Always test local offset

The reproduction requires tens of thousands of environment variables to
be set. Testing sets at most a handful. Let's try our luck.

tests/main.rs

@@ -1,6 +1,7 @@
 #![deny(
     anonymous_parameters,
     clippy::all,
+    clippy::undocumented_unsafe_blocks,
     illegal_floating_point_literal_pattern,
     late_bound_lifetime_arguments,
     path_statements,
@@ -9,7 +10,6 @@
     trivial_casts,
     trivial_numeric_casts,
     unreachable_pub,
-    unsafe_code,
     unused_extern_crates
 )]
 #![warn(
@@ -101,6 +101,12 @@ macro_rules! require_all_features {
 }
 
 require_all_features! {
+    use std::sync::Mutex;
+
+    /// A lock to ensure that certain tests don't run in parallel, which could lead to a test
+    /// unexpectedly failing.
+    static SOUNDNESS_LOCK: Mutex<()> = Mutex::new(());
+
     /// Construct a non-exhaustive modifier.
     macro_rules! modifier {
         ($name:ident {

tests/offset_date_time.rs

@@ -11,14 +11,19 @@ fn now_utc() {
     assert_eq!(OffsetDateTime::now_utc().offset(), offset!(UTC));
 }
 
+#[cfg_attr(miri, ignore)]
 #[test]
 fn now_local() {
-    #[cfg(not(target_family = "unix"))]
-    assert!(OffsetDateTime::now_local().is_ok());
+    use time::util::local_offset::*;
+
+    let _guard = crate::SOUNDNESS_LOCK.lock().unwrap();
 
-    // Include for test coverage.
-    #[cfg(target_family = "unix")]
-    let _ = OffsetDateTime::now_local();
+    // Safety: Technically not sound. However, this is a test, and it's highly improbable that we
+    // will run into issues with setting an environment variable a few times.
+    unsafe { set_soundness(Soundness::Unsound) };
+    assert!(OffsetDateTime::now_local().is_ok());
+    // Safety: We're setting it back to sound.
+    unsafe { set_soundness(Soundness::Sound) };
 }
 
 #[test]

tests/utc_offset.rs

@@ -144,24 +144,37 @@ fn neg() {
     assert_eq!(-offset!(-23:59:59), offset!(+23:59:59));
 }
 
+#[cfg_attr(miri, ignore)]
 #[test]
 fn local_offset_at() {
-    #[cfg(not(target_family = "unix"))]
-    assert!(UtcOffset::local_offset_at(OffsetDateTime::UNIX_EPOCH).is_ok());
+    use time::util::local_offset::*;
+
+    let _guard = crate::SOUNDNESS_LOCK.lock().unwrap();
 
-    #[cfg(target_family = "unix")]
-    let _ = UtcOffset::local_offset_at(OffsetDateTime::UNIX_EPOCH);
+    // Safety: Technically not sound. However, this is a test, and it's highly improbable that we
+    // will run into issues with setting an environment variable a few times.
+    unsafe { set_soundness(Soundness::Unsound) };
+    assert!(UtcOffset::local_offset_at(OffsetDateTime::UNIX_EPOCH).is_ok());
+    // Safety: We're setting it back to sound.
+    unsafe { set_soundness(Soundness::Sound) };
 }
 
+#[cfg_attr(miri, ignore)]
 #[test]
 fn current_local_offset() {
-    #[cfg(not(target_family = "unix"))]
-    assert!(UtcOffset::current_local_offset().is_ok());
+    use time::util::local_offset::*;
+
+    let _guard = crate::SOUNDNESS_LOCK.lock().unwrap();
 
-    #[cfg(target_family = "unix")]
-    let _ = UtcOffset::current_local_offset();
+    // Safety: Technically not sound. However, this is a test, and it's highly improbable that we
+    // will run into issues with setting an environment variable a few times.
+    unsafe { set_soundness(Soundness::Unsound) };
+    assert!(UtcOffset::current_local_offset().is_ok());
+    // Safety: We're setting it back to sound.
+    unsafe { set_soundness(Soundness::Sound) };
 }
 
+// Note: This behavior is not guaranteed and will hopefully be changed in the future.
 #[test]
 #[cfg_attr(
     any(

tests/util.rs

@@ -77,16 +77,18 @@ fn weeks_in_year() {
     }
 }
 
-#[allow(unsafe_code)]
 #[test]
 fn local_offset_soundness() {
     use time::util::local_offset::*;
 
-    // Safety: cargo does not mutate the environment while tests are running.
+    let _guard = crate::SOUNDNESS_LOCK.lock().unwrap();
 
     assert_eq!(get_soundness(), Soundness::Sound);
+    // Safety: Technically not sound. However, this is a test, and it's highly improbable that we
+    // will run into issues with setting an environment variable a few times.
     unsafe { set_soundness(Soundness::Unsound) };
     assert_eq!(get_soundness(), Soundness::Unsound);
+    // Safety: We're setting it back to sound.
     unsafe { set_soundness(Soundness::Sound) };
     assert_eq!(get_soundness(), Soundness::Sound);
 }

time/src/sys/local_offset_at/mod.rs

@@ -19,5 +19,10 @@ use crate::{OffsetDateTime, UtcOffset};
 /// Attempt to obtain the system's UTC offset. If the offset cannot be determined, `None` is
 /// returned.
 pub(crate) fn local_offset_at(datetime: OffsetDateTime) -> Option<UtcOffset> {
-    imp::local_offset_at(datetime)
+    // miri does not support tzset()
+    if cfg!(miri) {
+        None
+    } else {
+        imp::local_offset_at(datetime)
+    }
 }