The following attributes affect compile-time limits.
The recursion_limit
attribute may be applied at the crate level to set the
maximum depth for potentially infinitely-recursive compile-time operations
like macro expansion or auto-dereference. It uses the MetaNameValueStr
syntax to specify the recursion depth.
Note: The default in
rustc
is 128.
#![recursion_limit = "4"]
macro_rules! a {
() => { a!(1); };
(1) => { a!(2); };
(2) => { a!(3); };
(3) => { a!(4); };
(4) => { };
}
// This fails to expand because it requires a recursion depth greater than 4.
a!{}
#![recursion_limit = "1"]
// This fails because it requires two recursive steps to auto-dereference.
(|_: &u8| {})(&&&1);
Note
This limit is only enforced when the nightly -Zenforce-type-length-limit
flag is active.
For more information, see rust-lang/rust#127670.
The type_length_limit
attribute limits the maximum number of type
substitutions made when constructing a concrete type during monomorphization.
It is applied at the crate level, and uses the MetaNameValueStr syntax
to set the limit based on the number of type substitutions.
Note: The default in
rustc
is 1048576.
#![type_length_limit = "4"]
fn f<T>(x: T) {}
// This fails to compile because monomorphizing to
// `f::<((((i32,), i32), i32), i32)>` requires more than 4 type elements.
f(((((1,), 2), 3), 4));