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-rw-r--r--src/ci/docker/wasm32-unknown/Dockerfile6
-rw-r--r--src/liballoc/raw_vec.rs23
-rw-r--r--src/libcore/fmt/mod.rs6
-rw-r--r--src/libcore/panic.rs22
-rw-r--r--src/libpanic_abort/lib.rs2
-rw-r--r--src/libpanic_unwind/lib.rs12
-rw-r--r--src/libstd/lib.rs1
-rw-r--r--src/libstd/panicking.rs146
-rw-r--r--src/libstd/sys/cloudabi/stdio.rs4
-rw-r--r--src/libstd/sys/redox/stdio.rs4
-rw-r--r--src/libstd/sys/unix/stdio.rs4
-rw-r--r--src/libstd/sys/wasm/rwlock.rs4
-rw-r--r--src/libstd/sys/wasm/stdio.rs4
-rw-r--r--src/libstd/sys/windows/stdio.rs4
-rw-r--r--src/libstd/sys_common/backtrace.rs13
-rw-r--r--src/libstd/sys_common/mod.rs14
-rw-r--r--src/libstd/sys_common/thread_local.rs4
-rw-r--r--src/libstd/sys_common/util.rs5
-rw-r--r--src/libstd/thread/local.rs41
-rw-r--r--src/libstd/thread/mod.rs3
-rw-r--r--src/test/run-make/wasm-panic-small/Makefile17
-rw-r--r--src/test/run-make/wasm-panic-small/foo.rs29
22 files changed, 296 insertions, 72 deletions
diff --git a/src/ci/docker/wasm32-unknown/Dockerfile b/src/ci/docker/wasm32-unknown/Dockerfile
index 853923ad947..56eda548071 100644
--- a/src/ci/docker/wasm32-unknown/Dockerfile
+++ b/src/ci/docker/wasm32-unknown/Dockerfile
@@ -25,6 +25,12 @@ ENV RUST_CONFIGURE_ARGS \
   --set build.nodejs=/node-v9.2.0-linux-x64/bin/node \
   --set rust.lld
 
+# Some run-make tests have assertions about code size, and enabling debug
+# assertions in libstd causes the binary to be much bigger than it would
+# otherwise normally be. We already test libstd with debug assertions in lots of
+# other contexts as well
+ENV NO_DEBUG_ASSERTIONS=1
+
 ENV SCRIPT python2.7 /checkout/x.py test --target $TARGETS \
   src/test/run-make \
   src/test/ui \
diff --git a/src/liballoc/raw_vec.rs b/src/liballoc/raw_vec.rs
index 405814c021a..24b7cd3db0c 100644
--- a/src/liballoc/raw_vec.rs
+++ b/src/liballoc/raw_vec.rs
@@ -86,8 +86,8 @@ impl<T, A: Alloc> RawVec<T, A> {
         unsafe {
             let elem_size = mem::size_of::<T>();
 
-            let alloc_size = cap.checked_mul(elem_size).expect("capacity overflow");
-            alloc_guard(alloc_size).expect("capacity overflow");
+            let alloc_size = cap.checked_mul(elem_size).unwrap_or_else(|| capacity_overflow());
+            alloc_guard(alloc_size).unwrap_or_else(|_| capacity_overflow());
 
             // handles ZSTs and `cap = 0` alike
             let ptr = if alloc_size == 0 {
@@ -310,7 +310,7 @@ impl<T, A: Alloc> RawVec<T, A> {
                     // `from_size_align_unchecked`.
                     let new_cap = 2 * self.cap;
                     let new_size = new_cap * elem_size;
-                    alloc_guard(new_size).expect("capacity overflow");
+                    alloc_guard(new_size).unwrap_or_else(|_| capacity_overflow());
                     let ptr_res = self.a.realloc(NonNull::from(self.ptr).as_opaque(),
                                                  cur,
                                                  new_size);
@@ -369,7 +369,7 @@ impl<T, A: Alloc> RawVec<T, A> {
             // overflow and the alignment is sufficiently small.
             let new_cap = 2 * self.cap;
             let new_size = new_cap * elem_size;
-            alloc_guard(new_size).expect("capacity overflow");
+            alloc_guard(new_size).unwrap_or_else(|_| capacity_overflow());
             match self.a.grow_in_place(NonNull::from(self.ptr).as_opaque(), old_layout, new_size) {
                 Ok(_) => {
                     // We can't directly divide `size`.
@@ -441,7 +441,7 @@ impl<T, A: Alloc> RawVec<T, A> {
 
