Add From<T> for Lazy{Cell,Lock}<T, F>

Author: Jules-Bertholet

library/core/src/cell/lazy.rs

@@ -370,6 +370,16 @@ impl<T: fmt::Debug, F> fmt::Debug for LazyCell<T, F> {
     }
 }
 
+#[stable(feature = "from_wrapper_impls", since = "CURRENT_RUSTC_VERSION")]
+impl<T, F> From<T> for LazyCell<T, F> {
+    /// Constructs a `LazyCell` that starts already initialized
+    /// with the provided value.
+    #[inline]
+    fn from(value: T) -> Self {
+        Self { state: UnsafeCell::new(State::Init(value)) }
+    }
+}
+
 #[cold]
 #[inline(never)]
 const fn panic_poisoned() -> ! {

library/std/src/backtrace/tests.rs

@@ -44,10 +44,9 @@ fn generate_fake_frames() -> Vec<BacktraceFrame> {
 #[test]
 fn test_debug() {
     let backtrace = Backtrace {
-        inner: Inner::Captured(LazyLock::preinit(Capture {
-            actual_start: 1,
-            frames: generate_fake_frames(),
-        })),
+        inner: Inner::Captured(
+            (Capture { actual_start: 1, frames: generate_fake_frames() }).into(),
+        ),
     };
 
     #[rustfmt::skip]
@@ -66,10 +65,9 @@ fn test_debug() {
 #[test]
 fn test_frames() {
     let backtrace = Backtrace {
-        inner: Inner::Captured(LazyLock::preinit(Capture {
-            actual_start: 1,
-            frames: generate_fake_frames(),
-        })),
+        inner: Inner::Captured(
+            (Capture { actual_start: 1, frames: generate_fake_frames() }).into(),
+        ),
     };
 
     let frames = backtrace.frames();

library/std/src/sync/lazy_lock.rs

@@ -105,15 +105,6 @@ impl<T, F: FnOnce() -> T> LazyLock<T, F> {
         LazyLock { once: Once::new(), data: UnsafeCell::new(Data { f: ManuallyDrop::new(f) }) }
     }
 
-    /// Creates a new lazy value that is already initialized.
-    #[inline]
-    #[cfg(test)]
-    pub(crate) fn preinit(value: T) -> LazyLock<T, F> {
-        let once = Once::new();
-        once.call_once(|| {});
-        LazyLock { once, data: UnsafeCell::new(Data { value: ManuallyDrop::new(value) }) }
-    }
-
     /// Consumes this `LazyLock` returning the stored value.
     ///
     /// Returns `Ok(value)` if `Lazy` is initialized and `Err(f)` otherwise.
@@ -401,6 +392,19 @@ impl<T: fmt::Debug, F> fmt::Debug for LazyLock<T, F> {
     }
 }
 
+#[stable(feature = "from_wrapper_impls", since = "CURRENT_RUSTC_VERSION")]
+impl<T, F> From<T> for LazyLock<T, F> {
+    /// Constructs a `LazyLock` that starts already initialized
+    /// with the provided value.
+    #[inline]
+    fn from(value: T) -> Self {
+        LazyLock {
+            once: Once::new_complete(),
+            data: UnsafeCell::new(Data { value: ManuallyDrop::new(value) }),
+        }
+    }
+}
+
 #[cold]
 #[inline(never)]
 fn panic_poisoned() -> ! {

library/std/src/sync/poison/once.rs

@@ -82,6 +82,13 @@ impl Once {
         Once { inner: sys::Once::new() }
     }
 
+    /// Creates a new `Once` value that starts already completed.
+    #[inline]
+    #[must_use]
+    pub(crate) const fn new_complete() -> Once {
+        Once { inner: sys::Once::new_complete() }
+    }
+
     /// Performs an initialization routine once and only once. The given closure
     /// will be executed if this is the first time `call_once` has been called,
     /// and otherwise the routine will *not* be invoked.

library/std/src/sys/sync/once/futex.rs

@@ -75,6 +75,11 @@ impl Once {
         Once { state_and_queued: Futex::new(INCOMPLETE) }
     }
 
+    #[inline]
+    pub const fn new_complete() -> Once {
+        Once { state_and_queued: Futex::new(COMPLETE) }
+    }
+
     #[inline]
     pub fn is_completed(&self) -> bool {
         // Use acquire ordering to make all initialization changes visible to the

library/std/src/sys/sync/once/no_threads.rs

@@ -39,6 +39,11 @@ impl Once {
         Once { state: Cell::new(State::Incomplete) }
     }
 
+    #[inline]
+    pub const fn new_complete() -> Once {
+        Once { state: Cell::new(State::Complete) }
+    }
+
     #[inline]
     pub fn is_completed(&self) -> bool {
         self.state.get() == State::Complete

library/std/src/sys/sync/once/queue.rs

@@ -121,6 +121,11 @@ impl Once {
         Once { state_and_queue: AtomicPtr::new(ptr::without_provenance_mut(INCOMPLETE)) }
     }
 
+    #[inline]
+    pub const fn new_complete() -> Once {
+        Once { state_and_queue: AtomicPtr::new(ptr::without_provenance_mut(COMPLETE)) }
+    }
+
     #[inline]
     pub fn is_completed(&self) -> bool {
         // An `Acquire` load is enough because that makes all the initialization