# Objective
Implement `TryFrom<Vec2>`/`TryFrom<Vec3>` for direction primitives as
considered in #10857.
## Solution
Implement `TryFrom` for the direction primitives.
These are all equivalent:
```rust
let dir2d = Direction2d::try_from(Vec2::new(0.5, 0.5)).unwrap();
let dir2d = Vec2::new(0.5, 0.5).try_into().unwrap(); // (assumes that the type is inferred)
let dir2d = Direction2d::new(Vec2::new(0.5, 0.5)).unwrap();
```
For error cases, an `Err(InvalidDirectionError)` is returned. It
contains the type of failure:
```rust
/// An error indicating that a direction is invalid.
#[derive(Debug, PartialEq)]
pub enum InvalidDirectionError {
/// The length of the direction vector is zero or very close to zero.
Zero,
/// The length of the direction vector is `std::f32::INFINITY`.
Infinite,
/// The length of the direction vector is `NaN`.
NaN,
}
```
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