oh<woe, is>(me));

That is actually comparing the string "the" against the string "Turbofish" and the string "remains" against the string "undefeated". The test case is there to point out that if the "::" in the turbofish is removed (Vec<u32>::new() instead of Vec::<u32>::new()) there will be cases where it's ambiguous.

[1] https://twitter.com/whoisaldeka/status/914914008225816576?la...

EDIT: Fix my misspelling of ManishEarth