Add FromStr for FixedOffset

Closes chronotope#314

src/format/mod.rs

@@ -47,8 +47,9 @@ use std::error::Error;
 
 #[cfg(any(feature = "alloc", feature = "std", test))]
 use crate::naive::{NaiveDate, NaiveTime};
+use crate::offset::FixedOffset;
 #[cfg(any(feature = "alloc", feature = "std", test))]
-use crate::offset::{FixedOffset, Offset};
+use crate::offset::Offset;
 #[cfg(any(feature = "alloc", feature = "std", test))]
 use crate::{Datelike, Timelike};
 use crate::{Month, ParseMonthError, ParseWeekdayError, Weekday};
@@ -986,6 +987,15 @@ impl FromStr for Weekday {
     }
 }
 
+/// Parsing a `str` into a `FixedOffset` uses the format [`%z`](./format/strftime/index.html).
+impl FromStr for FixedOffset {
+    type Err = ParseError;
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let (_, offset) = scan::timezone_offset(s, scan::consume_colon_maybe)?;
+        Self::east_opt(offset).ok_or(OUT_OF_RANGE)
+    }
+}
+
 /// Formats single formatting item
 #[cfg(feature = "unstable-locales")]
 #[cfg_attr(docsrs, doc(cfg(feature = "unstable-locales")))]

src/format/scan.rs

@@ -221,7 +221,7 @@ pub(super) fn trim1(s: &str) -> &str {
 
 /// Consumes one colon char `:` if it is at the front of `s`.
 /// Always returns `Ok(s)`.
-pub(super) fn consume_colon_maybe(mut s: &str) -> ParseResult<&str> {
+pub(crate) fn consume_colon_maybe(mut s: &str) -> ParseResult<&str> {
     if s.is_empty() {
         // nothing consumed
         return Ok(s);
@@ -239,7 +239,7 @@ pub(super) fn consume_colon_maybe(mut s: &str) -> ParseResult<&str> {
 ///
 /// 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>,
 {