Implement --cfg emulate_second_only_system

Author: sunfishcode

cap-primitives/src/fs/system_time_spec.rs

@@ -15,8 +15,9 @@ pub enum SystemTimeSpec {
 
 impl SystemTimeSpec {
     /// Constructs a new instance of `Self` from the given `fs_set_times::SystemTimeSpec`.
+    // TODO: Make this a `const fn` once `SystemTime::from_std` is a `const fn`.
     #[inline]
-    pub const fn from_std(std: fs_set_times::SystemTimeSpec) -> Self {
+    pub fn from_std(std: fs_set_times::SystemTimeSpec) -> Self {
         match std {
             fs_set_times::SystemTimeSpec::SymbolicNow => SystemTimeSpec::SymbolicNow,
             fs_set_times::SystemTimeSpec::Absolute(time) => {

cap-primitives/src/time/system_clock.rs

@@ -17,7 +17,9 @@ impl SystemClock {
     /// This corresponds to [`std::time::SystemTime::UNIX_EPOCH`].
     ///
     /// [`std::time::SystemTime::UNIX_EPOCH`]: https://doc.rust-lang.org/std/time/constant.UNIX_EPOCH.html
-    pub const UNIX_EPOCH: SystemTime = SystemTime::from_std(time::SystemTime::UNIX_EPOCH);
+    pub const UNIX_EPOCH: SystemTime = SystemTime {
+        std: time::SystemTime::UNIX_EPOCH,
+    };
 
     /// Constructs a new instance of `Self`.
     ///

cap-primitives/src/time/system_time.rs

@@ -16,21 +16,38 @@ use std::{
 /// [`SystemClock::elapsed`] instead. The `UNIX_EPOCH` constant is at
 /// [`SystemClock::UNIX_EPOCH`].
 ///
+/// Similar to the [`filetime` crate], when
+/// `RUSTFLAGS=--cfg emulate_second_only_system` is set, `SystemTime` will
+/// round times from the operating system down to the second. This emulates
+/// the behavior of some file systems, mostly
+/// [HFS](https://en.wikipedia.org/wiki/HFS_Plus), allowing debugging on other
+/// hardware.
+///
 /// [`std::time::SystemTime`]: https://doc.rust-lang.org/std/time/struct.SystemTime.html
 /// [`SystemClock`]: struct.SystemClock.html
 /// [`SystemClock::now`]: struct.SystemClock.html#method.now
 /// [`SystemClock::elapsed`]: struct.SystemClock.html#method.elapsed
 /// [`SystemClock::UNIX_EPOCH`]: struct.SystemClock.html#associatedconstant.UNIX_EPOCH
+/// [`filetime` crate]: https://crates.io/crates/filetime
 #[derive(Clone, Copy, Eq, PartialEq, Hash, Ord, PartialOrd)]
 pub struct SystemTime {
     pub(crate) std: time::SystemTime,
 }
 
 impl SystemTime {
     /// Constructs a new instance of `Self` from the given `std::time::SystemTime`.
+    // TODO: Make this a `const fn` once `time::Duration::checked_add` is a `const fn`.
     #[inline]
-    pub const fn from_std(std: time::SystemTime) -> Self {
-        Self { std }
+    pub fn from_std(std: time::SystemTime) -> Self {
+        if cfg!(emulate_second_only_system) {
+            let duration = std.duration_since(time::SystemTime::UNIX_EPOCH).unwrap();
+            let secs = time::Duration::from_secs(duration.as_secs());
+            Self {
+                std: time::SystemTime::UNIX_EPOCH.checked_add(secs).unwrap(),
+            }
+        } else {
+            Self { std }
+        }
     }
 
     /// Constructs a new instance of `std::time::SystemTime` from the given `Self`.