README is wrong about when impl Trait was stabilized.

[rodrimati1992]

The readme claims that impl Trait was stabilized in Rust 1.31.0:

Strum is compatible with versions of rustc >= 1.31.0. That's the earliest version of stable rust that supports impl trait

But it actually was stabilized in Rust 1.26.0:
https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1260-2018-05-10

impl Trait is now stable allowing you to have abstract types in returns or in function parameters. E.g. fn foo() -> impl Iterator<Item=u8> or fn open(path: impl AsRef<Path>).

Example in godbolt:
https://rust.godbolt.org/z/1oqcej

[Peternator7]

Looks like you're correct. I believe 1.31 is rust 2018, and that's the actually reason for the MSRV.

Will accept a PR with the fix or change myself.

[rodrimati1992]

From looking at the dependencies, I discovered that syn 1.0 requires Rust 1.31.

Version requirement: Syn supports rustc 1.31 and up.

I guess all that needs to change is the README, mentioning syn's minimum supported Rust version.