Add FromStr for FixedOffset (#1157)

Co-authored-by: Yuxuan Shui <yshuiv7@gmail.com>

src/format/mod.rs

@@ -420,7 +420,7 @@ impl Error for ParseError {
 }
 
 // to be used in this module and submodules
-const OUT_OF_RANGE: ParseError = ParseError(ParseErrorKind::OutOfRange);
+pub(crate) const OUT_OF_RANGE: ParseError = ParseError(ParseErrorKind::OutOfRange);
 const IMPOSSIBLE: ParseError = ParseError(ParseErrorKind::Impossible);
 const NOT_ENOUGH: ParseError = ParseError(ParseErrorKind::NotEnough);
 const INVALID: ParseError = ParseError(ParseErrorKind::Invalid);
@@ -838,7 +838,7 @@ mod parsed;
 
 // due to the size of parsing routines, they are in separate modules.
 mod parse;
-mod scan;
+pub(crate) mod scan;
 
 pub mod strftime;
 

src/format/scan.rs

@@ -197,15 +197,15 @@ pub(super) fn space(s: &str) -> ParseResult<&str> {
 }
 
 /// Consumes any number (including zero) of colon or spaces.
-pub(super) fn colon_or_space(s: &str) -> ParseResult<&str> {
+pub(crate) fn colon_or_space(s: &str) -> ParseResult<&str> {
     Ok(s.trim_start_matches(|c: char| c == ':' || c.is_whitespace()))
 }
 
 /// Tries to parse `[-+]\d\d` continued by `\d\d`. Return an offset in seconds if possible.
 ///
 /// The additional `colon` may be used to parse a mandatory or optional `:`
 /// between hours and minutes, and should return either a new suffix or `Err` when parsing fails.
-pub(super) fn timezone_offset<F>(s: &str, consume_colon: F) -> ParseResult<(&str, i32)>
+pub(crate) fn timezone_offset<F>(s: &str, consume_colon: F) -> ParseResult<(&str, i32)>
 where
     F: FnMut(&str) -> ParseResult<&str>,
 {

src/offset/fixed.rs

@@ -5,14 +5,18 @@
 
 use core::fmt;
 use core::ops::{Add, Sub};
+use core::str::FromStr;
 
 #[cfg(feature = "rkyv")]
 use rkyv::{Archive, Deserialize, Serialize};
 
 use super::{LocalResult, Offset, TimeZone};
+use crate::format::scan;
+use crate::format::OUT_OF_RANGE;
 use crate::naive::{NaiveDate, NaiveDateTime, NaiveTime};
 use crate::oldtime::Duration as OldDuration;
 use crate::DateTime;
+use crate::ParseError;
 use crate::Timelike;
 
 /// The time zone with fixed offset, from UTC-23:59:59 to UTC+23:59:59.
@@ -113,6 +117,15 @@ impl FixedOffset {
     }
 }
 
+/// Parsing a `str` into a `FixedOffset` uses the format [`%z`](crate::format::strftime).
+impl FromStr for FixedOffset {
+    type Err = ParseError;
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let (_, offset) = scan::timezone_offset(s, scan::colon_or_space)?;
+        Self::east_opt(offset).ok_or(OUT_OF_RANGE)
+    }
+}
+
 impl TimeZone for FixedOffset {
     type Offset = FixedOffset;
 
@@ -246,6 +259,7 @@ impl<Tz: TimeZone> Sub<FixedOffset> for DateTime<Tz> {
 mod tests {
     use super::FixedOffset;
     use crate::offset::TimeZone;
+    use std::str::FromStr;
 
     #[test]
     fn test_date_extreme_offset() {
@@ -292,4 +306,14 @@ mod tests {
             "2012-03-04T05:06:07-23:59:59".to_string()
         );
     }
+
+    #[test]
+    fn test_parse_offset() {
+        let offset = FixedOffset::from_str("-0500").unwrap();
+        assert_eq!(offset.local_minus_utc, -5 * 3600);
+        let offset = FixedOffset::from_str("-08:00").unwrap();
+        assert_eq!(offset.local_minus_utc, -8 * 3600);
+        let offset = FixedOffset::from_str("+06:30").unwrap();
+        assert_eq!(offset.local_minus_utc, (6 * 3600) + 1800);
+    }
 }