Implement TryFrom<f64> for f32

src/libcore/num/mod.rs

@@ -4338,6 +4338,44 @@ mod ptr_try_from_impls {
     rev!(try_from_both_bounded, usize, i128);
 }
 
+/// The error type returned when a checked float type conversion fails.
+#[unstable(feature = "try_from_float", issue = "0")]
+#[derive(Debug, Copy, Clone)]
+pub struct TryFromFloatError(());
+
+impl TryFromFloatError {
+    #[unstable(feature = "float_error_internals",
+               reason = "available through Error trait and this method should \
+                         not be exposed publicly",
+               issue = "0")]
+    #[doc(hidden)]
+    pub fn __description(&self) -> &str {
+        "lossy float type conversion attempted"
+    }
+}
+
+#[unstable(feature = "try_from_float", issue = "0")]
+impl fmt::Display for TryFromFloatError {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        self.__description().fmt(fmt)
+    }
+}
+
+#[stable(feature = "try_from_float_impl", since = "1.28.0")]
+impl TryFrom<f64> for f32 {
+    type Error = TryFromFloatError;
+
+    #[inline]
+    fn try_from(x: f64) -> Result<f32, TryFromFloatError> {
+        let y = x as f32;
+        if y as f64 == x {
+            Ok(y)
+        } else {
+            Err(TryFromFloatError(()))
+        }
+    }
+}
+
 #[doc(hidden)]
 trait FromStrRadixHelper: PartialOrd + Copy {
     fn min_value() -> Self;

src/libstd/error.rs

@@ -278,6 +278,13 @@ impl Error for num::TryFromIntError {
     }
 }
 
+#[unstable(feature = "try_from_float", issue = "0")]
+impl Error for num::TryFromFloatError {
+    fn description(&self) -> &str {
+        self.__description()
+    }
+}
+
 #[unstable(feature = "try_from", issue = "33417")]
 impl Error for array::TryFromSliceError {
     fn description(&self) -> &str {

src/libstd/lib.rs

@@ -259,6 +259,7 @@
 #![feature(external_doc)]
 #![feature(fs_read_write)]
 #![feature(fixed_size_array)]
+#![feature(float_error_internals)]
 #![feature(float_from_str_radix)]
 #![cfg_attr(stage0, feature(float_internals))]
 #![feature(fn_traits)]
@@ -308,6 +309,7 @@
 #![feature(thread_local)]
 #![feature(toowned_clone_into)]
 #![feature(try_from)]
+#![feature(try_from_float)]
 #![feature(try_reserve)]
 #![feature(unboxed_closures)]
 #![feature(untagged_unions)]

src/libstd/num.rs

@@ -18,6 +18,8 @@
 
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::num::{FpCategory, ParseIntError, ParseFloatError, TryFromIntError};
+#[unstable(feature = "try_from_float", issue = "0")]
+pub use core::num::TryFromFloatError;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::num::Wrapping;