Merge #137

137: Document NumCast / ToPrimitive's sucess vs error rules r=cuviper a=ocstl

Closes #9 .

The added comments are pretty succinct, not going into details of the conversion (for example, truncation when converting from float to int). If something is missing, I'm not adverse to adding precision.

Co-authored-by: Olivier Chassé St-Laurent <[email protected]>

src/cast.rs

@@ -10,38 +10,45 @@ use float::FloatCore;
 
 /// A generic trait for converting a value to a number.
 pub trait ToPrimitive {
-    /// Converts the value of `self` to an `isize`.
+    /// Converts the value of `self` to an `isize`. If the value cannot be
+    /// represented by an `isize`, then `None` is returned.
     #[inline]
     fn to_isize(&self) -> Option<isize> {
         self.to_i64().as_ref().and_then(ToPrimitive::to_isize)
     }
 
-    /// Converts the value of `self` to an `i8`.
+    /// Converts the value of `self` to an `i8`. If the value cannot be
+    /// represented by an `i8`, then `None` is returned.
     #[inline]
     fn to_i8(&self) -> Option<i8> {
         self.to_i64().as_ref().and_then(ToPrimitive::to_i8)
     }
 
-    /// Converts the value of `self` to an `i16`.
+    /// Converts the value of `self` to an `i16`. If the value cannot be
+    /// represented by an `i16`, then `None` is returned.
     #[inline]
     fn to_i16(&self) -> Option<i16> {
         self.to_i64().as_ref().and_then(ToPrimitive::to_i16)
     }
 
-    /// Converts the value of `self` to an `i32`.
+    /// Converts the value of `self` to an `i32`. If the value cannot be
+    /// represented by an `i32`, then `None` is returned.
     #[inline]
     fn to_i32(&self) -> Option<i32> {
         self.to_i64().as_ref().and_then(ToPrimitive::to_i32)
     }
 
-    /// Converts the value of `self` to an `i64`.
+    /// Converts the value of `self` to an `i64`. If the value cannot be
+    /// represented by an `i64`, then `None` is returned.
     fn to_i64(&self) -> Option<i64>;
 
-    /// Converts the value of `self` to an `i128`.
+    /// Converts the value of `self` to an `i128`. If the value cannot be
+    /// represented by an `i128` (`i64` under the default implementation), then
+    /// `None` is returned.
     ///
     /// This method is only available with feature `i128` enabled on Rust >= 1.26.
     ///
-    /// The default implementation converts through `to_i64()`.  Types implementing
+    /// The default implementation converts through `to_i64()`. Types implementing
     /// this trait should override this method if they can represent a greater range.
     #[inline]
     #[cfg(has_i128)]
@@ -49,35 +56,42 @@ pub trait ToPrimitive {
         self.to_i64().map(From::from)
     }
 
-    /// Converts the value of `self` to a `usize`.
+    /// Converts the value of `self` to a `usize`. If the value cannot be
+    /// represented by a `usize`, then `None` is returned.
     #[inline]
     fn to_usize(&self) -> Option<usize> {
         self.to_u64().as_ref().and_then(ToPrimitive::to_usize)
     }
 
-    /// Converts the value of `self` to an `u8`.
+    /// Converts the value of `self` to a `u8`. If the value cannot be
+    /// represented by a `u8`, then `None` is returned.
     #[inline]
     fn to_u8(&self) -> Option<u8> {
         self.to_u64().as_ref().and_then(ToPrimitive::to_u8)
     }
 
-    /// Converts the value of `self` to an `u16`.
+    /// Converts the value of `self` to a `u16`. If the value cannot be
+    /// represented by a `u16`, then `None` is returned.
     #[inline]
     fn to_u16(&self) -> Option<u16> {
         self.to_u64().as_ref().and_then(ToPrimitive::to_u16)
     }
 
-    /// Converts the value of `self` to an `u32`.
+    /// Converts the value of `self` to a `u32`. If the value cannot be
+    /// represented by a `u32`, then `None` is returned.
     #[inline]
     fn to_u32(&self) -> Option<u32> {
         self.to_u64().as_ref().and_then(ToPrimitive::to_u32)
     }
 
