Repository navigation
[Feature Request] Handle if + cfg! in const context #1018
Description
Since the addition of if in const context, the usage of if + cfg! macro became a thing, since it reduces the need of replication of docs and definition.
pub const MYCONST: i32 = if cfg!(target_os = "linux") {
10
} else if cfg!(windows) {
5
} else {
0
}Currently, cbindgen ignores this kind of construction, but ideally, supporting that would make easier for maintaining C-exported constants that differ on targets.
Possible expected result:
#if defined(⟨USER_DEFINED_LINUX_FLAG_HERE⟩)
#define MYCONST 10
#elsif defined(⟨USER_DEFINED_WINDOWS_FLAG_HERE⟩)
#define MYCONST 5
#else
0
#endifIn const context, an if must always have an else, so it is also common to have panicking macros, like panic!, unimplemented!, todo! and unreacheble! in the else clause. That causes an compiler error with the panic message if it reaches the panic. I think this could generate an #error directive with the message.
Example
pub const MYCONST: i32 = if cfg!(target_os = "linux") {
10
} else if cfg!(windows) {
5
} else {
panic!("We only support linux and windows for now!")
}#if defined(⟨USER_DEFINED_LINUX_FLAG_HERE⟩)
#define MYCONST 10
#elsif defined(⟨USER_DEFINED_WINDOWS_FLAG_HERE⟩)
#define MYCONST 5
#else
#error "We only support linux and windows for now!"
#endif
Activity
GrayJack commentedon Oct 27, 2024
Its unclear to me how this would work with the macro expansion feature. I'm not using the expasion feature in none of my projects that uses cbindgen
GrayJack commentedon Oct 27, 2024
I currently have a working version for C/C++, but I have no idea how would be the equivalent for Cython to turn panics->
#errorchrysn commentedon Feb 3, 2025
Is there an established workaround for this?
Right now I'm tempted to just place a manual copy of the cfg tree into the
header, but then I'll also have to do the include guards manually.lakers-c: Manually reconstruct the buffer size selection for C
lakers-c: Manually reconstruct the buffer size selection for C
lakers-c: Manually reconstruct the buffer size selection for C
GrayJack commentedon Feb 9, 2025
I think the only workaround for this is to create a bunch of
#[cfg(...)]with the variable name for your const