https://doc.rust-lang.org/beta/nightly-rustc/rustc_span/source_map/monotonic/struct.MonotonicVec.html
MonotonicVec in rustc_span::source_map::monotonic - Rust
A `MonotonicVec` is a `Vec` which can only be grown. Once inserted, an element can never be removed or swapped, guaranteeing that any indices into a...
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