What are subtype lifetimes

Subtype lifetimes are the compiler's internal mechanism for verifying that one lifetime region outlives another during type checking. The Rust compiler uses HigherRankedSubtypeError diagnostics when it cannot prove that a generic lifetime constraint holds for all possible substitutions. This check occurs in the borrow checker's type relation logic, specifically when relating types with binders like for<'a> fn(&'a T). If the compiler cannot establish the required outlives relationship, it reports a higher-ranked subtype error.

Subtype lifetimes are the rules the compiler uses to ensure a reference doesn't outlive the data it points to. Think of it as a safety check that guarantees a borrowed item won't disappear while you are still using it. If the compiler can't prove this safety for every possible scenario, it stops the code from compiling.