Basically, the three most viable syntaxes were already discussed in various threads:
// The least noisy syntax, and well fitting because `@` has "location" semantics.
@a::b::c
// Slightly more noisy syntax, and memorable because `[ ### ]` is a *crate*.
[a]::b::c
// The "no sigils" variant, good if `use extern a;` will be used roughly like
// `extern crate a;` is used now, with "inline" uses being rare. Probably too
// wordy if used more often (e.g. in every `use` import).
extern a::b::c