Summary - Compiler internals

Combine HIR, MIR and LLVM IR to draw a compilation pipeline.

Compiler internals summary

The compilation pipeline: tokens → AST (macros, name resolution) → HIR (sugar removed, type checking, trait resolution) → MIR (control-flow graph, borrow checking, NLL, constant evaluation) → LLVM IR (optimization) → machine code. Type inference works by constraint unification within a function. Debug and release builds apply different checks and optimizations; for example, integer overflow checking is on by default only in debug.

Overflow with explicit intent

Integer methods make what happens on overflow the same in every build:

let x: u8 = 255;
println!("{:?}", x.checked_add(1));      // None
println!("{}", x.wrapping_add(1));       // 0
println!("{}", x.saturating_add(1));     // 255
println!("{:?}", x.overflowing_add(1));  // (0, true)

wrapping_* is the natural choice for counters and hashes, saturating_* for bounded values, checked_* for validation.

Exercise Overflow methods

For let x: u8 = 255;, print the results of checked_add(1) ({:?}), wrapping_add(1) and saturating_add(1) on one line separated by spaces.

Show example solution

fn main() {
    let x: u8 = 255;
    println!("{:?} {} {}", x.checked_add(1), x.wrapping_add(1), x.saturating_add(1));
}

Test yourself

What happens to <code>255u8 + 1</code> in a release build (with default settings)?
- Always panics
- It becomes 256
- Always a compile error
- It wraps to 0; in debug it panics
Show answer

It wraps to 0; in debug it panics — To keep behaviour independent of the build, state the intent explicitly: checked_add, wrapping_add, saturating_add. (The compiler already catches a constant expression as an error.)

Stay at the limit

let x: u8 = 255;
let y = x.____(1);   // 255

Show answer

let x: u8 = 255;
let y = x.saturating_add(1);   // 255

Method and 255u8 + 1 result

checked_add

None
wrapping_add

0
saturating_add

255
overflowing_add

(0, true)

Next lesson: Struct layout

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