Struggling to understand call syntax

help
1

Given:

impl From<A> for Vec<B> { ... }

I'm able to call it successfully like so:

...
    .and_then(Vec::<B>::from)
...

What are the grammatical rules for when to use something like Vec<B> vs Vec::<B>?

2

In expressions (i.e. the stuff inside functions) you need the :: to disambiguate it from a less-than comparison sign. Outside of that, the :: is optional. You can also put <> around something in an expression to use non-expression syntax inside an expression: .and_then(<Vec<B>>::from).

More here: Paths - The Rust Reference

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Also for completness and for some cool rust history here's the reason for the ::<> syntax, which is called the turbofish:

Bastion of the turbofish

//@ check-pass

// Bastion of the Turbofish
// ------------------------
// Beware travellers, lest you venture into waters callous and unforgiving,
// where hope must be abandoned, ere it is cruelly torn from you. For here
// stands the bastion of the Turbofish: an impenetrable fortress holding
// unshaking against those who would dare suggest the supererogation of the
// Turbofish.
//
// Once I was young and foolish and had the impudence to imagine that I could
// shake free from the coils by which that creature had us tightly bound. I
// dared to suggest that there was a better way: a brighter future, in which
// Rustaceans both new and old could be rid of that vile beast. But alas! In
// my foolhardiness my ignorance was unveiled and my dreams were dashed
// unforgivingly against the rock of syntactic ambiguity.
//
// This humble program, small and insignificant though it might seem,
// demonstrates that to which we had previously cast a blind eye: an ambiguity
// in permitting generic arguments to be provided without the consent of the
// Great Turbofish. Should you be so naïve as to try to revolt against its
// mighty clutches, here shall its wrath be indomitably displayed. This
// program must pass for all eternity: forever watched by the guardian angel
// which gave this beast its name, and stands fundamentally at odds with the
// impetuous rebellion against the Turbofish.
//
// My heart aches in sorrow, for I know I am defeated. Let this be a warning
// to all those who come after: for they too must overcome the impassible
// hurdle of defeating the great beast, championed by a resolute winged
// guardian.
//
// Here stands the Bastion of the Turbofish, a memorial to Anna Harren,
// Guardian Angel of these Hallowed Grounds. <3

// See https://github.com/rust-lang/rust/pull/53562
// and https://github.com/rust-lang/rfcs/pull/2527
// and https://web.archive.org/web/20211010063452/https://twitter.com/garblefart/status/1393236602856611843
// for context.

fn main() {
    let (the, guardian, stands, resolute) = ("the", "Turbofish", "remains", "undefeated");
    let _: (bool, bool) = (the<guardian, stands>(resolute));
}
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I'll just quietly leave this here for lore reasons.

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