Recursion limit silently ignored if it overflows a `usize`

[ecstatic-morse]

The following code fails to compile since it exceeds the default recursion limit. Using 2056 as the recursion limit works as expected.

#![recursion_limit = "99999999999999999999999999999999999999999999999999999"]
//#![recursion_limit = "2056"]

macro_rules! explode {
    (recurse; $head:tt $($tail:tt)*) => { explode!(recurse; $($tail)*); };
    (recurse;) => {};
    
    (0; $($tt:tt)*) => { explode!(1; $($tt)* $($tt)* $($tt)*); };
    (1; $($tt:tt)*) => { explode!(2; $($tt)* $($tt)* $($tt)*); };
    (2; $($tt:tt)*) => { explode!(3; $($tt)* $($tt)* $($tt)*); };
    (3; $($tt:tt)*) => { explode!(recurse; $($tt)* $($tt)* $($tt)*); };
}

explode!(0; a b c);

This is because we don't check for integer overflow errors when parsing limits:

rust/src/librustc/middle/recursion_limit.rs

Lines 19 to 33 in e9469a6

	fn update_limit(krate: &ast::Crate, limit: &Once<usize>, name: Symbol, default: usize) {
	for attr in &krate.attrs {
	if !attr.check_name(name) {
	continue;
	}
	if let Some(s) = attr.value_str() {
	if let Some(n) = s.as_str().parse().ok() {
	limit.set(n);
	return;
	}
	}
	}
	limit.set(default);
	}

There's a few options here. My preference would be to silently default to usize::MAX when overflow occurs.

This issue has been assigned to @fisherdarling via this comment.

[fisherdarling]

Hi! I'd like to try and handle this. If I understand this correctly, we should check if there is an ParseIntError, and if that error is an overflow. If it's an overflow, we default to usize::MAX?

@rustbot claim

[estebank]

@fisherdarling correct