     pub fn reserve_exact(&mut self, used_cap: usize, needed_extra_cap: usize) {
         match self.try_reserve_exact(used_cap, needed_extra_cap) {
-            Err(CapacityOverflow) => panic!("capacity overflow"),
+            Err(CapacityOverflow) => capacity_overflow(),
             Err(AllocErr) => self.a.oom(),
             Ok(()) => { /* yay */ }
          }
@@ -551,7 +551,7 @@ impl<T, A: Alloc> RawVec<T, A> {
     /// The same as try_reserve, but errors are lowered to a call to oom().
     pub fn reserve(&mut self, used_cap: usize, needed_extra_cap: usize) {
         match self.try_reserve(used_cap, needed_extra_cap) {
-            Err(CapacityOverflow) => panic!("capacity overflow"),
+            Err(CapacityOverflow) => capacity_overflow(),
             Err(AllocErr) => self.a.oom(),
             Ok(()) => { /* yay */ }
          }
@@ -592,7 +592,7 @@ impl<T, A: Alloc> RawVec<T, A> {
             }
 
             let new_cap = self.amortized_new_size(used_cap, needed_extra_cap)
-                              .expect("capacity overflow");
+                .unwrap_or_else(|_| capacity_overflow());
 
             // Here, `cap < used_cap + needed_extra_cap <= new_cap`
             // (regardless of whether `self.cap - used_cap` wrapped).
@@ -600,7 +600,7 @@ impl<T, A: Alloc> RawVec<T, A> {
 
             let new_layout = Layout::new::<T>().repeat(new_cap).unwrap().0;
             // FIXME: may crash and burn on over-reserve
-            alloc_guard(new_layout.size()).expect("capacity overflow");
+            alloc_guard(new_layout.size()).unwrap_or_else(|_| capacity_overflow());
             match self.a.grow_in_place(
                 NonNull::from(self.ptr).as_opaque(), old_layout, new_layout.size(),
             ) {
@@ -732,6 +732,13 @@ fn alloc_guard(alloc_size: usize) -> Result<(), CollectionAllocErr> {
     }
 }
 
+// One central function responsible for reporting capacity overflows. This'll
+// ensure that the code generation related to these panics is minimal as there's
+// only one location which panics rather than a bunch throughout the module.
+fn capacity_overflow() -> ! {
+    panic!("capacity overflow")
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
diff --git a/src/libcore/fmt/mod.rs b/src/libcore/fmt/mod.rs
index d55219d7226..277bef2bf66 100644
--- a/src/libcore/fmt/mod.rs
+++ b/src/libcore/fmt/mod.rs
@@ -1212,7 +1212,11 @@ impl<'a> Formatter<'a> {
             // truncation. However other flags like `fill`, `width` and `align`
             // must act as always.
             if let Some((i, _)) = s.char_indices().skip(max).next() {
-                &s[..i]
+                // LLVM here can't prove that `..i` won't panic `&s[..i]`, but
+                // we know that it can't panic. Use `get` + `unwrap_or` to avoid
+                // `unsafe` and otherwise don't emit any panic-related code
+                // here.
+                s.get(..i).unwrap_or(&s)
             } else {
                 &s
             }
diff --git a/src/libcore/panic.rs b/src/libcore/panic.rs
index 1720c9d8c60..27ec4aaac75 100644
--- a/src/libcore/panic.rs
+++ b/src/libcore/panic.rs
@@ -49,11 +49,17 @@ impl<'a> PanicInfo<'a> {
                           and related macros",
                 issue = "0")]
     #[doc(hidden)]
-    pub fn internal_constructor(payload: &'a (Any + Send),
-                                message: Option<&'a fmt::Arguments<'a>>,
+    #[inline]
+    pub fn internal_constructor(message: Option<&'a fmt::Arguments<'a>>,
                                 location: Location<'a>)
                                 -> Self {
-        PanicInfo { payload, location, message }
+        PanicInfo { payload: &(), location, message }
+    }
+
+    #[doc(hidden)]
+    #[inline]
+    pub fn set_payload(&mut self, info: &'a (Any + Send)) {
+        self.payload = info;
     }
 
