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.

6 files changed, 22 insertions, 2 deletions

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
+}