diff options
Diffstat (limited to 'syn/examples/trace-var/README.md')
-rw-r--r-- | syn/examples/trace-var/README.md | 61 |
1 files changed, 61 insertions, 0 deletions
diff --git a/syn/examples/trace-var/README.md b/syn/examples/trace-var/README.md new file mode 100644 index 0000000..b93fae2 --- /dev/null +++ b/syn/examples/trace-var/README.md @@ -0,0 +1,61 @@ +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`](trace-var/src/lib.rs) +- [`example/src/main.rs`](example/src/main.rs) + +Consider the following factorial implementation. + +```rust +#[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: + +[`Fold`]: https://docs.rs/syn/1.0/syn/fold/trait.Fold.html + +```rust +// Before +let VAR = INIT; + +// After +let VAR = { let VAR = INIT; println!("VAR = {:?}", VAR); VAR }; +``` + +```rust +// Before +VAR = INIT + +// After +{ VAR = INIT; println!("VAR = {:?}", VAR); } +``` |