     /// Returns the payload associated with the panic.
@@ -251,3 +257,13 @@ impl<'a> fmt::Display for Location<'a> {
         write!(formatter, "{}:{}:{}", self.file, self.line, self.col)
     }
 }
+
+/// An internal trait used by libstd to pass data from libstd to `panic_unwind`
+/// and other panic runtimes. Not intended to be stabilized any time soon, do
+/// not use.
+#[unstable(feature = "std_internals", issue = "0")]
+#[doc(hidden)]
+pub unsafe trait BoxMeUp {
+    fn box_me_up(&mut self) -> *mut (Any + Send);
+    fn get(&mut self) -> &(Any + Send);
+}
diff --git a/src/libpanic_abort/lib.rs b/src/libpanic_abort/lib.rs
index 43c5bbbc618..392bf17968f 100644
--- a/src/libpanic_abort/lib.rs
+++ b/src/libpanic_abort/lib.rs
@@ -52,7 +52,7 @@ pub unsafe extern fn __rust_maybe_catch_panic(f: fn(*mut u8),
 // now hopefully.
 #[no_mangle]
 #[rustc_std_internal_symbol]
-pub unsafe extern fn __rust_start_panic(_data: usize, _vtable: usize) -> u32 {
+pub unsafe extern fn __rust_start_panic(_payload: usize) -> u32 {
     abort();
 
     #[cfg(any(unix, target_os = "cloudabi"))]
diff --git a/src/libpanic_unwind/lib.rs b/src/libpanic_unwind/lib.rs
index 9321d6917d1..6c52c0fa10c 100644
--- a/src/libpanic_unwind/lib.rs
+++ b/src/libpanic_unwind/lib.rs
@@ -29,6 +29,7 @@
        html_root_url = "https://doc.rust-lang.org/nightly/",
        issue_tracker_base_url = "https://github.com/rust-lang/rust/issues/")]
 
+#![feature(allocator_api)]
 #![feature(alloc)]
 #![feature(core_intrinsics)]
 #![feature(lang_items)]
@@ -36,6 +37,7 @@
 #![feature(panic_unwind)]
 #![feature(raw)]
 #![feature(staged_api)]
+#![feature(std_internals)]
 #![feature(unwind_attributes)]
 #![cfg_attr(target_env = "msvc", feature(raw))]
 
@@ -47,9 +49,11 @@ extern crate libc;
 #[cfg(not(any(target_env = "msvc", all(windows, target_arch = "x86_64", target_env = "gnu"))))]
 extern crate unwind;
 
+use alloc::boxed::Box;
 use core::intrinsics;
 use core::mem;
 use core::raw;
+use core::panic::BoxMeUp;
 
 // Rust runtime's startup objects depend on these symbols, so make them public.
 #[cfg(all(target_os="windows", target_arch = "x86", target_env="gnu"))]
@@ -112,9 +116,7 @@ pub unsafe extern "C" fn __rust_maybe_catch_panic(f: fn(*mut u8),
 // implementation.
 #[no_mangle]
 #[unwind(allowed)]
-pub unsafe extern "C" fn __rust_start_panic(data: usize, vtable: usize) -> u32 {
-    imp::panic(mem::transmute(raw::TraitObject {
-        data: data as *mut (),
-        vtable: vtable as *mut (),
-    }))
+pub unsafe extern "C" fn __rust_start_panic(payload: usize) -> u32 {
+    let payload = payload as *mut &mut BoxMeUp;
+    imp::panic(Box::from_raw((*payload).box_me_up()))
 }
diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index a34fcb5a7f9..dd96c57538c 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -292,6 +292,7 @@
 #![feature(rand)]
 #![feature(raw)]
 #![feature(rustc_attrs)]
+#![feature(std_internals)]
 #![feature(stdsimd)]
 #![feature(shrink_to)]
 #![feature(slice_bytes)]
diff --git a/src/libstd/panicking.rs b/src/libstd/panicking.rs
index fba3269204e..24eae6a4c82 100644
--- a/src/libstd/panicking.rs
+++ b/src/libstd/panicking.rs
@@ -17,6 +17,8 @@
 //! * Executing a panic up to doing the actual implementation
 //! * Shims around "try"
 
