How do I match enum values with an integer?

You can derive FromPrimitive. Using Rust 2018 simplified imports syntax:

use num_derive::FromPrimitive;    
use num_traits::FromPrimitive;

#[derive(FromPrimitive)]
enum MyEnum {
    A = 1,
    B,
    C,
}

fn main() {
    let x = 2;

    match FromPrimitive::from_i32(x) {
        Some(MyEnum::A) => println!("Got A"),
        Some(MyEnum::B) => println!("Got B"),
        Some(MyEnum::C) => println!("Got C"),
        None            => println!("Couldn't convert {}", x),
    }
}

In your Cargo.toml:

[dependencies]
num-traits = "0.2"
num-derive = "0.2"

More details in num-derive crate, see esp. sample uses in tests.

Since Rust 1.34, I recommend implementing TryFrom:

use std::convert::TryFrom;

impl TryFrom<i32> for MyEnum {
    type Error = ();

    fn try_from(v: i32) -> Result<Self, Self::Error> {
        match v {
            x if x == MyEnum::A as i32 => Ok(MyEnum::A),
            x if x == MyEnum::B as i32 => Ok(MyEnum::B),
            x if x == MyEnum::C as i32 => Ok(MyEnum::C),
            _ => Err(()),
        }
    }
}

Then you can use TryInto and handle the possible error:

use std::convert::TryInto;

fn main() {
    let x = MyEnum::C as i32;

    match x.try_into() {
        Ok(MyEnum::A) => println!("a"),
        Ok(MyEnum::B) => println!("b"),
        Ok(MyEnum::C) => println!("c"),
        Err(_) => eprintln!("unknown number"),
    }
}

If you have a great number of variants, a macro can be used to create a parallel implementation of TryFrom automatically based on the definition of the enum:

macro_rules! back_to_enum {
    ($(#[$meta:meta])* $vis:vis enum $name:ident {
        $($(#[$vmeta:meta])* $vname:ident $(= $val:expr)?,)*
    }) => {
        $(#[$meta])*
        $vis enum $name {
            $($(#[$vmeta])* $vname $(= $val)?,)*
        }

        impl std::convert::TryFrom<i32> for $name {
            type Error = ();

            fn try_from(v: i32) -> Result<Self, Self::Error> {
                match v {
                    $(x if x == $name::$vname as i32 => Ok($name::$vname),)*
                    _ => Err(()),
                }
            }
        }
    }
}

back_to_enum! {
    enum MyEnum {
        A = 1,
        B,
        C,
    }
}

See also:

• How to ensure every enum variant can be returned from a specific function at compile time?
• Generating documentation in macros