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Co-authored-by: Camelid <camelidcamel@gmail.com>
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Léo Lanteri Thauvin 2021-03-09 08:23:59 +01:00 committed by Joshua Nelson
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<!-- toc --> <!-- toc -->
The THIR ("Typed High-Level Intermediate Representation"), previously HAIR for 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 "High-Level Abstract IR", is another IR used by rustc that is generated after
[type checking]. It is (as of <!-- date: 2021-03 --> March 2021) only used for [type checking]. It is (as of <!-- date: 2021-03 --> March 2021) only used for
[MIR construction] and [exhaustiveness checking], but [MIR construction] and [exhaustiveness checking], but
@ -16,14 +16,17 @@ for the current MIR unsafety checker.
As the name might suggest, the THIR is a lowered version of the [HIR] where all 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. 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 HIR: 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
- 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 function bodies, but also `const` initializers, for example. Consequently, the THIR
has no representation for items like `struct`s or `trait`s. has no representation for items like `struct`s or `trait`s.
- a body of THIR is only stored temporarily and is dropped as soon as it's no longer
- 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 needed, as opposed to being stored until the end of the compilation process (which
is what is done with the HIR). is what is done with the HIR).
- besides making the types of all nodes available, the THIR also has additional
- Besides making the types of all nodes available, the THIR also has additional
desugaring compared to the HIR. For example, automatic references and dereferences desugaring compared to the HIR. For example, automatic references and dereferences
are made explicit, and method calls and overloaded operators are converted into are made explicit, and method calls and overloaded operators are converted into
plain function calls. Destruction scopes are also made explicit. plain function calls. Destruction scopes are also made explicit.
@ -32,8 +35,8 @@ But it has some other interesting features that distinguish it from HIR:
The THIR lives in [`rustc_mir_build::thir`][thir]. To construct a `thir::Expr`, The THIR lives in [`rustc_mir_build::thir`][thir]. To construct a `thir::Expr`,
you can use the `build_thir` function, passing in the memory arena where the THIR 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: will be allocated. Dropping this arena will result in the THIR being destroyed,
this is useful to keep peak memory in check, as having a THIR representation of 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. all bodies of a crate in memory at the same time would be very heavy.
[thir]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_build/thir/index.html [thir]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_build/thir/index.html