HIR
- Rust course
- Expert
- Lesson 1/46
- 4 min
Learn what High-level IR (HIR) is and what it does.
HIR
rustc passes code through several intermediate representations: text → tokens → AST (macro expansion, name resolution) → HIR → THIR → MIR → LLVM IR → machine code. HIR (High-level IR) is the AST with its "sugar" removed: for, while let, ? and async are lowered to more basic constructs. Type checking and trait resolution happen on HIR.
for x in v { .. }
// roughly, in HIR:
match IntoIterator::into_iter(v) {
mut it => loop {
match Iterator::next(&mut it) {
Some(x) => { .. }
None => break,
}
},
}Lowering the ? operator
? also becomes a match in HIR; the From conversion is added at this step:
let n = s.parse::<i32>()?;
// roughly:
let n = match s.parse::<i32>() {
Ok(v) => v,
Err(e) => return Err(From::from(e)),
};That's why sugared and lowered code produce the same machine code; syntactic conveniences have no runtime cost.
Practice Hand-lowered for
Sum the vector vec![1, 2, 3] without for, as in HIR, using IntoIterator::into_iter, loop and Iterator::next, and print the result.
Starter code
fn main() {
// buraya yaz
}Sample solution
fn main() {
let v = vec![1, 2, 3];
let mut toplam = 0;
let mut it = IntoIterator::into_iter(v);
loop {
match Iterator::next(&mut it) {
Some(x) => toplam += x,
None => break,
}
}
println!("{}", toplam);
}Expected output
6
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is the easiest way to see a macro's expanded form?
- cargo expand (or rustc -Zunpretty=expanded) Correct answer
- cargo build --verbose
- rustfmt
- cargo doc
Explanation: Macro expansion happens at the AST stage; by the time HIR is reached, macros no longer exist.
2 Lowered loop
let mut it = IntoIterator::into_iter(v);
loop {
match Iterator::____(&mut it) {
Some(x) => toplam += x,
None => break,
}
}let mut it = IntoIterator::into_iter(v);
loop {
match Iterator::next(&mut it) {
Some(x) => toplam += x,
None => break,
}
}Explanation: A for loop calls Iterator::next each round.
3 Stage and role
| Term | Meaning |
|---|---|
AST |
Macro expansion |
HIR |
Type checking |
MIR |
Borrow checking |
LLVM IR |
Optimization and codegen |