Function rustc_metadata::rmeta::encoder::should_encode_mir

fn should_encode_mir(
    tcx: TyCtxt<'_>,
    reachable_set: &LocalDefIdSet,
    def_id: LocalDefId,
) -> (bool, bool)

Whether we should encode MIR. Return a pair, resp. for CTFE and for LLVM.

Computing, optimizing and encoding the MIR is a relatively expensive operation. We want to avoid this work when not required. Therefore:

• we only compute mir_for_ctfe on items with const-eval semantics;
• we skip optimized_mir for check runs.
• we only encode optimized_mir that could be generated in other crates, that is, a code that is either generic or has inline hint, and is reachable from the other crates (contained in reachable set).

Note: Reachable set describes definitions that might be generated or referenced from other crates and it can be used to limit optimized MIR that needs to be encoded. On the other hand, the reachable set doesn’t have much to say about which definitions might be evaluated at compile time in other crates, so it cannot be used to omit CTFE MIR. For example, f below is unreachable and yet it can be evaluated in other crates:

const fn f() -> usize { 0 }
pub struct S { pub a: [usize; f()] }