Don't call rust_begin_unwind directly

`builtins-test-intrinsics` has long included a call to an unmangled
`rust_begin_unwind` (the name `rustc` gives the `#[panic_handler]`),
since [1], which I believe was intended to ensure panic machinery links
correctly. However, since [2], `rust_begin_unwind` is mangled and
unavailable for calling directly, which explains the recent CI failures.

Instead of calling the function directly, we can just panic; do so here.
Additionally, put this call behind `black_box(false)` rather than
unconditional, which means we can run the binary and ensure there are no
runtime issues.

[1]: rust-lang#360
[2]: rust-lang/rust#127173

Author: tgross35

builtins-test-intrinsics/build.rs

@@ -8,4 +8,8 @@ fn main() {
     let target = builtins_configure::Target::from_env();
     builtins_configure::configure_f16_f128(&target);
     builtins_configure::configure_aliases(&target);
+
+    if target.os == "windows" {
+        println!("cargo::rustc-link-arg=/ENTRY:mainCRTStartup");
+    }
 }

builtins-test-intrinsics/src/main.rs

@@ -626,12 +626,9 @@ fn run() {
 
     something_with_a_dtor(&|| assert_eq!(bb(1), 1));
 
-    extern "C" {
-        fn rust_begin_unwind(x: usize);
-    }
-
-    unsafe {
-        rust_begin_unwind(0);
+    // Ensure panic machinery gets linked, but still allow this to run to completion.
+    if bb(false) {
+        panic!();
     }
 }
 
@@ -648,15 +645,23 @@ fn something_with_a_dtor(f: &dyn Fn()) {
 }
 
 #[no_mangle]
-#[cfg(not(thumb))]
-fn main(_argc: core::ffi::c_int, _argv: *const *const u8) -> core::ffi::c_int {
+#[cfg(not(any(thumb, windows)))]
+extern "C" fn main(_argc: core::ffi::c_int, _argv: *const *const u8) -> core::ffi::c_int {
+    run();
+    0
+}
+
+#[no_mangle]
+#[cfg(windows)]
+#[allow(non_snake_case)]
+extern "C" fn mainCRTStartup() -> core::ffi::c_int {
     run();
     0
 }
 
 #[no_mangle]
 #[cfg(thumb)]
-pub fn _start() -> ! {
+extern "C" fn _start() -> ! {
     run();
     loop {}
 }

ci/run.sh

@@ -120,22 +120,26 @@ done
 
 rm -f "${rlib_paths[@]}"
 
-build_intrinsics_test() {
-    cargo build --target "$target" -v --package builtins-test-intrinsics "$@"
+run_intrinsics_test() {
+    if [ "${NO_STD:-}" = "1" ]; then
+        cmd=build
+    else
+        cmd=run
+    fi
+    
+    cargo "$cmd" --target "$target" -v --package builtins-test-intrinsics "$@"
 }
 
 # Verify that we haven't dropped any intrinsics/symbols
-build_intrinsics_test
-build_intrinsics_test --release
-build_intrinsics_test --features c
-build_intrinsics_test --features c --release
+run_intrinsics_test
+run_intrinsics_test --release
+run_intrinsics_test --features c
+run_intrinsics_test --features c --release
 
 # Verify that there are no undefined symbols to `panic` within our
 # implementations
-CARGO_PROFILE_DEV_LTO=true \
-    cargo build --target "$target" --package builtins-test-intrinsics
-CARGO_PROFILE_RELEASE_LTO=true \
-    cargo build --target "$target" --package builtins-test-intrinsics --release
+CARGO_PROFILE_DEV_LTO=true run_intrinsics_test
+CARGO_PROFILE_RELEASE_LTO=true run_intrinsics_test --release
 
 # Ensure no references to any symbols from core
 update_rlib_paths