compiler/rustc_mir_dataflow/src/storage.rs

use rustc_index::bit_set::BitSet;
use rustc_middle::mir::{self, Local};

/// The set of locals in a MIR body that do not have `StorageLive`/`StorageDead` annotations.
///
/// These locals have fixed storage for the duration of the body.
//
// FIXME: Currently, we need to traverse the entire MIR to compute this. We should instead store it
// as a field in the `LocalDecl` for each `Local`.
pub fn always_live_locals(body: &mir::Body<'_>) -> BitSet<Local> {
    let mut always_live_locals = BitSet::new_filled(body.local_decls.len());

    for block in body.basic_blocks() {
        for statement in &block.statements {
            use mir::StatementKind::{StorageDead, StorageLive};
            if let StorageLive(l) | StorageDead(l) = statement.kind {
                always_live_locals.remove(l);
            }
        }
    }

    always_live_locals
}