Constant RETPOLINE_ENABLED 

pub const RETPOLINE_ENABLED: bool = false;

Whether retpoline (compiler-level Spectre v2 mitigation) is active.

Retpoline replaces indirect calls (call *%rax) with a construct that traps speculation in an infinite pause loop. It is enabled at the compiler level (e.g., -Cllvm-args=-x86-speculative-load-hardening or equivalent). Because Rust does not expose a target_feature for retpoline, this is a compile-time constant that should be updated when the build system enables retpoline flags.