std: Minimize size of panicking on wasm

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 \

src/liballoc/raw_vec.rs

@@ -85,8 +85,8 @@ 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 {
@@ -309,7 +309,7 @@ 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);
@@ -368,7 +368,7 @@ 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`.
@@ -440,7 +440,7 @@ 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 */ }
          }
@@ -550,7 +550,7 @@ 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 */ }
          }
@@ -591,15 +591,15 @@ 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).
             // Therefore we can safely call grow_in_place.
 
             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(),
             ) {
@@ -731,6 +731,13 @@ 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::*;

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
             }

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);
+}

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"))]

src/libpanic_unwind/lib.rs

@@ -29,13 +29,15 @@
        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)]
 #![feature(libc)]
 #![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()))
 }

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)]