6 files changed, 29 insertions, 8 deletions
diff --git a/src/libstd/sys/cloudabi/thread.rs b/src/libstd/sys/cloudabi/thread.rs
index 8cca47efd22..a76e1fa3345 100644
--- a/src/libstd/sys/cloudabi/thread.rs
+++ b/src/libstd/sys/cloudabi/thread.rs
@@ -32,7 +32,8 @@ unsafe impl Send for Thread {}
 unsafe impl Sync for Thread {}
 
 impl Thread {
-    pub unsafe fn new<'a>(stack: usize, p: Box<dyn FnBox() + 'a>) -> io::Result<Thread> {
+    // unsafe: see thread::Builder::spawn_unchecked for safety requirements
+    pub unsafe fn new(stack: usize, p: Box<dyn FnBox()>) -> io::Result<Thread> {
         let p = box p;
         let mut native: libc::pthread_t = mem::zeroed();
         let mut attr: libc::pthread_attr_t = mem::zeroed();
diff --git a/src/libstd/sys/redox/thread.rs b/src/libstd/sys/redox/thread.rs
index bab91b16e6c..ff861805382 100644
--- a/src/libstd/sys/redox/thread.rs
+++ b/src/libstd/sys/redox/thread.rs
@@ -28,7 +28,8 @@ unsafe impl Send for Thread {}
 unsafe impl Sync for Thread {}
 
 impl Thread {
-    pub unsafe fn new<'a>(_stack: usize, p: Box<dyn FnBox() + 'a>) -> io::Result<Thread> {
+    // unsafe: see thread::Builder::spawn_unchecked for safety requirements
+    pub unsafe fn new(_stack: usize, p: Box<dyn FnBox()>) -> io::Result<Thread> {
         let p = box p;
 
         let id = cvt(syscall::clone(syscall::CLONE_VM | syscall::CLONE_FS | syscall::CLONE_FILES))?;
diff --git a/src/libstd/sys/unix/thread.rs b/src/libstd/sys/unix/thread.rs
index f3a45d24657..4ff060018ae 100644
--- a/src/libstd/sys/unix/thread.rs
+++ b/src/libstd/sys/unix/thread.rs
@@ -49,7 +49,8 @@ unsafe fn pthread_attr_setstacksize(_attr: *mut libc::pthread_attr_t,
 }
 
 impl Thread {
-    pub unsafe fn new<'a>(stack: usize, p: Box<dyn FnBox() + 'a>)
+    // unsafe: see thread::Builder::spawn_unchecked for safety requirements
+    pub unsafe fn new(stack: usize, p: Box<dyn FnBox()>)
                           -> io::Result<Thread> {
         let p = box p;
         let mut native: libc::pthread_t = mem::zeroed();
diff --git a/src/libstd/sys/wasm/thread.rs b/src/libstd/sys/wasm/thread.rs
index 4ad89c42b92..3d74ffdc14a 100644
--- a/src/libstd/sys/wasm/thread.rs
+++ b/src/libstd/sys/wasm/thread.rs
@@ -19,7 +19,8 @@ pub struct Thread(Void);
 pub const DEFAULT_MIN_STACK_SIZE: usize = 4096;
 
 impl Thread {
-    pub unsafe fn new<'a>(_stack: usize, _p: Box<dyn FnBox() + 'a>)
+    // unsafe: see thread::Builder::spawn_unchecked for safety requirements
+    pub unsafe fn new(_stack: usize, _p: Box<dyn FnBox()>)
         -> io::Result<Thread>
     {
         unsupported()
diff --git a/src/libstd/sys/windows/thread.rs b/src/libstd/sys/windows/thread.rs
index 85588cc6c8e..1a97dd10ced 100644
--- a/src/libstd/sys/windows/thread.rs
+++ b/src/libstd/sys/windows/thread.rs
@@ -28,7 +28,8 @@ pub struct Thread {
 }
 
 impl Thread {
-    pub unsafe fn new<'a>(stack: usize, p: Box<dyn FnBox() + 'a>)
+    // unsafe: see thread::Builder::spawn_unchecked for safety requirements
+    pub unsafe fn new(stack: usize, p: Box<dyn FnBox()>)
                           -> io::Result<Thread> {
         let p = box p;
 
diff --git a/src/libstd/thread/mod.rs b/src/libstd/thread/mod.rs
index 3a9f3ec5c6f..d15b4902412 100644
--- a/src/libstd/thread/mod.rs
+++ b/src/libstd/thread/mod.rs
@@ -167,10 +167,12 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use any::Any;
+use boxed::FnBox;
 use cell::UnsafeCell;
 use ffi::{CStr, CString};
 use fmt;
 use io;
+use mem;
 use panic;
 use panicking;
 use str;
@@ -452,8 +454,8 @@ impl Builder {
     /// [`io::Result`]: ../../std/io/type.Result.html
     /// [`JoinHandle`]: ../../std/thread/struct.JoinHandle.html
     #[unstable(feature = "thread_spawn_unchecked", issue = "55132")]
-    pub unsafe fn spawn_unchecked<F, T>(self, f: F) -> io::Result<JoinHandle<T>> where
-        F: FnOnce() -> T, F: Send, T: Send
+    pub unsafe fn spawn_unchecked<'a, F, T>(self, f: F) -> io::Result<JoinHandle<T>> where
+        F: FnOnce() -> T, F: Send + 'a, T: Send + 'a
     {
         let Builder { name, stack_size } = self;
 
@@ -482,7 +484,21 @@ impl Builder {
         };
 
         Ok(JoinHandle(JoinInner {
-            native: Some(imp::Thread::new(stack_size, Box::new(main))?),
+            // `imp::Thread::new` takes a closure with a `'static` lifetime, since it's passed
+            // through FFI or otherwise used with low-level threading primitives that have no
+            // notion of or way to enforce lifetimes.
+            //
+            // As mentioned in the `Safety` section of this function's documentation, the caller of
+            // this function needs to guarantee that the passed-in lifetime is sufficiently long
+            // for the lifetime of the thread.
+            //
+            // Similarly, the `sys` implementation must guarantee that no references to the closure
+            // exist after the thread has terminated, which is signaled by `Thread::join`
+            // returning.
+            native: Some(imp::Thread::new(
+                stack_size,
+                mem::transmute::<Box<dyn FnBox() + 'a>, Box<dyn FnBox() + 'static>>(Box::new(main))
+            )?),
             thread: my_thread,
             packet: Packet(my_packet),
         }))