Add basic backtrace functionality

Whenever a failure happens, if a program is run with
`RUST_LOG=std::rt::backtrace` a backtrace will be printed to the task's stderr
handle. Stack traces are uncondtionally printed on double-failure and
rtabort!().

This ended up having a nontrivial implementation, and here's some highlights of
it:

* We're bundling libbacktrace for everything but OSX and Windows
* We use libgcc_s and its libunwind apis to get a backtrace of instruction
  pointers
* On OSX we use dladdr() to go from an instruction pointer to a symbol
* On unix that isn't OSX, we use libbacktrace to get symbols
* Windows, as usual, has an entirely separate implementation

Lots more fun details and comments can be found in the source itself.

Closes #10128

mk/crates.mk

@@ -55,7 +55,7 @@ HOST_CRATES := syntax rustc rustdoc fourcc hexfloat
 CRATES := $(TARGET_CRATES) $(HOST_CRATES)
 TOOLS := compiletest rustdoc rustc
 
-DEPS_std := native:rustrt native:compiler-rt
+DEPS_std := native:rustrt native:compiler-rt native:backtrace
 DEPS_extra := std term sync serialize getopts collections time rand
 DEPS_green := std rand native:context_switch
 DEPS_rustuv := std native:uv native:uv_support

mk/rt.mk

@@ -249,6 +249,73 @@ $$(COMPRT_LIB_$(1)): $$(COMPRT_DEPS) $$(MKFILE_DEPS)
 		triple-runtime
 	$$(Q)cp $$(COMPRT_BUILD_DIR_$(1))/triple/runtime/libcompiler_rt.a $$(COMPRT_LIB_$(1))
 
+################################################################################
+# libbacktrace
+#
+# We use libbacktrace on linux to get symbols in backtraces, but only on linux.
+# Elsewhere we use other system utilities, so this library is only built on
+# linux.
+################################################################################
+
+BACKTRACE_NAME_$(1) := $$(call CFG_STATIC_LIB_NAME_$(1),backtrace)
+BACKTRACE_LIB_$(1) := $$(RT_OUTPUT_DIR_$(1))/$$(BACKTRACE_NAME_$(1))
+BACKTRACE_BUILD_DIR_$(1) := $$(RT_OUTPUT_DIR_$(1))/libbacktrace
+
+ifeq ($$(findstring darwin,$$(OSTYPE_$(1))),darwin)
+
+# We don't use this on platforms that aren't linux-based, so just make the file
+# available, the compilation of libstd won't actually build it.
+$$(BACKTRACE_LIB_$(1)):
+	touch $$@
+
+else
+ifeq ($$(CFG_WINDOWSY_$(1)),1)
+$$(BACKTRACE_LIB_$(1)):
+	touch $$@
+else
+
+ifdef CFG_ENABLE_FAST_MAKE
+BACKTRACE_DEPS := $(S)/.gitmodules
+else
+BACKTRACE_DEPS := $(wildcard $(S)src/libbacktrace/*)
+endif
+
+# We need to export CFLAGS because otherwise it doesn't pick up cross compile
+# builds. If libbacktrace doesn't realize this, it will attempt to read 64-bit
+# elf headers when compiled for a 32-bit system, yielding blank backtraces.
+#
+# This also removes the -Werror flag specifically to prevent errors during
+# configuration.
+#
+# Down below you'll also see echos into the config.h generated by the
+# ./configure script. This is done to force libbacktrace to *not* use the
+# atomic/sync functionality because it pulls in unnecessary dependencies and we
+# never use it anyway.
+$$(BACKTRACE_BUILD_DIR_$(1))/Makefile: \
+		export CFLAGS:=$$(CFG_GCCISH_CFLAGS_$(1):-Werror=) \
+				-fno-stack-protector
+$$(BACKTRACE_BUILD_DIR_$(1))/Makefile: export CC:=$$(CC_$(1))
+$$(BACKTRACE_BUILD_DIR_$(1))/Makefile: export AR:=$$(AR_$(1))
+$$(BACKTRACE_BUILD_DIR_$(1))/Makefile: export RANLIB:=$$(AR_$(1)) s
+$$(BACKTRACE_BUILD_DIR_$(1))/Makefile: $$(BACKTRACE_DEPS) $$(MKFILE_DEPS)
+	$$(Q)rm -rf $$(BACKTRACE_BUILD_DIR_$(1))
+	$$(Q)mkdir -p $$(BACKTRACE_BUILD_DIR_$(1))
+	$$(Q)(cd $$(BACKTRACE_BUILD_DIR_$(1)) && \
+	      $(S)src/libbacktrace/configure --target=$(1) --host=$(CFG_BUILD))
+	$$(Q)echo '#undef HAVE_ATOMIC_FUNCTIONS' >> \
+	      $$(BACKTRACE_BUILD_DIR_$(1))/config.h
+	$$(Q)echo '#undef HAVE_SYNC_FUNCTIONS' >> \
+	      $$(BACKTRACE_BUILD_DIR_$(1))/config.h
+
+$$(BACKTRACE_LIB_$(1)): $$(BACKTRACE_BUILD_DIR_$(1))/Makefile $$(MKFILE_DEPS)
+	@$$(call E, make: libbacktrace)
+	$$(Q)$$(MAKE) -C $$(BACKTRACE_BUILD_DIR_$(1)) \
+		INCDIR=$(S)src/libbacktrace
+	$$(Q)cp $$(BACKTRACE_BUILD_DIR_$(1))/.libs/libbacktrace.a $$@
+
+endif # endif for windowsy
+endif # endif for darwin
+
 endef
 
 # Instantiate template for all stages/targets

mk/tests.mk

@@ -240,6 +240,7 @@ tidy:
 		| grep '^$(S)src/libuv' -v \
 		| grep '^$(S)src/llvm' -v \
 		| grep '^$(S)src/gyp' -v \
+		| grep '^$(S)src/libbacktrace' -v \
 		| xargs -n 10 $(CFG_PYTHON) $(S)src/etc/tidy.py
 		$(Q)find $(S)src/etc -name '*.py' \
 		| xargs -n 10 $(CFG_PYTHON) $(S)src/etc/tidy.py
@@ -266,6 +267,7 @@ tidy:
 		| grep '^$(S)src/etc' -v \
 		| grep '^$(S)src/doc' -v \
 		| grep '^$(S)src/compiler-rt' -v \
+		| grep '^$(S)src/libbacktrace' -v \
 		| xargs $(CFG_PYTHON) $(S)src/etc/check-binaries.py
 
 endif