Clarify localtime and oldtime's docs

src/lib.rs

@@ -38,15 +38,22 @@
 //!
 //! Default features:
 //!
-//! - `alloc`: Enable features that depend on allocation (primarily string formatting)
 //! - `std`: Enables functionality that depends on the standard library. This
 //!   is a superset of `alloc` and adds interoperation with standard library types
 //!   and traits.
 //! - `clock`: enables reading the system time (`now`), independent of whether
 //!   `std::time::SystemTime` is present, depends on having a libc.
+//! - `oldtime`: **deprecated** Cause the `chrono::Duration` time to be exactly
+//!   the `Duration` in `time` 0.1's Duration.
+//!
+//!    It is recommended to disable default features and to not rely on this,
+//!    instead manually converting between the two Duration types in your own
+//!    code if necessary.
 //!
 //! Optional features:
 //!
+//! - `alloc`: If the `std` feature is disabled, this can be used to re-enable string
+//!   formatting and some other items that require the alloc crate.
 //! - `wasmbind`: Enable integration with [wasm-bindgen][] and its `js-sys` project
 //! - [`serde`][]: Enable serialization/deserialization via serde.
 //! - `unstable-locales`: Enable localization. This adds various methods with a

src/offset/local.rs

@@ -97,12 +97,32 @@ impl Local {
     }
 
     /// Returns a `DateTime` which corresponds to the current date.
+    ///
+    /// # Thread Safety Warning
+    ///
+    /// If, on a Unix OS, a thread sets any environment variable while another
+    /// thread tries to determine the local time your program may get a
+    /// segfault or crash in other interesting ways. Conversation in [chrono]
+    /// and [rust-std].
+    ///
+    /// [chrono]: https://github.com/chronotope/chrono/pull/578
+    /// [rust-std]: https://github.com/rust-lang/rust/issues/27970
     #[cfg(not(all(target_arch = "wasm32", not(target_os = "wasi"), feature = "wasmbind")))]
     pub fn now() -> DateTime<Local> {
         tm_to_datetime(Timespec::now().local())
     }
 
     /// Returns a `DateTime` which corresponds to the current date.
+    ///
+    /// # Thread Safety Warning
+    ///
+    /// If, on a Unix OS, a thread sets any environment variable while another
+    /// thread tries to determine the local time your program may get a
+    /// segfault or crash in other interesting ways. Conversation in [chrono]
+    /// and [rust-std].
+    ///
+    /// [chrono]: https://github.com/chronotope/chrono/pull/578
+    /// [rust-std]: https://github.com/rust-lang/rust/issues/27970
     #[cfg(all(target_arch = "wasm32", not(target_os = "wasi"), feature = "wasmbind"))]
     pub fn now() -> DateTime<Local> {
         use super::Utc;

src/sys.rs

@@ -47,6 +47,16 @@ impl Timespec {
     }
 
     /// Converts this timespec into the system's local time.
+    ///
+    /// # Thread Safety Warning
+    ///
+    /// If, on a Unix OS, a thread sets any environment variable while another
+    /// thread tries to determine the local time your program may get a
+    /// segfault or crash in other interesting ways. Conversation in [chrono]
+    /// and [rust-std].
+    ///
+    /// [chrono]: https://github.com/chronotope/chrono/pull/578
+    /// [rust-std]: https://github.com/rust-lang/rust/issues/27970
     pub fn local(self) -> Tm {
         let mut tm = Tm {
             tm_sec: 0,