+use core::panic::BoxMeUp;
+
 use io::prelude::*;
 
 use any::Any;
@@ -27,7 +29,7 @@ use intrinsics;
 use mem;
 use ptr;
 use raw;
-use sys::stdio::Stderr;
+use sys::stdio::{Stderr, stderr_prints_nothing};
 use sys_common::rwlock::RWLock;
 use sys_common::thread_info;
 use sys_common::util;
@@ -56,7 +58,7 @@ extern {
                                 data_ptr: *mut usize,
                                 vtable_ptr: *mut usize) -> u32;
     #[unwind(allowed)]
-    fn __rust_start_panic(data: usize, vtable: usize) -> u32;
+    fn __rust_start_panic(payload: usize) -> u32;
 }
 
 #[derive(Copy, Clone)]
@@ -177,9 +179,6 @@ fn default_hook(info: &PanicInfo) {
     };
 
     let location = info.location().unwrap();  // The current implementation always returns Some
-    let file = location.file();
-    let line = location.line();
-    let col = location.column();
 
     let msg = match info.payload().downcast_ref::<&'static str>() {
         Some(s) => *s,
@@ -193,8 +192,8 @@ fn default_hook(info: &PanicInfo) {
     let name = thread.as_ref().and_then(|t| t.name()).unwrap_or("<unnamed>");
 
     let write = |err: &mut ::io::Write| {
-        let _ = writeln!(err, "thread '{}' panicked at '{}', {}:{}:{}",
-                         name, msg, file, line, col);
+        let _ = writeln!(err, "thread '{}' panicked at '{}', {}",
+                         name, msg, location);
 
         #[cfg(feature = "backtrace")]
         {
@@ -212,15 +211,15 @@ fn default_hook(info: &PanicInfo) {
 
     let prev = LOCAL_STDERR.with(|s| s.borrow_mut().take());
     match (prev, err.as_mut()) {
-        (Some(mut stderr), _) => {
-            write(&mut *stderr);
-            let mut s = Some(stderr);
-            LOCAL_STDERR.with(|slot| {
-                *slot.borrow_mut() = s.take();
-            });
-        }
-        (None, Some(ref mut err)) => { write(err) }
-        _ => {}
+       (Some(mut stderr), _) => {
+           write(&mut *stderr);
+           let mut s = Some(stderr);
+           LOCAL_STDERR.with(|slot| {
+               *slot.borrow_mut() = s.take();
+           });
+       }
+       (None, Some(ref mut err)) => { write(err) }
+       _ => {}
     }
 }
 
@@ -342,9 +341,38 @@ pub fn begin_panic_fmt(msg: &fmt::Arguments,
     // panic + OOM properly anyway (see comment in begin_panic
     // below).
 
-    let mut s = String::new();
-    let _ = s.write_fmt(*msg);
-    rust_panic_with_hook(Box::new(s), Some(msg), file_line_col)
+    rust_panic_with_hook(&mut PanicPayload::new(msg), Some(msg), file_line_col);
+
+    struct PanicPayload<'a> {
+        inner: &'a fmt::Arguments<'a>,
+        string: Option<String>,
+    }
+
+    impl<'a> PanicPayload<'a> {
+        fn new(inner: &'a fmt::Arguments<'a>) -> PanicPayload<'a> {
+            PanicPayload { inner, string: None }
+        }
+
+        fn fill(&mut self) -> &mut String {
+            let inner = self.inner;
+            self.string.get_or_insert_with(|| {
+                let mut s = String::new();
+                drop(s.write_fmt(*inner));
+                s
+            })
+        }
+    }
+
+    unsafe impl<'a> BoxMeUp for PanicPayload<'a> {
+        fn box_me_up(&mut self) -> *mut (Any + Send) {
+            let contents = mem::replace(self.fill(), String::new());
+            Box::into_raw(Box::new(contents))
+        }
+
+        fn get(&mut self) -> &(Any + Send) {
+            self.fill()
+        }
+    }
 }
 
 /// This is the entry point of panicking for panic!() and assert!().
@@ -360,18 +388,42 @@ pub fn begin_panic<M: Any + Send>(msg: M, file_line_col: &(&'static str, u32, u3
     // be performed in the parent of this thread instead of the thread that's
     // panicking.
 
