Constant STAGE0_MISSING_TARGETS 

const STAGE0_MISSING_TARGETS: &[&str];

During sanity checks, we search for target tuples to determine if they exist in the compiler’s built-in target list (rustc --print target-list). While a target tuple may be present in the in-tree compiler, the stage 0 compiler might not yet know about it (assuming not operating with local-rebuild). In such cases, we handle the targets missing from stage 0 in this list.

Targets can be removed from this list during the usual release process bootstrap compiler bumps, when the newly-bumped stage 0 compiler now knows about the formerly-missing targets.