std: Minimize size of panicking on wasm

This commit applies a few code size optimizations for the wasm target to
the standard library, namely around panics. We notably know that in most
configurations it's impossible for us to print anything in
wasm32-unknown-unknown so we can skip larger portions of panicking that
are otherwise simply informative. This allows us to get quite a nice
size reduction.

Finally we can also tweak where the allocation happens for the
`Box<Any>` that we panic with. By only allocating once unwinding starts
we can reduce the size of a panicking wasm module from 44k to 350 bytes.

4 files changed, 22 insertions, 14 deletions

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));
 }