Make time::Instant::actually_monotonic() a const fn.

src/libstd/sys/cloudabi/time.rs

@@ -25,7 +25,7 @@ impl Instant {
         }
     }
 
-    pub fn actually_monotonic() -> bool {
+    pub const fn actually_monotonic() -> bool {
         true
     }
 

src/libstd/sys/hermit/time.rs

@@ -128,7 +128,7 @@ impl Instant {
         Instant { t: Timespec::zero() }
     }
 
-    pub fn actually_monotonic() -> bool {
+    pub const fn actually_monotonic() -> bool {
         true
     }
 

src/libstd/sys/sgx/time.rs

@@ -26,7 +26,7 @@ impl Instant {
         Some(Instant(self.0.checked_sub(*other)?))
     }
 
-    pub fn actually_monotonic() -> bool {
+    pub const fn actually_monotonic() -> bool {
         false
     }
 

src/libstd/sys/unix/time.rs

@@ -158,7 +158,7 @@ mod inner {
             Instant { t: 0 }
         }
 
-        pub fn actually_monotonic() -> bool {
+        pub const fn actually_monotonic() -> bool {
             true
         }
 
@@ -308,11 +308,21 @@ mod inner {
             }
         }
 
-        pub fn actually_monotonic() -> bool {
-            (cfg!(target_os = "linux") && cfg!(target_arch = "x86_64")) ||
-            (cfg!(target_os = "linux") && cfg!(target_arch = "x86")) ||
-            cfg!(target_os = "fuchsia") ||
-            false // last clause, used so `||` is always trailing above
+        pub const fn actually_monotonic() -> bool {
+            // note: duplicate attributes may be optimized when
+            // conditional expressions in constant functions hits stable.
+            #[cfg(any(
+                all(target_os = "linux", target_arch = "x86_64"),
+                all(target_os = "linux", target_arch = "x86"),
+                all(target_os = "fuchsia"),
+            ))]
+            { true }
+            #[cfg(not(any(
+                all(target_os = "linux", target_arch = "x86_64"),
+                all(target_os = "linux", target_arch = "x86"),
+                all(target_os = "fuchsia"),
+            )))]
+            { false }
         }
 
         pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {

src/libstd/sys/vxworks/time.rs

@@ -147,7 +147,7 @@ mod inner {
             }
         }
 
-        pub fn actually_monotonic() -> bool {
+        pub const fn actually_monotonic() -> bool {
             true
         }
 

src/libstd/sys/wasi/time.rs

@@ -29,7 +29,7 @@ impl Instant {
         Instant(Duration::from_secs(0))
     }
 
-    pub fn actually_monotonic() -> bool {
+    pub const fn actually_monotonic() -> bool {
         true
     }
 

src/libstd/sys/wasm/time.rs

@@ -17,7 +17,7 @@ impl Instant {
         Instant(Duration::from_secs(0))
     }
 
-    pub fn actually_monotonic() -> bool {
+    pub const fn actually_monotonic() -> bool {
         false
     }
 

src/libstd/sys/windows/time.rs

@@ -41,7 +41,7 @@ impl Instant {
         perf_counter::PerformanceCounterInstant::now().into()
     }
 
-    pub fn actually_monotonic() -> bool {
+    pub const fn actually_monotonic() -> bool {
         false
     }