Add Method::from_static

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

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

Fixes: #587

Author: WhyNotHugo

Diff for: src/method.rs

@@ -15,6 +15,8 @@
 //! assert_eq!(Method::POST.as_str(), "POST");
 //! ```
 
+use extension::StaticExtension;
+
 use self::extension::{AllocatedExtension, InlineExtension};
 use self::Inner::*;
 
@@ -64,6 +66,8 @@ enum Inner {
     ExtensionInline(InlineExtension),
     // Otherwise, allocate it
     ExtensionAllocated(AllocatedExtension),
+    // Statically allocated data
+    ExtensionStatic(StaticExtension),
 }
 
 impl Method {
@@ -134,6 +138,30 @@ impl Method {
         }
     }
 
+    /// Convert static bytes into a `Method`.
+    pub const fn from_static(src: &'static [u8]) -> Method {
+        match src {
+            b"OPTIONS" => Method::OPTIONS,
+            b"GET" => Method::GET,
+            b"POST" => Method::POST,
+            b"PUT" => Method::PUT,
+            b"DELETE" => Method::DELETE,
+            b"HEAD" => Method::HEAD,
+            b"TRACE" => Method::TRACE,
+            b"CONNECT" => Method::CONNECT,
+            b"PATCH" => Method::PATCH,
+            src => {
+                if src.len() <= 15 {
+                    let inline = InlineExtension::from_static(src);
+                    Method(ExtensionInline(inline))
+                } else {
+                    let allocated = StaticExtension::from_static(src);
+                    Method(ExtensionStatic(allocated))
+                }
+            }
+        }
+    }
+
     fn extension_inline(src: &[u8]) -> Result<Method, InvalidMethod> {
         let inline = InlineExtension::new(src)?;
 
@@ -176,6 +204,7 @@ impl Method {
             Patch => "PATCH",
             ExtensionInline(ref inline) => inline.as_str(),
             ExtensionAllocated(ref allocated) => allocated.as_str(),
+            ExtensionStatic(ref s) => s.as_str(),
         }
     }
 }
@@ -316,6 +345,9 @@ mod extension {
     // Invariant: self.0 contains valid UTF-8.
     pub struct AllocatedExtension(Box<[u8]>);
 
+    #[derive(Clone, PartialEq, Eq, Hash)]
+    pub struct StaticExtension(&'static [u8]);
+
     impl InlineExtension {
         // Method::from_bytes() assumes this is at least 7
         pub const MAX: usize = 15;
@@ -330,6 +362,34 @@ mod extension {
             Ok(InlineExtension(data, src.len() as u8))
         }
 
+        /// Convert static bytes into an `InlineExtension`.
+        ///
+        /// # Panics
+        ///
+        /// If the input bytes are not a valid method name or if the method name is over 15 bytes.
+        pub const fn from_static(src: &'static [u8]) -> InlineExtension {
+            let mut i = 0;
+            let mut dst = [0u8; 15];
+            if src.len() > 15 {
+                // panicking in const requires Rust 1.57.0
+                #[allow(unconditional_panic)]
+                ([] as [u8; 0])[0];
+            }
+            while i < src.len() {
+                let byte = src[i];
+                let v = METHOD_CHARS[byte as usize];
+                if v == 0 {
+                    // panicking in const requires Rust 1.57.0
+                    #[allow(unconditional_panic)]
+                    ([] as [u8; 0])[0];
+                }
+                dst[i] = byte;
+                i += 1;
+            }
+
+            InlineExtension(dst, i as u8)
+        }
+
         pub fn as_str(&self) -> &str {
             let InlineExtension(ref data, len) = self;
             // Safety: the invariant of InlineExtension ensures that the first
@@ -356,6 +416,32 @@ mod extension {
         }
     }
 
+    impl StaticExtension {
+        pub const fn from_static(src: &'static [u8]) -> StaticExtension {
+            let mut i = 0;
+            while i < src.len() {
+                let byte = src[i];
+                let v = METHOD_CHARS[byte as usize];
+                if v == 0 {
+                    // panicking in const requires Rust 1.57.0
+                    #[allow(unconditional_panic)]
+                    ([] as [u8; 0])[0];
+                }
+                i += 1;
+            }
+
+            // Invariant: data is exactly src.len() long and write_checked
+            // ensures that the first src.len() bytes of data are valid UTF-8.
+            StaticExtension(src)
+        }
+
+        pub fn as_str(&self) -> &str {
+            // Safety: the invariant of StaticExtension ensures that self.0
+            // contains valid UTF-8.
+            unsafe { str::from_utf8_unchecked(&self.0) }
+        }
+    }
+
     // From the RFC 9110 HTTP Semantics, section 9.1, the HTTP method is case-sensitive and can
     // contain the following characters:
     //
@@ -436,6 +522,38 @@ mod test {
         assert_eq!(Method::GET, &Method::GET);
     }
 
+    #[test]
+    fn test_from_static() {
+        // First class variant
+        assert_eq!(
+            Method::from_static(b"GET"),
+            Method::from_bytes(b"GET").unwrap()
+        );
+        // Inline, len < 15
+        assert_eq!(
+            Method::from_static(b"PROPFIND"),
+            Method::from_bytes(b"PROPFIND").unwrap()
+        );
+        // Inline, len == 15
+        assert_eq!(Method::from_static(b"GET"), Method::GET);
+        assert_eq!(
+            Method::from_static(b"123456789012345").to_string(),
+            "123456789012345".to_string()
+        );
+        // Ref, len > 15
+        Method::from_static(b"1234567890123456");
+        assert_eq!(
+            Method::from_static(b"1234567890123456").to_string(),
+            "1234567890123456".to_string()
+        );
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_from_static_bad() {
+        Method::from_static(b"\0");
+    }
+
     #[test]
     fn test_invalid_method() {
         assert!(Method::from_str("").is_err());