src/thir.md - rust-lang/rustc-dev-guide - Git at Google

 # The THIR

 <!-- toc -->

 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 <!-- date-check --> January 2024) used for
 [MIR construction], [exhaustiveness checking], and [unsafety checking].

 [type checking]: ./type-checking.md
 [MIR construction]: ./mir/construction.md
 [exhaustiveness checking]: ./pat-exhaustive-checking.md
 [unsafety checking]: ./unsafety-checking.md

 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. Specifically, all [body owners] have
   THIR created. Consequently, the THIR has no representation for items like `struct`s or `trait`s.

 - 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.

 - Statements, expressions, and match arms are stored separately. For example, statements in the
   `stmts` array reference expressions by their index (represented as a [`ExprId`]) in the `exprs`
   array.

 [HIR]: ./hir.md
 [`ExprId`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/thir/struct.ExprId.html
 [body owners]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/enum.BodyOwnerKind.html

 The THIR lives in [`rustc_mir_build::thir`][thir-docs]. 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:

 ```rust
 fn main() {
     let x = 1 + 2;
 }
 ```

 Here is how that gets represented in THIR (as of <!-- date-check --> Aug 2022):

 ```rust,no_run
 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),
             ),
         },
     ],
 }
 ```

 [thir-docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_build/thir/index.html
 [`thir::Expr`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/thir/struct.Expr.html
 [`thir_body`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.thir_body
	# The THIR

	<!-- toc -->

	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 <!-- date-check --> January 2024) used for
	[MIR construction], [exhaustiveness checking], and [unsafety checking].

	[type checking]: ./type-checking.md
	[MIR construction]: ./mir/construction.md
	[exhaustiveness checking]: ./pat-exhaustive-checking.md
	[unsafety checking]: ./unsafety-checking.md

	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. Specifically, all [body owners] have
	THIR created. Consequently, the THIR has no representation for items like `struct`s or `trait`s.

	- 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.

	- Statements, expressions, and match arms are stored separately. For example, statements in the
	`stmts` array reference expressions by their index (represented as a [`ExprId`]) in the `exprs`
	array.

	[HIR]: ./hir.md
	[`ExprId`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/thir/struct.ExprId.html
	[body owners]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/enum.BodyOwnerKind.html

	The THIR lives in [`rustc_mir_build::thir`][thir-docs]. 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:

	```rust
	fn main() {
	let x = 1 + 2;
	}
	```

	Here is how that gets represented in THIR (as of <!-- date-check --> Aug 2022):

	```rust,no_run
	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),
	),
	},
	],
	}
	```

	[thir-docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_build/thir/index.html
	[`thir::Expr`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/thir/struct.Expr.html
	[`thir_body`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.thir_body