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+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); }
+```