-    rust_panic_with_hook(Box::new(msg), None, file_line_col)
+    rust_panic_with_hook(&mut PanicPayload::new(msg), None, file_line_col);
+
+    struct PanicPayload<A> {
+        inner: Option<A>,
+    }
+
+    impl<A: Send + 'static> PanicPayload<A> {
+        fn new(inner: A) -> PanicPayload<A> {
+            PanicPayload { inner: Some(inner) }
+        }
+    }
+
+    unsafe impl<A: Send + 'static> BoxMeUp for PanicPayload<A> {
+        fn box_me_up(&mut self) -> *mut (Any + Send) {
+            let data = match self.inner.take() {
+                Some(a) => Box::new(a) as Box<Any + Send>,
+                None => Box::new(()),
+            };
+            Box::into_raw(data)
+        }
+
+        fn get(&mut self) -> &(Any + Send) {
+            match self.inner {
+                Some(ref a) => a,
+                None => &(),
+            }
+        }
+    }
 }
 
-/// Executes the primary logic for a panic, including checking for recursive
-/// panics and panic hooks.
+/// Central point for dispatching panics.
 ///
-/// This is the entry point or panics from libcore, formatted panics, and
-/// `Box<Any>` panics. Here we'll verify that we're not panicking recursively,
-/// run panic hooks, and then delegate to the actual implementation of panics.
-#[inline(never)]
-#[cold]
-fn rust_panic_with_hook(payload: Box<Any + Send>,
+/// Executes the primary logic for a panic, including checking for recursive
+/// panics, panic hooks, and finally dispatching to the panic runtime to either
+/// abort or unwind.
+fn rust_panic_with_hook(payload: &mut BoxMeUp,
                         message: Option<&fmt::Arguments>,
                         file_line_col: &(&'static str, u32, u32)) -> ! {
     let (file, line, col) = *file_line_col;
@@ -390,15 +442,24 @@ fn rust_panic_with_hook(payload: Box<Any + Send>,
     }
 
     unsafe {
-        let info = PanicInfo::internal_constructor(
-            &*payload,
+        let mut info = PanicInfo::internal_constructor(
             message,
             Location::internal_constructor(file, line, col),
         );
         HOOK_LOCK.read();
         match HOOK {
-            Hook::Default => default_hook(&info),
-            Hook::Custom(ptr) => (*ptr)(&info),
+            // Some platforms know that printing to stderr won't ever actually
+            // print anything, and if that's the case we can skip the default
+            // hook.
+            Hook::Default if stderr_prints_nothing() => {}
+            Hook::Default => {
+                info.set_payload(payload.get());
+                default_hook(&info);
+            }
+            Hook::Custom(ptr) => {
+                info.set_payload(payload.get());
+                (*ptr)(&info);
+            }
         }
         HOOK_LOCK.read_unlock();
     }
@@ -419,16 +480,29 @@ fn rust_panic_with_hook(payload: Box<Any + Send>,
 /// Shim around rust_panic. Called by resume_unwind.
 pub fn update_count_then_panic(msg: Box<Any + Send>) -> ! {
     update_panic_count(1);
-    rust_panic(msg)
+
+    struct RewrapBox(Box<Any + Send>);
+
+    unsafe impl BoxMeUp for RewrapBox {
+        fn box_me_up(&mut self) -> *mut (Any + Send) {
+            Box::into_raw(mem::replace(&mut self.0, Box::new(())))
+        }
+
+        fn get(&mut self) -> &(Any + Send) {
+            &*self.0
+        }
+    }
+
+    rust_panic(&mut RewrapBox(msg))
 }
 
 /// A private no-mangle function on which to slap yer breakpoints.
 #[no_mangle]
 #[allow(private_no_mangle_fns)] // yes we get it, but we like breakpoints
-pub fn rust_panic(msg: Box<Any + Send>) -> ! {
+pub fn rust_panic(mut msg: &mut BoxMeUp) -> ! {
     let code = unsafe {
-        let obj = mem::transmute::<_, raw::TraitObject>(msg);
-        __rust_start_panic(obj.data as usize, obj.vtable as usize)
+        let obj = &mut msg as *mut &mut BoxMeUp;
+        __rust_start_panic(obj as usize)
     };
     rtabort!("failed to initiate panic, error {}", code)
 }
diff --git a/src/libstd/sys/cloudabi/stdio.rs b/src/libstd/sys/cloudabi/stdio.rs
index 9519a926471..1d7344f921c 100644
--- a/src/libstd/sys/cloudabi/stdio.rs
+++ b/src/libstd/sys/cloudabi/stdio.rs
@@ -77,3 +77,7 @@ pub fn is_ebadf(err: &io::Error) -> bool {
 }
 
