MIR construction

The lowering of HIR to MIR occurs for the following (probably incomplete) list of items:

  • Function and Closure bodies
  • Initializers of static and const items
  • Initializers of enum discriminants
  • Glue and Shims of any kind
    • Tuple struct initializer functions
    • Drop code (the Drop::drop function is not called directly)
    • Drop implementations of types without an explicit Drop implementation

The lowering is triggered by calling the mir_built query. There is an intermediate representation between HIR and MIR called the HAIR that is only used during the lowering. The HAIR's most important feature is that the various adjustments (which happen without explicit syntax) like coercions, autoderef, autoref and overloaded method calls have become explicit casts, deref operations, reference expressions or concrete function calls.

The HAIR has datatypes that mirror the HIR datatypes, but instead of e.g. -x being a hair::ExprKind::Neg(hair::Expr) it is a hair::ExprKind::Neg(hir::Expr). This shallowness enables the HAIR to represent all datatypes that HIR has, but without having to create an in-memory copy of the entire HIR. MIR lowering will first convert the topmost expression from HIR to HAIR (in rustc_mir::hair::cx::expr) and then process the HAIR expressions recursively.

The lowering creates local variables for every argument as specified in the signature. Next it creates local variables for every binding specified (e.g. (a, b): (i32, String)) produces 3 bindings, one for the argument, and two for the bindings. Next it generates field accesses that read the fields from the argument and writes the value to the binding variable.

With this initialization out of the way, the lowering triggers a recursive call to a function that generates the MIR for the body (a Block expression) and writes the result into the RETURN_PLACE.

unpack! all the things

Functions that generate MIR tend to fall into one of two patterns. First, if the function generates only statements, then it will take a basic block as argument onto which those statements should be appended. It can then return a result as normal:

fn generate_some_mir(&mut self, block: BasicBlock) -> ResultType {

But there are other functions that may generate new basic blocks as well. For example, lowering an expression like if foo { 22 } else { 44 } requires generating a small "diamond-shaped graph". In this case, the functions take a basic block where their code starts and return a (potentially) new basic block where the code generation ends. The BlockAnd type is used to represent this:

fn generate_more_mir(&mut self, block: BasicBlock) -> BlockAnd<ResultType> {

When you invoke these functions, it is common to have a local variable block that is effectively a "cursor". It represents the point at which we are adding new MIR. When you invoke generate_more_mir, you want to update this cursor. You can do this manually, but it's tedious:

let mut block;
let v = match self.generate_more_mir(..) {
    BlockAnd { block: new_block, value: v } => {
        block = new_block;

For this reason, we offer a macro that lets you write let v = unpack!(block = self.generate_more_mir(...)). It simply extracts the new block and overwrites the variable block that you named in the unpack!.

Lowering expressions into the desired MIR

There are essentially four kinds of representations one might want of an expression:

  • Place refers to a (or part of a) preexisting memory location (local, static, promoted)
  • Rvalue is something that can be assigned to a Place
  • Operand is an argument to e.g. a + operation or a function call
  • a temporary variable containing a copy of the value

These following image depicts a general overview of the interactions between the representations:

Click here for a more detailed view

We start out with lowering the function body to an Rvalue so we can create an assignment to RETURN_PLACE, This Rvalue lowering will in turn trigger lowering to Operand for its arguments (if any). Operand lowering either produces a const operand, or moves/copies out of a Place, thus triggering a Place lowering. An expression being lowered to a Place can in turn trigger a temporary to be created if the expression being lowered contains operations. This is where the snake bites its own tail and we need to trigger an Rvalue lowering for the expression to be written into the local.

Operator lowering

Operators on builtin types are not lowered to function calls (which would end up being infinite recursion calls, because the trait impls just contain the operation itself again). Instead there are Rvalues for binary and unary operators and index operations. These Rvalues later get codegened to llvm primitive operations or llvm intrinsics.

Operators on all other types get lowered to a function call to their impl of the operator's corresponding trait.

Regardless of the lowering kind, the arguments to the operator are lowered to Operands. This means all arguments are either constants, or refer to an already existing value somewhere in a local or static.

Method call lowering

Method calls are lowered to the same TerminatorKind that function calls are. In MIR there is no difference between method calls and function calls anymore.


if conditions and match statements for enums without variants with fields are lowered to TerminatorKind::SwitchInt. Each possible value (so 0 and 1 for if conditions) has a corresponding BasicBlock to which the code continues. The argument being branched on is (again) an Operand representing the value of the if condition.

Pattern matching

match statements for enums with variants that have fields are lowered to TerminatorKind::SwitchInt, too, but the Operand refers to a Place where the discriminant of the value can be found. This often involves reading the discriminant to a new temporary variable.

Aggregate construction

Aggregate values of any kind (e.g. structs or tuples) are built via Rvalue::Aggregate. All fields are lowered to Operators. This is essentially equivalent to one assignment statement per aggregate field plus an assignment to the discriminant in the case of enums.