Compiler error when combined with cfg-if in particular ways.

[jonhoo]

Prerequisites

• Rust version : rustc 1.43.1 (8d69840ab 2020-05-04)
• nom version : 5.1.1

Test case

This is a manifestation of Alexhuszagh/rust-lexical#38, and I gave a bit of context in #1115 (comment). The issue specifically is that nom 5 depends on lexical-core 0.6, which now pins cfg-if 0.1.9. And cfg-if fails to compile all sorts valid Rust code on newer Rust versions (this is fixed in cfg-if 0.1.10). See the lexical-core issue linked above for more details.

Normally, this would be none of nom's business (since nom is doing nothing wrong here). However, since the bump from lexical-core 0.6 to lexical-core 0.7 presented no API changes, as @shepmaster pointed out in #1115, it should be possible to issue a new minor release of nom 5 that allows lexical-core 0.7 (basically by backporting @shepmaster's PR).

The error cases are pretty bad:

Example test case 1:

[dependencies]
nom = "5"
time = "0.2"

fn main() {}

Note that this applies if nom 5 and time 0.2 every appear together in any dependency graph.

Example test case 2:

[dependencies]
nom = "5"
cfg-if = "0.1"

All sorts of different things fail, but here's an example:

fn main() {
    cfg_if::cfg_if! {
        if #[cfg(unix)] {
            fn bar() {}
            let tm = ();
        }
    }
}

[Geal]

closing as it's been fixed by moving to minimal-lexical

[jonhoo]

Note that this was specifically about backporting the change to nom5. Was the move to minimal-lexical also done for 5.x?

[Stargateur]

@jonhoo Have you try rename ?

[dependencies.cfg_if_1]
package = "cfg-if"
version = "0.1"

Or a patch ?

[Geal]

@jonhoo nom 5.1.2 allows lexical-core 0.7: https://crates.io/crates/nom/5.1.2/dependencies since commit e73a4d4

is there still an issue with it?

[jonhoo]

Ah, sorry, you're entirely right ­— I missed that change! Sorry for the noise :)