How do you convert between Substrate specific types and Rust primitive types?

Using the Substrate blockchain framework, how can I convert between Substrate specific types and Rust primitive types and vice versa?

For example:

Convert a time (T::Moment) to a u64
Convert a u64 to a T::Balance

etc...

Solution

For the latest in Substrate master

Substrate has removed As in favor of From/Into. An assumption is made that all types are at least u32.

From the trait SimpleArithmatic, the following are implemented:

From: u8, u16, u32
TryFrom: u64, u128, usize
TryInto: u8, u16, u32, u64, u128, usize

Another trait is also provided to provide ergonomic infallible conversion when you don't care if the value saturates.

UniqueSaturatedInto: u8, u16, u32, u64, u128
UniqueSaturatedFrom: u64, u128

NOTE on SaturatedConversion from Gav

SaturatedConversion (saturated_into and saturated_from) should not be used unless you know what you're doing, you've thought and considered all options and your use-case implies that saturation is fundamentally correct. The only time I imagine this is the case is deep in runtime arithmetic where you are logically certain it will not overflow, but can't provide a proof because it would depend on consistent pre-existing state.

This means that working from u32 to Substrate specific types should be easy:

pub fn u32_to_balance(input: u32) -> T::Balance {
    input.into()
}

For larger types, you need to handle the case where the Balance type for a runtime is smaller than what is available:

pub fn u64_to_balance_option(input: u64) -> Option<T::Balance> {
    input.try_into().ok()
}

// Note the warning above about saturated conversions
pub fn u64_to_balance_saturated(input: u64) -> T::Balance {
    input.saturated_into()
}

When converting from T::Balance to a rust primitive, you need to also handle conversion between incompatible types:

pub fn balance_to_u64(input: T::Balance) -> Option<u64> {
    TryInto::<u64>::try_into(input).ok()
}

// Note the warning above about saturated conversions
pub fn balance_to_u64_saturated(input: T::Balance) -> u64 {
    input.saturated_into::<u64>()
}

For Substrate v1.0

Substrate provides pub trait As<T> in the sr-primitives crate:

/// Simple trait similar to `Into`, except that it can be used to convert numerics between each
/// other.
pub trait As<T> {
    /// Convert forward (ala `Into::into`).
    fn as_(self) -> T;
    /// Convert backward (ala `From::from`).
    fn sa(_: T) -> Self;
}

Here are some working examples of how it can be used:

impl<T: Trait> Module<T> {
    // `as_` will turn T::Balance into a u64
    pub fn balance_to_u64(input: T::Balance) -> u64 {
        input.as_()
    }

    // Being explicit, you can convert a `u64` to a T::Balance
    // using the `As` trait, with `T: u64`, and then calling `sa`
    pub fn u64_to_balance(input: u64) -> T::Balance {
        <T::Balance as As<u64>>::sa(input)
    }

    // You can also let Rust figure out what `T` is
    pub fn u64_to_balance_implied(input: u64) -> T::Balance {
        <T::Balance as As<_>>::sa(input)
    }

    // You can also let Rust figure out where `sa` is implemented
    pub fn u64_to_balance_implied_more(input: u64) -> T::Balance {
        T::Balance::sa(input)
    }
}