An example of an attribute procedural macro. The #[trace_var(...)] attribute prints the value of the given variables each time they are reassigned.

• trace-var/src/lib.rs
• example/src/main.rs

Consider the following factorial implementation.

#[trace_var(p, n)]
fn factorial(mut n: u64) -> u64 {
    let mut p = 1;
    while n > 1 {
        p *= n;
        n -= 1;
    }
    p
}

Invoking this with factorial(8) prints all the values of p and n during the execution of the function.

p = 1
p = 8
n = 7
p = 56
n = 6
p = 336
n = 5
p = 1680
n = 4
p = 6720
n = 3
p = 20160
n = 2
p = 40320
n = 1

The procedural macro uses a syntax tree Fold to rewrite every let statement and assignment expression in the following way:

// Before
let VAR = INIT;

// After
let VAR = { let VAR = INIT; println!("VAR = {:?}", VAR); VAR };

// Before
VAR = INIT

// After
{ VAR = INIT; println!("VAR = {:?}", VAR); }