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The THIR

The THIR ("Typed High-Level Intermediate Representation"), previously called HAIR for "High-Level Abstract IR", is another IR used by rustc that is generated after type checking. It is (as of March 2021) only used for MIR construction and exhaustiveness checking, but it may also soon be used for unsafety checking as a replacement for the current MIR unsafety checker.

As the name might suggest, the THIR is a lowered version of the HIR where all the types have been filled in, which is possible after type checking has completed. But it has some other interesting features that distinguish it from the HIR:

  • Like the MIR, the THIR only represents bodies, i.e. "executable code"; this includes function bodies, but also const initializers, for example. Consequently, the THIR has no representation for items like structs or traits.

  • Each body of THIR is only stored temporarily and is dropped as soon as it's no longer needed, as opposed to being stored until the end of the compilation process (which is what is done with the HIR).

  • Besides making the types of all nodes available, the THIR also has additional desugaring compared to the HIR. For example, automatic references and dereferences are made explicit, and method calls and overloaded operators are converted into plain function calls. Destruction scopes are also made explicit.

The THIR lives in rustc_mir_build::thir. To construct a thir::Expr, you can use the build_thir function, passing in the memory arena where the THIR will be allocated. Dropping this arena will result in the THIR being destroyed, which is useful to keep peak memory in check. Having a THIR representation of all bodies of a crate in memory at the same time would be very heavy.