Add Method::from_static

Allows creating constant `Method` instances, e.g.:

    const PROPFIND: Method = Method::from_static(b"PROPFIND");

Fixes: hyperium#587

src/method.rs

@@ -136,6 +136,18 @@ impl Method {
         }
     }
 
+    /// Convert static bytes into a `Method`.
+    ///
+    /// The method name can only be up to 15 bytes long.
+    ///
+    /// # Panics
+    ///
+    /// If the input bytes are not a valid method name.
+    pub const fn from_static(src: &'static [u8]) -> Method {
+        let inline = InlineExtension::from_static(src);
+        Method(ExtensionInline(inline))
+    }
+
     fn extension_inline(src: &[u8]) -> Result<Method, InvalidMethod> {
         let inline = InlineExtension::new(src)?;
 
@@ -335,6 +347,29 @@ mod extension {
             Ok(InlineExtension(data, src.len() as u8))
         }
 
+        /// Convert static bytes into an `InlineExtension`.
+        ///
+        /// Only the first 15 characters are used.
+        ///
+        /// # Panics
+        ///
+        /// If the input bytes are not a valid method name.
+        pub const fn from_static(src: &'static [u8]) -> InlineExtension {
+            let mut i = 0;
+            let mut dst = [0u8;15];
+            while i < src.len() {
+                let byte = src[i] ;
+                let v = METHOD_CHARS[byte as usize];
+                if v == 0 {
+                    panic!("Invalid byte for method name.");
+                }
+                dst[i] = byte;
+                i += 1;
+            }
+
+            InlineExtension(dst, src.len() as u8)
+        }
+
         pub fn as_str(&self) -> &str {
             let InlineExtension(ref data, len) = self;
             // Safety: the invariant of InlineExtension ensures that the first
@@ -440,6 +475,17 @@ mod test {
         assert_eq!(Method::GET, &Method::GET);
     }
 
+    #[test]
+    fn test_from_static() {
+        assert_eq!(Method::from_static(b"PROPFIND"), Method::from_bytes(b"PROPFIND").unwrap());
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_from_static_bad() {
+        Method::from_static(b"\0");
+    }
+
     #[test]
     fn test_invalid_method() {
         assert!(Method::from_str("").is_err());