Change:
<ins> // Copyright (c) 2015 Anders Kaseorg <andersk@mit.edu> // Permission is hereby granted, free of charge, to any person obtaining // a copy of this software and associated documentation files (the // “Software”), to deal in the Software without restriction, including // without limitation the rights to use, copy, modify, merge, publish, // distribute, sublicense, and/or sell copies of the Software, and to // permit persons to whom the Software is furnished to do so, subject to // the following conditions: // The above copyright notice and this permission notice shall be // included in all copies or substantial portions of the Software. // THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, // EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF // MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. // IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY // CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, // TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE // SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. //! This crate exports a macro `enum_from_primitive!` that wraps an //! `enum` declaration and automatically adds an implementation of //! `num::FromPrimitive` (reexported here), to allow conversion from //! primitive integers to the enum.  It therefore provides an //! alternative to the built-in `#[derive(FromPrimitive)]`, which //! requires the unstable `std::num::FromPrimitive` and is disabled in //! Rust 1.0. //! //! # Example //! //! ``` //! #[macro_use] extern crate enum_primitive; //! extern crate num_traits; //! use num_traits::FromPrimitive; //! //! enum_from_primitive! { //! #[derive(Debug, PartialEq)] //! enum FooBar { //!     Foo = 17, //!     Bar = 42, //!     Baz, //! } //! } //! //! fn main() { //!     assert_eq!(FooBar::from_i32(17), Some(FooBar::Foo)); //!     assert_eq!(FooBar::from_i32(42), Some(FooBar::Bar)); //!     assert_eq!(FooBar::from_i32(43), Some(FooBar::Baz)); //!     assert_eq!(FooBar::from_i32(91), None); //! } //! ``` pub mod num_traits { pub trait FromPrimitive: Sized { fn from_i64(n: i64) -> Option<Self>; fn from_u64(n: u64) -> Option<Self>; } } pub use std::option::Option; pub use num_traits::FromPrimitive; /// Helper macro for internal use by `enum_from_primitive!`. #[macro_export] macro_rules! enum_from_primitive_impl_ty { ($meth:ident, $ty:ty, $name:ident, $( $variant:ident )*) => { #[allow(non_upper_case_globals, unused)] fn $meth(n: $ty) -> $crate::Option<Self> { $( if n == $name::$variant as $ty { $crate::Option::Some($name::$variant) } else )* { $crate::Option::None } } }; } /// Helper macro for internal use by `enum_from_primitive!`. #[macro_export] #[macro_use(enum_from_primitive_impl_ty)] macro_rules! enum_from_primitive_impl { ($name:ident, $( $variant:ident )*) => { impl $crate::FromPrimitive for $name { enum_from_primitive_impl_ty! { from_i64, i64, $name, $( $variant )* } enum_from_primitive_impl_ty! { from_u64, u64, $name, $( $variant )* } } }; } /// Wrap this macro around an `enum` declaration to get an /// automatically generated implementation of `num::FromPrimitive`. #[macro_export] #[macro_use(enum_from_primitive_impl)] macro_rules! enum_from_primitive { ( $( #[$enum_attr:meta] )* enum $name:ident { $( $( #[$variant_attr:meta] )* $variant:ident ),+ $( = $discriminator:expr, $( $( #[$variant_two_attr:meta] )* $variant_two:ident ),+ )* } ) => { $( #[$enum_attr] )* enum $name { $( $( #[$variant_attr] )* $variant ),+ $( = $discriminator, $( $( #[$variant_two_attr] )* $variant_two ),+ )* } enum_from_primitive_impl! { $name, $( $variant )+ $( $( $variant_two )+ )* } }; ( $( #[$enum_attr:meta] )* enum $name:ident { $( $( $( #[$variant_attr:meta] )* $variant:ident ),+ = $discriminator:expr ),* } ) => { $( #[$enum_attr] )* enum $name { $( $( $( #[$variant_attr] )* $variant ),+ = $discriminator ),* } enum_from_primitive_impl! { $name, $( $( $variant )+ )* } }; ( $( #[$enum_attr:meta] )* enum $name:ident { $( $( #[$variant_attr:meta] )* $variant:ident ),+ $( = $discriminator:expr, $( $( #[$variant_two_attr:meta] )* $variant_two:ident ),+ )*, } ) => { $( #[$enum_attr] )* enum $name { $( $( #[$variant_attr] )* $variant ),+ $( = $discriminator, $( $( #[$variant_two_attr] )* $variant_two ),+ )*, } enum_from_primitive_impl! { $name, $( $variant )+ $( $( $variant_two )+ )* } }; ( $( #[$enum_attr:meta] )* enum $name:ident { $( $( $( #[$variant_attr:meta] )* $variant:ident ),+ = $discriminator:expr ),+, } ) => { $( #[$enum_attr] )* enum $name { $( $( $( #[$variant_attr] )* $variant ),+ = $discriminator ),+, } enum_from_primitive_impl! { $name, $( $( $variant )+ )+ } }; ( $( #[$enum_attr:meta] )* pub enum $name:ident { $( $( #[$variant_attr:meta] )* $variant:ident ),+ $( = $discriminator:expr, $( $( #[$variant_two_attr:meta] )* $variant_two:ident ),+ )* } ) => { $( #[$enum_attr] )* pub enum $name { $( $( #[$variant_attr] )* $variant ),+ $( = $discriminator, $( $( #[$variant_two_attr] )* $variant_two ),+ )* } enum_from_primitive_impl! { $name, $( $variant )+ $( $( $variant_two )+ )* } }; ( $( #[$enum_attr:meta] )* pub enum $name:ident { $( $( $( #[$variant_attr:meta] )* $variant:ident ),+ = $discriminator:expr ),* } ) => { $( #[$enum_attr] )* pub enum $name { $( $( $( #[$variant_attr] )* $variant ),+ = $discriminator ),* } enum_from_primitive_impl! { $name, $( $( $variant )+ )* } }; ( $( #[$enum_attr:meta] )* pub enum $name:ident { $( $( #[$variant_attr:meta] )* $variant:ident ),+ $( = $discriminator:expr, $( $( #[$variant_two_attr:meta] )* $variant_two:ident ),+ )*, } ) => { $( #[$enum_attr] )* pub enum $name { $( $( #[$variant_attr] )* $variant ),+ $( = $discriminator, $( $( #[$variant_two_attr] )* $variant_two ),+ )*, } enum_from_primitive_impl! { $name, $( $variant )+ $( $( $variant_two )+ )* } }; ( $( #[$enum_attr:meta] )* pub enum $name:ident { $( $( $( #[$variant_attr:meta] )* $variant:ident ),+ = $discriminator:expr ),+, } ) => { $( #[$enum_attr] )* pub enum $name { $( $( $( #[$variant_attr] )* $variant ),+ = $discriminator ),+, } enum_from_primitive_impl! { $name, $( $( $variant )+ )+ } }; } </ins>

Discussion:
Running on the following code results in a stack overflow: With significantly fewer enum variants, it works. If I remove the use of , it works. This is breaking on one of my PRs (bjz/glfw-rs).
This seems to be fixed.
In my tests, you now need to have around 460+ enum variants in order to get the stack overflow. Can we assume nobody will ever have that many?
should a test still be written for this? Panicking is not correct, better would be that the number of variants is limited explicitly and that this is documented. Better still if there is no limit, of course.
In theory yeah if there's something that overflowed previously and no longer does, we should have a test for that :)
Triage: not aware of a test yet