Macro recursion occurs when a macro invocation expands into another invocation of the same macro. In Rust declarative macros, expansion recursion differs from function recursion: expressions such as `$val - 1` are not evaluated while the macro expands, so a seemingly valid terminal condition may never be reached. section 2.2.3
Recursive delegation
A `generate_get_value!` macro can delegate its one-argument form to a more general two-argument form: `generate_get_value!($struct_type)` expands to `generate_get_value!($struct_type, String)`. This reuses one implementation for both default and explicit return types. section 2.2.3
Listing 2.11 A recursive fumble
macro_rules! count {
($val: expr) => {
if $val == 1 {
1
} else {
count!($val - 1)
}
}
}
The `count!` example demonstrates a recursive fumble: expansion repeatedly appends `- 1` to the expression instead of evaluating it, eventually producing a compile-time recursion-limit error. section 2.2.3