 pub const STDIN_BUF_SIZE: usize = ::sys_common::io::DEFAULT_BUF_SIZE;
+
+pub fn stderr_prints_nothing() -> bool {
+    false
+}
diff --git a/src/libstd/sys/redox/stdio.rs b/src/libstd/sys/redox/stdio.rs
index 3abb094ac34..7a4d11b0ecb 100644
--- a/src/libstd/sys/redox/stdio.rs
+++ b/src/libstd/sys/redox/stdio.rs
@@ -75,3 +75,7 @@ pub fn is_ebadf(err: &io::Error) -> bool {
 }
 
 pub const STDIN_BUF_SIZE: usize = ::sys_common::io::DEFAULT_BUF_SIZE;
+
+pub fn stderr_prints_nothing() -> bool {
+    false
+}
diff --git a/src/libstd/sys/unix/stdio.rs b/src/libstd/sys/unix/stdio.rs
index e9b3d4affc7..87ba2aef4f1 100644
--- a/src/libstd/sys/unix/stdio.rs
+++ b/src/libstd/sys/unix/stdio.rs
@@ -75,3 +75,7 @@ pub fn is_ebadf(err: &io::Error) -> bool {
 }
 
 pub const STDIN_BUF_SIZE: usize = ::sys_common::io::DEFAULT_BUF_SIZE;
+
+pub fn stderr_prints_nothing() -> bool {
+    false
+}
diff --git a/src/libstd/sys/wasm/rwlock.rs b/src/libstd/sys/wasm/rwlock.rs
index 8b06f541674..6516010af47 100644
--- a/src/libstd/sys/wasm/rwlock.rs
+++ b/src/libstd/sys/wasm/rwlock.rs
@@ -30,7 +30,7 @@ impl RWLock {
         if *mode >= 0 {
             *mode += 1;
         } else {
-            panic!("rwlock locked for writing");
+            rtabort!("rwlock locked for writing");
         }
     }
 
@@ -51,7 +51,7 @@ impl RWLock {
         if *mode == 0 {
             *mode = -1;
         } else {
-            panic!("rwlock locked for reading")
+            rtabort!("rwlock locked for reading")
         }
     }
 
diff --git a/src/libstd/sys/wasm/stdio.rs b/src/libstd/sys/wasm/stdio.rs
index beb19c0ed2c..023f29576a2 100644
--- a/src/libstd/sys/wasm/stdio.rs
+++ b/src/libstd/sys/wasm/stdio.rs
@@ -69,3 +69,7 @@ pub const STDIN_BUF_SIZE: usize = 0;
 pub fn is_ebadf(_err: &io::Error) -> bool {
     true
 }
+
+pub fn stderr_prints_nothing() -> bool {
+    !cfg!(feature = "wasm_syscall")
+}
diff --git a/src/libstd/sys/windows/stdio.rs b/src/libstd/sys/windows/stdio.rs
index b43df20bddd..81b89da21d3 100644
--- a/src/libstd/sys/windows/stdio.rs
+++ b/src/libstd/sys/windows/stdio.rs
@@ -227,3 +227,7 @@ pub fn is_ebadf(err: &io::Error) -> bool {
 // idea is that on windows we use a slightly smaller buffer that's
 // been seen to be acceptable.
 pub const STDIN_BUF_SIZE: usize = 8 * 1024;
+
+pub fn stderr_prints_nothing() -> bool {
+    false
+}
diff --git a/src/libstd/sys_common/backtrace.rs b/src/libstd/sys_common/backtrace.rs
index 1955f3ec9a2..20109d2d0d5 100644
--- a/src/libstd/sys_common/backtrace.rs
+++ b/src/libstd/sys_common/backtrace.rs
@@ -139,10 +139,10 @@ pub fn __rust_begin_short_backtrace<F, T>(f: F) -> T
 /// Controls how the backtrace should be formatted.
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
 pub enum PrintFormat {
-    /// Show all the frames with absolute path for files.
-    Full = 2,
     /// Show only relevant data from the backtrace.
-    Short = 3,
+    Short = 2,
+    /// Show all the frames with absolute path for files.
+    Full = 3,
 }
 
