Limits
The following attributes affect compile-time limits.
The recursion_limit
attribute
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 64.
# #![allow(unused_variables)] #![recursion_limit = "4"] #fn main() { 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!{} #}
# #![allow(unused_variables)] #![recursion_limit = "1"] #fn main() { // This fails because it requires two recursive steps to auto-derefence. (|_: &u8| {})(&&1); #}
The type_length_limit
attribute
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.
# #![allow(unused_variables)] #![type_length_limit = "8"] #fn main() { fn f<T>(x: T) {} // This fails to compile because monomorphizing to // `f::<(i32, i32, i32, i32, i32, i32, i32, i32, i32)>>` requires more // than 8 type elements. f(((1, 2, 3, 4, 5, 6, 7, 8, 9)); #}