What Is MIR (Mid-Level IR) in Rust?

MIR (Mid-Level Intermediate Representation) is an internal, platform-independent representation of Rust code used by the compiler for optimization and analysis, not a user-facing tool or syntax. It sits between the high-level HIR and the low-level LLVM IR, allowing the compiler to perform complex transformations like monomorphization and borrow checking before generating machine code. You interact with MIR indirectly through compiler flags like -Z unpretty=expanded or tools like cargo miri for unsafe code analysis, but you do not write or edit MIR directly.

MIR is like a translator's rough draft that the Rust compiler uses to check your code for errors and make it faster before turning it into a final program. You don't write in MIR; the compiler creates it automatically to ensure your code is safe and efficient. Think of it as the compiler's internal scratchpad where it figures out exactly how your code should run.