 // For now logging is turned off by default, and this function checks to see
@@ -150,11 +150,10 @@ pub enum PrintFormat {
 pub fn log_enabled() -> Option<PrintFormat> {
     static ENABLED: atomic::AtomicIsize = atomic::AtomicIsize::new(0);
     match ENABLED.load(Ordering::SeqCst) {
-        0 => {},
+        0 => {}
         1 => return None,
-        2 => return Some(PrintFormat::Full),
-        3 => return Some(PrintFormat::Short),
-        _ => unreachable!(),
+        2 => return Some(PrintFormat::Short),
+        _ => return Some(PrintFormat::Full),
     }
 
     let val = match env::var_os("RUST_BACKTRACE") {
diff --git a/src/libstd/sys_common/mod.rs b/src/libstd/sys_common/mod.rs
index 27504d374dd..d0c4d6a7737 100644
--- a/src/libstd/sys_common/mod.rs
+++ b/src/libstd/sys_common/mod.rs
@@ -28,6 +28,16 @@
 use sync::Once;
 use sys;
 
+macro_rules! rtabort {
+    ($($t:tt)*) => (::sys_common::util::abort(format_args!($($t)*)))
+}
+
+macro_rules! rtassert {
+    ($e:expr) => (if !$e {
+        rtabort!(concat!("assertion failed: ", stringify!($e)));
+    })
+}
+
 pub mod at_exit_imp;
 #[cfg(feature = "backtrace")]
 pub mod backtrace;
@@ -101,10 +111,6 @@ pub fn at_exit<F: FnOnce() + Send + 'static>(f: F) -> Result<(), ()> {
     if at_exit_imp::push(Box::new(f)) {Ok(())} else {Err(())}
 }
 
-macro_rules! rtabort {
-    ($($t:tt)*) => (::sys_common::util::abort(format_args!($($t)*)))
-}
-
 /// One-time runtime cleanup.
 pub fn cleanup() {
     static CLEANUP: Once = Once::new();
diff --git a/src/libstd/sys_common/thread_local.rs b/src/libstd/sys_common/thread_local.rs
index a4aa3d96d25..d0d6224de0a 100644
--- a/src/libstd/sys_common/thread_local.rs
+++ b/src/libstd/sys_common/thread_local.rs
@@ -169,7 +169,7 @@ impl StaticKey {
                 self.key.store(key, Ordering::SeqCst);
             }
             INIT_LOCK.unlock();
-            assert!(key != 0);
+            rtassert!(key != 0);
             return key
         }
 
@@ -190,7 +190,7 @@ impl StaticKey {
             imp::destroy(key1);
             key2
         };
-        assert!(key != 0);
+        rtassert!(key != 0);
         match self.key.compare_and_swap(0, key as usize, Ordering::SeqCst) {
             // The CAS succeeded, so we've created the actual key
             0 => key as usize,
diff --git a/src/libstd/sys_common/util.rs b/src/libstd/sys_common/util.rs
index a391c7cc6ef..a373e980b97 100644
--- a/src/libstd/sys_common/util.rs
+++ b/src/libstd/sys_common/util.rs
@@ -10,10 +10,13 @@
 
 use fmt;
 use io::prelude::*;
-use sys::stdio::Stderr;
+use sys::stdio::{Stderr, stderr_prints_nothing};
 use thread;
 
 pub fn dumb_print(args: fmt::Arguments) {
+    if stderr_prints_nothing() {
+        return
+    }
     let _ = Stderr::new().map(|mut stderr| stderr.write_fmt(args));
 }
 
diff --git a/src/libstd/thread/local.rs b/src/libstd/thread/local.rs
index 99479bc56ef..40d3280baa6 100644
--- a/src/libstd/thread/local.rs
+++ b/src/libstd/thread/local.rs
@@ -172,12 +172,16 @@ macro_rules! __thread_local_inner {
                 &'static $crate::cell::UnsafeCell<
                     $crate::option::Option<$t>>>
             {
+                #[cfg(target_arch = "wasm32")]
+                static __KEY: $crate::thread::__StaticLocalKeyInner<$t> =
+                    $crate::thread::__StaticLocalKeyInner::new();
+
                 #[thread_local]
-                #[cfg(target_thread_local)]
+                #[cfg(all(target_thread_local, not(target_arch = "wasm32")))]
                 static __KEY: $crate::thread::__FastLocalKeyInner<$t> =
                     $crate::thread::__FastLocalKeyInner::new();
 
