HIR

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.

Exercise 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.

Show example 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);
}

Test yourself

What is the easiest way to see a macro's expanded form?
- cargo expand (or rustc -Zunpretty=expanded)
- cargo build --verbose
- rustfmt
- cargo doc
Show answer

cargo expand (or rustc -Zunpretty=expanded) — Macro expansion happens at the AST stage; by the time HIR is reached, macros no longer exist.

Lowered loop

let mut it = IntoIterator::into_iter(v);
loop {
    match Iterator::____(&mut it) {
        Some(x) => toplam += x,
        None => break,
    }
}

Show answer

let mut it = IntoIterator::into_iter(v);
loop {
    match Iterator::next(&mut it) {
        Some(x) => toplam += x,
        None => break,
    }
}

Stage and role

AST

Macro expansion
HIR

Type checking
MIR

Borrow checking
LLVM IR

Optimization and codegen

Next lesson: MIR

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