Correctly format HTTPDate using time-rs

Format strings for time-rs are an original and more complex syntax than
any of the strptime inspired examples that preceeded it.

This syntax must be parsed before it can be used in a formatter, so we
invoke a proc-macro to do that at compile time rather than every time
the formatter is required.

As time-rs requires an MSRV of 1.51, we must also increase our minimum
supported version.

.github/workflows/ci.yaml

@@ -32,7 +32,7 @@ jobs:
         rust:
           - stable
           - nightly
-          - 1.48
+          - 1.51
         features:
           - default
           - ssl

Cargo.toml

@@ -20,7 +20,7 @@ chunked_transfer = "1"
 openssl = { version = "0.10", optional = true }
 url = "2"
 log = "0.4"
-time = { version = "0.3", features = [ "std", "formatting" ] }
+time = { version = "0.3", features = [ "std", "formatting", "macros", "parsing" ] }
 
 [dev-dependencies]
 rustc-serialize = "0.3"

src/common.rs

@@ -2,7 +2,7 @@ use ascii::{AsciiStr, AsciiString, FromAsciiError};
 use std::cmp::Ordering;
 use std::fmt::{self, Display, Formatter};
 use std::str::FromStr;
-use time::{format_description, OffsetDateTime};
+use time::OffsetDateTime;
 
 /// Status code of a request or response.
 #[derive(Eq, PartialEq, Copy, Clone, Debug, Ord, PartialOrd)]
@@ -394,7 +394,7 @@ impl From<(u8, u8)> for HTTPVersion {
     }
 }
 
-/// Represents the current date, expressed in RFC 1123 format, e.g. Sun, 06 Nov 1994 08:49:37 GMT
+/// Represents the current date, expressed in RFC 7231 (IMF-fixdate) format, e.g. Sun, 06 Nov 1994 08:49:37 GMT
 #[allow(clippy::upper_case_acronyms)]
 pub struct HTTPDate {
     d: OffsetDateTime,
@@ -408,20 +408,27 @@ impl HTTPDate {
     }
 }
 
+/// Format description for emitting a [`time::PrimitiveDateTime`] or [`time::OffsetDateTime`] in the format required by RFC7231
+///
+/// `format_description!` is a procedural macro, but since `time-rs` doesn't provide any other way
+/// to construct the format description slice in a `const` context we haven't much choice.
+const IMF_FIXDATE_FORMAT: &[time::format_description::FormatItem<'static>] = time::macros::format_description!(
+    "[weekday repr:short], [day padding:zero] [month repr:short] [year repr:full] [hour repr:24 padding:zero]:[minute padding:zero]:[second padding:zero] GMT"
+);
+
 impl ToString for HTTPDate {
     fn to_string(&self) -> String {
-        // Note: This can probably be made Self::format however parse is not a const function, making this difficult.
-        let format = format_description::parse("%a, %e %b %Y %H:%M:%S GMT")
-            .expect("Cannot fail.  The format string is correct.");
         self.d
-            .format(&format)
+            .format(&IMF_FIXDATE_FORMAT)
             .expect("Cannot fail with this format under any reasonable conditions.")
     }
 }
 
 #[cfg(test)]
 mod test {
     use super::Header;
+    use crate::common::HTTPDate;
+    use time::OffsetDateTime;
 
     #[test]
     fn test_parse_header() {
@@ -433,6 +440,15 @@ mod test {
         assert!("hello world".parse::<Header>().is_err());
     }
 
+    #[test]
+    fn formats_date_correctly() {
+        let http_date = HTTPDate {
+            d: OffsetDateTime::from_unix_timestamp(420895020).unwrap(),
+        };
+
+        assert_eq!(http_date.to_string(), "Wed, 04 May 1983 11:17:00 GMT")
+    }
+
     #[test]
     fn test_parse_header_with_doublecolon() {
         let header: Header = "Time: 20: 34".parse().unwrap();