-                #[cfg(not(target_thread_local))]
+                #[cfg(all(not(target_thread_local), not(target_arch = "wasm32")))]
                 static __KEY: $crate::thread::__OsLocalKeyInner<$t> =
                     $crate::thread::__OsLocalKeyInner::new();
 
@@ -295,6 +299,39 @@ impl<T: 'static> LocalKey<T> {
     }
 }
 
+/// On some platforms like wasm32 there's no threads, so no need to generate
+/// thread locals and we can instead just use plain statics!
+#[doc(hidden)]
+#[cfg(target_arch = "wasm32")]
+pub mod statik {
+    use cell::UnsafeCell;
+    use fmt;
+
+    pub struct Key<T> {
+        inner: UnsafeCell<Option<T>>,
+    }
+
+    unsafe impl<T> ::marker::Sync for Key<T> { }
+
+    impl<T> fmt::Debug for Key<T> {
+        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+            f.pad("Key { .. }")
+        }
+    }
+
+    impl<T> Key<T> {
+        pub const fn new() -> Key<T> {
+            Key {
+                inner: UnsafeCell::new(None),
+            }
+        }
+
+        pub unsafe fn get(&self) -> Option<&'static UnsafeCell<Option<T>>> {
+            Some(&*(&self.inner as *const _))
+        }
+    }
+}
+
 #[doc(hidden)]
 #[cfg(target_thread_local)]
 pub mod fast {
diff --git a/src/libstd/thread/mod.rs b/src/libstd/thread/mod.rs
index 71aee673cfe..1b976b79b4c 100644
--- a/src/libstd/thread/mod.rs
+++ b/src/libstd/thread/mod.rs
@@ -203,6 +203,9 @@ pub use self::local::{LocalKey, AccessError};
 // where available, but both are needed.
 
 #[unstable(feature = "libstd_thread_internals", issue = "0")]
+#[cfg(target_arch = "wasm32")]
+#[doc(hidden)] pub use self::local::statik::Key as __StaticLocalKeyInner;
+#[unstable(feature = "libstd_thread_internals", issue = "0")]
 #[cfg(target_thread_local)]
 #[doc(hidden)] pub use self::local::fast::Key as __FastLocalKeyInner;
 #[unstable(feature = "libstd_thread_internals", issue = "0")]
diff --git a/src/test/run-make/wasm-panic-small/Makefile b/src/test/run-make/wasm-panic-small/Makefile
new file mode 100644
index 00000000000..330ae300c44
--- /dev/null
+++ b/src/test/run-make/wasm-panic-small/Makefile
@@ -0,0 +1,17 @@
+-include ../../run-make-fulldeps/tools.mk
+
+ifeq ($(TARGET),wasm32-unknown-unknown)
+all:
+	$(RUSTC) foo.rs -C lto -O --target wasm32-unknown-unknown --cfg a
+	wc -c < $(TMPDIR)/foo.wasm
+	[ "`wc -c < $(TMPDIR)/foo.wasm`" -lt "1024" ]
+	$(RUSTC) foo.rs -C lto -O --target wasm32-unknown-unknown --cfg b
+	wc -c < $(TMPDIR)/foo.wasm
+	[ "`wc -c < $(TMPDIR)/foo.wasm`" -lt "5120" ]
+	$(RUSTC) foo.rs -C lto -O --target wasm32-unknown-unknown --cfg c
+	wc -c < $(TMPDIR)/foo.wasm
+	[ "`wc -c < $(TMPDIR)/foo.wasm`" -lt "5120" ]
+else
+all:
+endif
+
diff --git a/src/test/run-make/wasm-panic-small/foo.rs b/src/test/run-make/wasm-panic-small/foo.rs
new file mode 100644
index 00000000000..1ea724ca94d
--- /dev/null
+++ b/src/test/run-make/wasm-panic-small/foo.rs
@@ -0,0 +1,29 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![crate_type = "cdylib"]
+
+#[no_mangle]
+#[cfg(a)]
+pub fn foo() {
+    panic!("test");
+}
+
+#[no_mangle]
+#[cfg(b)]
+pub fn foo() {
+    panic!("{}", 1);
+}
+
+#[no_mangle]
+#[cfg(c)]
+pub fn foo() {
+    panic!("{}", "a");
+}
generated by cgit-pink 1.4.1-3-g733a5 (git 2.37.1) at 2026-04-24 14:12:58 +0000