`rustc_scalable_vector(N)`
Supercedes rust-lang/rust#118917.
Initial experimental implementation of rust-lang/rfcs#3838. Introduces a `rustc_scalable_vector(N)` attribute that can be applied to types with a single `[$ty]` field (for `u{16,32,64}`, `i{16,32,64}`, `f{32,64}`, `bool`). `rustc_scalable_vector` types are lowered to scalable vectors in the codegen backend.
As with any unstable feature, there will necessarily be follow-ups as we experiment and find cases that we've not considered or still need some logic to handle, but this aims to be a decent baseline to start from.
See rust-lang/rust#145052 for request for a lang experiment.
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