Merge #176

176: Document cast traits about possible precision loss and truncation r=cuviper a=Enet4

Resolves #175.
Document the traits `ToPrimitive`, `FromPrimitive` and `NumCast` that
value can be represented by the target type
when they are within the range admitted by the target type,
and so truncation or precision loss may happen.

One question worth reflecting on is whether this documentation be normalized somewhere at module level,
and whether it should be replicated to all methods or simply stay at each trait.
I would also be open to adding examples with doctests if deemed useful.

Co-authored-by: Eduardo Pinho <enet4mikeenet@gmail.com>

src/cast.rs

@@ -9,6 +9,14 @@ use core::{u16, u32, u64, u8, usize};
 use float::FloatCore;
 
 /// A generic trait for converting a value to a number.
+///
+/// A value can be represented by the target type when it lies within
+/// the range of scalars supported by the target type.
+/// For example, a negative integer cannot be represented by an unsigned
+/// integer type, and an `f64` with a very high magnitude might not be
+/// convertible to an `f32`.
+/// On the other hand, conversions with possible precision loss or truncation
+/// (e.g. an `f32` with a decimal part to an integer type) are admitted.
 pub trait ToPrimitive {
     /// Converts the value of `self` to an `isize`. If the value cannot be
     /// represented by an `isize`, then `None` is returned.
@@ -392,6 +400,14 @@ impl_to_primitive_float!(f32);
 impl_to_primitive_float!(f64);
 
 /// A generic trait for converting a number to a value.
+///
+/// A value can be represented by the target type when it lies within
+/// the range of scalars supported by the target type.
+/// For example, a negative integer cannot be represented by an unsigned
+/// integer type, and an `f64` with a very high magnitude might not be
+/// convertible to an `f32`.
+/// On the other hand, conversions with possible precision loss or truncation
+/// (e.g. an `f32` with a decimal part to an integer type) are admitted.
 pub trait FromPrimitive: Sized {
     /// Converts an `isize` to return an optional value of this type. If the
     /// value cannot be represented by this type, then `None` is returned.
@@ -672,6 +688,14 @@ pub trait NumCast: Sized + ToPrimitive {
     /// Creates a number from another value that can be converted into
     /// a primitive via the `ToPrimitive` trait. If the source value cannot be
     /// represented by the target type, then `None` is returned.
+    ///
+    /// A value can be represented by the target type when it lies within
+    /// the range of scalars supported by the target type.
+    /// For example, a negative integer cannot be represented by an unsigned
+    /// integer type, and an `f64` with a very high magnitude might not be
+    /// convertible to an `f32`.
+    /// On the other hand, conversions with possible precision loss or truncation
+    /// (e.g. an `f32` with a decimal part to an integer type) are admitted.
     fn from<T: ToPrimitive>(n: T) -> Option<Self>;
 }