Use the correct stderr when testing libstd

src/libstd/io/impls.rs

@@ -165,6 +165,20 @@ impl<B: BufRead + ?Sized> BufRead for Box<B> {
     }
 }
 
+// Used by panicking::default_hook
+#[cfg(test)]
+/// This impl is only used by printing logic, so any error returned is always
+/// of kind `Other`, and should be ignored.
+impl Write for Box<dyn (::realstd::io::Write) + Send> {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        (**self).write(buf).map_err(|_| ErrorKind::Other.into())
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        (**self).flush().map_err(|_| ErrorKind::Other.into())
+    }
+}
+
 // =============================================================================
 // In-memory buffer implementations
 

src/libstd/io/stdio.rs

@@ -1,3 +1,5 @@
+#![cfg_attr(test, allow(unused))]
+
 use crate::io::prelude::*;
 
 use crate::cell::RefCell;
@@ -16,6 +18,13 @@ thread_local! {
     }
 }
 
+/// Stderr used by eprint! and eprintln! macros, and panics
+thread_local! {
+    static LOCAL_STDERR: RefCell<Option<Box<dyn Write + Send>>> = {
+        RefCell::new(None)
+    }
+}
+
 /// A handle to a raw instance of the standard input stream of this process.
 ///
 /// This handle is not synchronized or buffered in any fashion. Constructed via
@@ -668,7 +677,6 @@ impl fmt::Debug for StderrLock<'_> {
            issue = "0")]
 #[doc(hidden)]
 pub fn set_panic(sink: Option<Box<dyn Write + Send>>) -> Option<Box<dyn Write + Send>> {
-    use crate::panicking::LOCAL_STDERR;
     use crate::mem;
     LOCAL_STDERR.with(move |slot| {
         mem::replace(&mut *slot.borrow_mut(), sink)
@@ -740,6 +748,7 @@ where
            reason = "implementation detail which may disappear or be replaced at any time",
            issue = "0")]
 #[doc(hidden)]
+#[cfg(not(test))]
 pub fn _print(args: fmt::Arguments) {
     print_to(args, &LOCAL_STDOUT, stdout, "stdout");
 }
@@ -748,11 +757,14 @@ pub fn _print(args: fmt::Arguments) {
            reason = "implementation detail which may disappear or be replaced at any time",
            issue = "0")]
 #[doc(hidden)]
+#[cfg(not(test))]
 pub fn _eprint(args: fmt::Arguments) {
-    use crate::panicking::LOCAL_STDERR;
     print_to(args, &LOCAL_STDERR, stderr, "stderr");
 }
 
+#[cfg(test)]
+pub use realstd::io::{_eprint, _print};
+
 #[cfg(test)]
 mod tests {
     use crate::panic::{UnwindSafe, RefUnwindSafe};

src/libstd/lib.rs

@@ -219,7 +219,7 @@
 // std may use features in a platform-specific way
 #![allow(unused_features)]
 
-#![cfg_attr(test, feature(test, update_panic_count))]
+#![cfg_attr(test, feature(print_internals, set_stdio, test, update_panic_count))]
 #![cfg_attr(all(target_vendor = "fortanix", target_env = "sgx"),
             feature(global_asm, range_contains, slice_index_methods,
                     decl_macro, coerce_unsized, sgx_platform, ptr_wrapping_offset_from))]

src/libstd/panicking.rs

@@ -7,13 +7,9 @@
 //! * Executing a panic up to doing the actual implementation
 //! * Shims around "try"
 
-use core::panic::BoxMeUp;
-use core::panic::{PanicInfo, Location};
-
-use crate::io::prelude::*;
+use core::panic::{BoxMeUp, PanicInfo, Location};
 
 use crate::any::Any;
-use crate::cell::RefCell;
 use crate::fmt;
 use crate::intrinsics;
 use crate::mem;
@@ -25,11 +21,12 @@ use crate::sys_common::thread_info;
 use crate::sys_common::util;
 use crate::thread;
 
-thread_local! {
-    pub static LOCAL_STDERR: RefCell<Option<Box<dyn Write + Send>>> = {
-        RefCell::new(None)
-    }
-}
+#[cfg(not(test))]
+use crate::io::set_panic;
+// make sure to use the stderr output configured
+// by libtest in the real copy of std
+#[cfg(test)]
+use realstd::io::set_panic;
 
 // Binary interface to the panic runtime that the standard library depends on.
 //
@@ -205,12 +202,11 @@ fn default_hook(info: &PanicInfo) {
         }
     };
 
-    if let Some(mut local) = LOCAL_STDERR.with(|s| s.borrow_mut().take()) {
-       write(&mut *local);
-       let mut s = Some(local);
-       LOCAL_STDERR.with(|slot| {
-           *slot.borrow_mut() = s.take();
-       });
+    if let Some(mut local) = set_panic(None) {
+        // NB. In `cfg(test)` this uses the forwarding impl
+        // for `Box<dyn (::realstd::io::Write) + Send>`.
+        write(&mut local);
+        set_panic(Some(local));
     } else if let Some(mut out) = panic_output() {
         write(&mut out);
     }