Auto merge of #49488 - alexcrichton:small-wasm-panic, r=sfackler

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.

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
+

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");
+}