-    /// Converts the value of `self` to an `u64`.
+    /// Converts the value of `self` to a `u64`. If the value cannot be
+    /// represented by a `u64`, then `None` is returned.
     #[inline]
     fn to_u64(&self) -> Option<u64>;
 
-    /// Converts the value of `self` to an `u128`.
+    /// Converts the value of `self` to a `u128`. If the value cannot be
+    /// represented by a `u128` (`u64` under the default implementation), then
+    /// `None` is returned.
     ///
     /// This method is only available with feature `i128` enabled on Rust >= 1.26.
     ///
@@ -89,13 +103,15 @@ pub trait ToPrimitive {
         self.to_u64().map(From::from)
     }
 
-    /// Converts the value of `self` to an `f32`.
+    /// Converts the value of `self` to an `f32`. If the value cannot be
+    /// represented by an `f32`, then `None` is returned.
     #[inline]
     fn to_f32(&self) -> Option<f32> {
         self.to_f64().as_ref().and_then(ToPrimitive::to_f32)
     }
 
-    /// Converts the value of `self` to an `f64`.
+    /// Converts the value of `self` to an `f64`. If the value cannot be
+    /// represented by an `f64`, then `None` is returned.
     #[inline]
     fn to_f64(&self) -> Option<f64> {
         match self.to_i64() {
@@ -362,44 +378,44 @@ impl_to_primitive_float!(f64);
 
 /// A generic trait for converting a number to a value.
 pub trait FromPrimitive: Sized {
-    /// Convert an `isize` to return an optional value of this type. If the
-    /// value cannot be represented by this value, then `None` is returned.
+    /// Converts an `isize` to return an optional value of this type. If the
+    /// value cannot be represented by this type, then `None` is returned.
     #[inline]
     fn from_isize(n: isize) -> Option<Self> {
         n.to_i64().and_then(FromPrimitive::from_i64)
     }
 
-    /// Convert an `i8` to return an optional value of this type. If the
-    /// type cannot be represented by this value, then `None` is returned.
+    /// Converts an `i8` to return an optional value of this type. If the
+    /// value cannot be represented by this type, then `None` is returned.
     #[inline]
     fn from_i8(n: i8) -> Option<Self> {
         FromPrimitive::from_i64(From::from(n))
     }
 
-    /// Convert an `i16` to return an optional value of this type. If the
-    /// type cannot be represented by this value, then `None` is returned.
+    /// Converts an `i16` to return an optional value of this type. If the
+    /// value cannot be represented by this type, then `None` is returned.
     #[inline]
     fn from_i16(n: i16) -> Option<Self> {
         FromPrimitive::from_i64(From::from(n))
     }
 
-    /// Convert an `i32` to return an optional value of this type. If the
-    /// type cannot be represented by this value, then `None` is returned.
+    /// Converts an `i32` to return an optional value of this type. If the
+    /// value cannot be represented by this type, then `None` is returned.
     #[inline]
     fn from_i32(n: i32) -> Option<Self> {
         FromPrimitive::from_i64(From::from(n))
     }
 
-    /// Convert an `i64` to return an optional value of this type. If the
-    /// type cannot be represented by this value, then `None` is returned.
+    /// Converts an `i64` to return an optional value of this type. If the
+    /// value cannot be represented by this type, then `None` is returned.
     fn from_i64(n: i64) -> Option<Self>;
 
-    /// Convert an `i128` to return an optional value of this type. If the
-    /// type cannot be represented by this value, then `None` is returned.
+    /// Converts an `i128` to return an optional value of this type. If the
+    /// value cannot be represented by this type, then `None` is returned.
     ///
     /// This method is only available with feature `i128` enabled on Rust >= 1.26.
     ///
-    /// The default implementation converts through `from_i64()`.  Types implementing
+    /// The default implementation converts through `from_i64()`. Types implementing
     /// this trait should override this method if they can represent a greater range.
     #[inline]
     #[cfg(has_i128)]
@@ -407,44 +423,44 @@ pub trait FromPrimitive: Sized {
         n.to_i64().and_then(FromPrimitive::from_i64)
     }
 
