This CL adds a procedural macro to generate functions for converting a
primitive integer into the corresponding variant of an enum.
Loosely based on https://docs.rs/enum-primitive-derive but implemented
against a newer version of Syn and without the dependency on num-traits.
The generated function is named `n` and has the following signature:
impl YourEnum {
pub fn n(value: Repr) -> Option<Self>;
}
where `Repr` is an integer type of the right size as described in more
detail below.
EXAMPLE
extern crate enumn;
#[derive(PartialEq, Debug, enumn::N)]
enum Status {
LegendaryTriumph,
QualifiedSuccess,
FortuitousRevival,
IndeterminateStalemate,
RecoverableSetback,
DireMisadventure,
AbjectFailure,
}
fn main() {
let s = Status::n(1);
assert_eq!(s, Some(Status::QualifiedSuccess));
let s = Status::n(9);
assert_eq!(s, None);
}
SIGNATURE
The generated signature depends on whether the enum has a `#[repr(..)]`
attribute. If a `repr` is specified, the input to `n` will be required
to be of that type.
#[derive(enumn::N)]
#[repr(u8)]
enum E {
/* ... */
}
// expands to:
impl E {
pub fn n(value: u8) -> Option<Self> {
/* ... */
}
}
On the other hand if no `repr` is specified then we get a signature that
is generic over a variety of possible types.
impl E {
pub fn n<REPR: Into<i64>>(value: REPR) -> Option<Self> {
/* ... */
}
}
DISCRIMINANTS
The conversion respects explictly specified enum discriminants. Consider
this enum:
#[derive(enumn::N)]
enum Letter {
A = 65,
B = 66,
}
Here `Letter::n(65)` would return `Some(Letter::A)`.
TEST=`cargo test` against the new crate
Change-Id: I4286a816828c83507b35185fe497455ee30ae9e8
Reviewed-on: https://chromium-review.googlesource.com/1365114
Commit-Ready: David Tolnay <dtolnay@chromium.org>
Tested-by: David Tolnay <dtolnay@chromium.org>
Reviewed-by: Chirantan Ekbote <chirantan@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>
