Add trait IntoAscii to replace AsciiCast<Target=Ascii>

`AsciiCast` requires an explicit lifetime, which doesn't make sense for `Target=Ascii`:
`fn by_value<'a,C:AsciiCast<'a,Target=Ascii>>(ch: C) {…}` doesn't compile (`'a` doesn't appear anywhere)
so you have to borrow ch: `fn by_ref<'a,C:AsciiCast<'a,Target=Ascii>>(c: &'a C) {…}`.
But then you have to explicitly borrow it in the caller: `by_ref(& 'a');` which looks bad.

Names are also more in line with what std uses: xxx_unchecked() and Into/From.

Author: tormol

src/ascii.rs

@@ -1,6 +1,6 @@
 use std::mem::transmute;
 use std::fmt;
-#[cfg(feature="unstable")]
+use std::error::Error;
 use std::ascii::AsciiExt;
 
 use AsciiCast;
@@ -279,7 +279,7 @@ pub enum Ascii {
 }
 
 impl Ascii {
-    /// Constructs an Ascii character from a `char`.
+    /// Constructs an Ascii character from a `char` or `u8`.
     ///
     /// # Failure
     ///
@@ -292,11 +292,13 @@ impl Ascii {
     /// assert_eq!(a.as_char(), 'g');
     /// ```
     #[inline]
-    pub fn from(ch: char) -> Result<Ascii, ()> {
-        unsafe{if ch as u32 <= 0x7F {
-            return Ok(ch.to_ascii_nocheck());
-        }}
-        Err(())
+    pub fn from<C:IntoAscii>(ch: C) -> Result<Self, ()> {
+        ch.into_ascii().map_err(|_| () )
+    }
+
+    /// Constructs an Ascii character from a `char` or `u8` without any checks.
+    pub unsafe fn from_unchecked<C:IntoAscii>(ch: C) -> Self {
+        ch.into_ascii_unchecked()
     }
 
     /// Constructs an Ascii character from a `u8`.
@@ -530,10 +532,73 @@ impl<'a> AsciiCast<'a> for char {
     }
 }
 
+
+
+const ERRORMSG_CHAR: &'static str = "not an ASCII character";
+/// Error returned by `IntoAscii.into_ascii()` when the parameter is greater than 127.
+#[derive(PartialEq)]
+pub struct IntoAsciiError(());
+impl fmt::Debug for IntoAsciiError {
+    fn fmt(&self,  fmtr: &mut fmt::Formatter) -> fmt::Result {
+        write!(fmtr, "{}", ERRORMSG_CHAR)
+    }
+}
+impl fmt::Display for IntoAsciiError {
+    fn fmt(&self,  fmtr: &mut fmt::Formatter) -> fmt::Result {
+        write!(fmtr, "{}", ERRORMSG_CHAR)
+    }
+}
+impl Error for IntoAsciiError {
+    fn description(&self) -> &'static str {
+        ERRORMSG_CHAR
+    }
+}
+
+
+/// Trait for converting `char` and `u8` into `Ascii`.
+pub trait IntoAscii : AsciiExt {
+    /// Convert to `Ascii` without checking if it's less than 128.
+    unsafe fn into_ascii_unchecked(self) -> Ascii;
+    /// Convert to `Ascii`.
+    fn into_ascii(self) -> Result<Ascii,IntoAsciiError>;
+}
+
+#[cfg(feature = "unstable")]
+impl IntoAscii for Ascii {
+    fn into_ascii(self) -> Result<Ascii,IntoAsciiError> {
+        Ok(self)
+    }
+    unsafe fn into_ascii_unchecked(self) -> Ascii {
+        self
+    }
+}
+impl IntoAscii for u8 {
+    fn into_ascii(self) -> Result<Ascii,IntoAsciiError> {
+        unsafe{if self <= 0x7F {
+            return Ok(self.into_ascii_unchecked());
+        }}
+        Err(IntoAsciiError(()))
+    }
+    unsafe fn into_ascii_unchecked(self) -> Ascii {
+        transmute(self)
+    }
+}
+impl IntoAscii for char {
+    fn into_ascii(self) -> Result<Ascii,IntoAsciiError> {
+        unsafe{if self as u32 <= 0x7F {
+            return Ok(self.into_ascii_unchecked());
+        }}
+        Err(IntoAsciiError(()))
+    }
+    unsafe fn into_ascii_unchecked(self) -> Ascii {
+        (self as u8).into_ascii_unchecked()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use AsciiCast;
-    use super::Ascii;
+    use super::{Ascii,IntoAscii,IntoAsciiError};
 
     #[test]
     fn to_ascii() {
@@ -544,6 +609,15 @@ mod tests {
         assert_eq!('λ'.to_ascii(), Err(()));
     }
 
+    #[test]
+    fn into_ascii() {
+        fn generic<C:IntoAscii>(c: C) -> Result<Ascii,IntoAsciiError> {
+            c.into_ascii()
+        }
+        assert_eq!(generic('A'), Ok(Ascii::A));
+        assert_eq!(generic(b'A'), Ok(Ascii::A));
+    }
+
     #[test]
     fn as_byte() {
         assert_eq!(65u8.to_ascii().unwrap().as_byte(), 65u8);

src/lib.rs

@@ -21,7 +21,7 @@ mod ascii_str;
 use std::borrow::Borrow;
 use std::ascii::AsciiExt;
 
-pub use ascii::Ascii;
+pub use ascii::{Ascii, IntoAscii, IntoAsciiError};
 pub use ascii_string::AsciiString;
 pub use ascii_str::AsciiStr;