-    /// Convert a `usize` to return an optional value of this type. If the
-    /// type cannot be represented by this value, then `None` is returned.
+    /// Converts a `usize` to return an optional value of this type. If the
+    /// value cannot be represented by this type, then `None` is returned.
     #[inline]
     fn from_usize(n: usize) -> Option<Self> {
         n.to_u64().and_then(FromPrimitive::from_u64)
     }
 
-    /// Convert an `u8` to return an optional value of this type. If the
-    /// type cannot be represented by this value, then `None` is returned.
+    /// Converts an `u8` to return an optional value of this type. If the
+    /// value cannot be represented by this type, then `None` is returned.
     #[inline]
     fn from_u8(n: u8) -> Option<Self> {
         FromPrimitive::from_u64(From::from(n))
     }
 
-    /// Convert an `u16` to return an optional value of this type. If the
-    /// type cannot be represented by this value, then `None` is returned.
+    /// Converts an `u16` to return an optional value of this type. If the
+    /// value cannot be represented by this type, then `None` is returned.
     #[inline]
     fn from_u16(n: u16) -> Option<Self> {
         FromPrimitive::from_u64(From::from(n))
     }
 
-    /// Convert an `u32` to return an optional value of this type. If the
-    /// type cannot be represented by this value, then `None` is returned.
+    /// Converts an `u32` to return an optional value of this type. If the
+    /// value cannot be represented by this type, then `None` is returned.
     #[inline]
     fn from_u32(n: u32) -> Option<Self> {
         FromPrimitive::from_u64(From::from(n))
     }
 
-    /// Convert an `u64` to return an optional value of this type. If the
-    /// type cannot be represented by this value, then `None` is returned.
+    /// Converts an `u64` to return an optional value of this type. If the
+    /// value cannot be represented by this type, then `None` is returned.
     fn from_u64(n: u64) -> Option<Self>;
 
-    /// Convert an `u128` to return an optional value of this type. If the
-    /// type cannot be represented by this value, then `None` is returned.
+    /// Converts an `u128` to return an optional value of this type. If the
+    /// value cannot be represented by this type, then `None` is returned.
     ///
     /// This method is only available with feature `i128` enabled on Rust >= 1.26.
     ///
-    /// The default implementation converts through `from_u64()`.  Types implementing
+    /// The default implementation converts through `from_u64()`. Types implementing
     /// this trait should override this method if they can represent a greater range.
     #[inline]
     #[cfg(has_i128)]
@@ -452,15 +468,15 @@ pub trait FromPrimitive: Sized {
         n.to_u64().and_then(FromPrimitive::from_u64)
     }
 
-    /// Convert a `f32` to return an optional value of this type. If the
-    /// type cannot be represented by this value, then `None` is returned.
+    /// Converts a `f32` to return an optional value of this type. If the
+    /// value cannot be represented by this type, then `None` is returned.
     #[inline]
     fn from_f32(n: f32) -> Option<Self> {
         FromPrimitive::from_f64(From::from(n))
     }
 
-    /// Convert a `f64` to return an optional value of this type. If the
-    /// type cannot be represented by this value, then `None` is returned.
+    /// Converts a `f64` to return an optional value of this type. If the
+    /// value cannot be represented by this type, then `None` is returned.
     #[inline]
     fn from_f64(n: f64) -> Option<Self> {
         match n.to_i64() {
@@ -639,7 +655,8 @@ pub fn cast<T: NumCast, U: NumCast>(n: T) -> Option<U> {
 /// An interface for casting between machine scalars.
 pub trait NumCast: Sized + ToPrimitive {
     /// Creates a number from another value that can be converted into
-    /// a primitive via the `ToPrimitive` trait.
+    /// a primitive via the `ToPrimitive` trait. If the source value cannot be
+    /// represented by the target type, then `None` is returned.
     fn from<T: ToPrimitive>(n: T) -> Option<Self>;
 }