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 January 2024) used for MIR construction, exhaustiveness checking, and unsafety checking.
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:
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Like the MIR, the THIR only represents bodies, i.e. "executable code"; this includes function bodies, but also
constinitializers, for example. Specifically, all body owners have THIR created. Consequently, the THIR has no representation for items likestructs ortraits. -
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).
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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.
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Statements, expressions, and match arms are stored separately. For example, statements in the
stmtsarray reference expressions by their index (represented as aExprId) in theexprsarray.
The THIR lives in rustc_mir_build::thir. To construct a thir::Expr,
you can use the thir_body 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.
You can get a debug representation of the THIR by passing the -Zunpretty=thir-tree flag
to rustc.
To demonstrate, let's use the following example:
fn main() { let x = 1 + 2; }
Here is how that gets represented in THIR (as of Aug 2022):
#![allow(unused)] fn main() { Thir { // no match arms arms: [], exprs: [ // expression 0, a literal with a value of 1 Expr { ty: i32, temp_lifetime: Some( Node(1), ), span: oneplustwo.rs:2:13: 2:14 (#0), kind: Literal { lit: Spanned { node: Int( 1, Unsuffixed, ), span: oneplustwo.rs:2:13: 2:14 (#0), }, neg: false, }, }, // expression 1, scope surrounding literal 1 Expr { ty: i32, temp_lifetime: Some( Node(1), ), span: oneplustwo.rs:2:13: 2:14 (#0), kind: Scope { // reference to expression 0 above region_scope: Node(3), lint_level: Explicit( HirId { owner: DefId(0:3 ~ oneplustwo[6932]::main), local_id: 3, }, ), value: e0, }, }, // expression 2, literal 2 Expr { ty: i32, temp_lifetime: Some( Node(1), ), span: oneplustwo.rs:2:17: 2:18 (#0), kind: Literal { lit: Spanned { node: Int( 2, Unsuffixed, ), span: oneplustwo.rs:2:17: 2:18 (#0), }, neg: false, }, }, // expression 3, scope surrounding literal 2 Expr { ty: i32, temp_lifetime: Some( Node(1), ), span: oneplustwo.rs:2:17: 2:18 (#0), kind: Scope { region_scope: Node(4), lint_level: Explicit( HirId { owner: DefId(0:3 ~ oneplustwo[6932]::main), local_id: 4, }, ), // reference to expression 2 above value: e2, }, }, // expression 4, represents 1 + 2 Expr { ty: i32, temp_lifetime: Some( Node(1), ), span: oneplustwo.rs:2:13: 2:18 (#0), kind: Binary { op: Add, // references to scopes surrounding literals above lhs: e1, rhs: e3, }, }, // expression 5, scope surrounding expression 4 Expr { ty: i32, temp_lifetime: Some( Node(1), ), span: oneplustwo.rs:2:13: 2:18 (#0), kind: Scope { region_scope: Node(5), lint_level: Explicit( HirId { owner: DefId(0:3 ~ oneplustwo[6932]::main), local_id: 5, }, ), value: e4, }, }, // expression 6, block around statement Expr { ty: (), temp_lifetime: Some( Node(9), ), span: oneplustwo.rs:1:11: 3:2 (#0), kind: Block { body: Block { targeted_by_break: false, region_scope: Node(8), opt_destruction_scope: None, span: oneplustwo.rs:1:11: 3:2 (#0), // reference to statement 0 below stmts: [ s0, ], expr: None, safety_mode: Safe, }, }, }, // expression 7, scope around block in expression 6 Expr { ty: (), temp_lifetime: Some( Node(9), ), span: oneplustwo.rs:1:11: 3:2 (#0), kind: Scope { region_scope: Node(9), lint_level: Explicit( HirId { owner: DefId(0:3 ~ oneplustwo[6932]::main), local_id: 9, }, ), value: e6, }, }, // destruction scope around expression 7 Expr { ty: (), temp_lifetime: Some( Node(9), ), span: oneplustwo.rs:1:11: 3:2 (#0), kind: Scope { region_scope: Destruction(9), lint_level: Inherited, value: e7, }, }, ], stmts: [ // let statement Stmt { kind: Let { remainder_scope: Remainder { block: 8, first_statement_index: 0}, init_scope: Node(1), pattern: Pat { ty: i32, span: oneplustwo.rs:2:9: 2:10 (#0), kind: Binding { mutability: Not, name: "x", mode: ByValue, var: LocalVarId( HirId { owner: DefId(0:3 ~ oneplustwo[6932]::main), local_id: 7, }, ), ty: i32, subpattern: None, is_primary: true, }, }, initializer: Some( e5, ), else_block: None, lint_level: Explicit( HirId { owner: DefId(0:3 ~ oneplustwo[6932]::main), local_id: 6, }, ), }, opt_destruction_scope: Some( Destruction(1), ), }, ], } }