implement unwinding abi's (RFC 2945)

 ### Changes

    This commit implements unwind ABI's, specified in RFC 2945.

    We adjust the `rustc_middle::ty::layout::fn_can_unwind` function,
    used to compute whether or not a `FnAbi` object represents a
    function that should be able to unwind when `panic=unwind` is in
    use.

    Changes are also made to
    `rustc_mir_build::build::should_abort_on_panic` so that the
    function ABI is used to determind whether it should abort, assuming
    that the `panic=unwind` strategy is being used, and no explicit
    unwind attribute was provided.

 ### Tests

    Unit tests, checking that the behavior is correct for `C-unwind`,
    `stdcall-unwind`, `system-unwind`, and `thiscall-unwind`, are
    included. These alternative `unwind` ABI strings are specified in
    RFC 2945, in the "_Other `unwind` ABI strings_" section.

    Additionally, a test case is included to assert that the LLVM IR
    generated for an external function defined with the `C-unwind` ABI
    will be appropriately labeled with the `nounwind` LLVM attribute
    when the `panic=abort` compilation flag is used.

 ### Ignore Directives

    This commit uses `ignore-*` directives in two of our `*-unwind` ABI
    test cases.

    Specifically, the `stdcall-unwind` and `thiscall-unwind` test cases
    ignore architectures that do not support `stdcall` and `thiscall`,
    respectively.

    These directives are cribbed from
    `src/test/ui/c-variadic/variadic-ffi-1.rs` for `stdcall`, and
    `src/test/ui/extern/extern-thiscall.rs` for `thiscall`.

5 files changed, 141 insertions, 0 deletions

diff --git a/src/test/codegen/unwind-abis/c-unwind-abi-panic-abort.rs b/src/test/codegen/unwind-abis/c-unwind-abi-panic-abort.rs
new file mode 100644
index 00000000000..afd65ff6741
--- /dev/null
+++ b/src/test/codegen/unwind-abis/c-unwind-abi-panic-abort.rs
@@ -0,0 +1,18 @@
+// compile-flags: -C panic=abort -C opt-level=0
+
+// Test that `nounwind` atributes are applied to `C-unwind` extern functions when the
+// code is compiled with `panic=abort`.  We disable optimizations above to prevent LLVM from
+// inferring the attribute.
+
+#![crate_type = "lib"]
+#![feature(c_unwind)]
+
+// CHECK: @rust_item_that_can_unwind() unnamed_addr #0 {
+#[no_mangle]
+pub extern "C-unwind" fn rust_item_that_can_unwind() {
+}
+
+// Now, make sure that the LLVM attributes for this functions are correct.  First, make
+// sure that the first item is correctly marked with the `nounwind` attribute:
+//
+// CHECK: attributes #0 = { {{.*}}nounwind{{.*}} }
diff --git a/src/test/codegen/unwind-abis/c-unwind-abi.rs b/src/test/codegen/unwind-abis/c-unwind-abi.rs
new file mode 100644
index 00000000000..f1576536753
--- /dev/null
+++ b/src/test/codegen/unwind-abis/c-unwind-abi.rs
@@ -0,0 +1,29 @@
+// compile-flags: -C opt-level=0
+
+// Test that `nounwind` atributes are correctly applied to exported `C` and `C-unwind` extern
+// functions. `C-unwind` functions MUST NOT have this attribute. We disable optimizations above
+// to prevent LLVM from inferring the attribute.
+
+#![crate_type = "lib"]
+#![feature(c_unwind)]
+
+// CHECK: @rust_item_that_cannot_unwind() unnamed_addr #0 {
+#[no_mangle]
+pub extern "C" fn rust_item_that_cannot_unwind() {
+}
+
+// CHECK: @rust_item_that_can_unwind() unnamed_addr #1 {
+#[no_mangle]
+pub extern "C-unwind" fn rust_item_that_can_unwind() {
+}
+
+// Now, make some assertions that the LLVM attributes for these functions are correct.  First, make
+// sure that the first item is correctly marked with the `nounwind` attribute:
+//
+// CHECK: attributes #0 = { {{.*}}nounwind{{.*}} }
+//
+// Next, let's assert that the second item, which CAN unwind, does not have this attribute.
+//
+// CHECK: attributes #1 = {
+// CHECK-NOT: nounwind
+// CHECK: }
diff --git a/src/test/codegen/unwind-abis/stdcall-unwind-abi.rs b/src/test/codegen/unwind-abis/stdcall-unwind-abi.rs
new file mode 100644
index 00000000000..ed804ca278d
--- /dev/null
+++ b/src/test/codegen/unwind-abis/stdcall-unwind-abi.rs
@@ -0,0 +1,32 @@
+// compile-flags: -C opt-level=0
+// ignore-arm stdcall isn't supported
+// ignore-aarch64 stdcall isn't supported
+// ignore-riscv64 stdcall isn't supported
+
+// Test that `nounwind` atributes are correctly applied to exported `stdcall` and `stdcall-unwind`
+// extern functions. `stdcall-unwind` functions MUST NOT have this attribute. We disable
+// optimizations above to prevent LLVM from inferring the attribute.
+
+#![crate_type = "lib"]
+#![feature(c_unwind)]
+
+// CHECK: @rust_item_that_cannot_unwind() unnamed_addr #0 {
+#[no_mangle]
+pub extern "stdcall" fn rust_item_that_cannot_unwind() {
+}
+
+// CHECK: @rust_item_that_can_unwind() unnamed_addr #1 {
+#[no_mangle]
+pub extern "stdcall-unwind" fn rust_item_that_can_unwind() {
+}
+
+// Now, make some assertions that the LLVM attributes for these functions are correct.  First, make
+// sure that the first item is correctly marked with the `nounwind` attribute:
+//
+// CHECK: attributes #0 = { {{.*}}nounwind{{.*}} }
+//
+// Next, let's assert that the second item, which CAN unwind, does not have this attribute.
+//
+// CHECK: attributes #1 = {
+// CHECK-NOT: nounwind
+// CHECK: }
diff --git a/src/test/codegen/unwind-abis/system-unwind-abi.rs b/src/test/codegen/unwind-abis/system-unwind-abi.rs
new file mode 100644
index 00000000000..c4d51328352
--- /dev/null
+++ b/src/test/codegen/unwind-abis/system-unwind-abi.rs
@@ -0,0 +1,29 @@
+// compile-flags: -C opt-level=0
+
+// Test that `nounwind` atributes are correctly applied to exported `system` and `system-unwind`
+// extern functions. `system-unwind` functions MUST NOT have this attribute. We disable
+// optimizations above to prevent LLVM from inferring the attribute.
+
+#![crate_type = "lib"]
+#![feature(c_unwind)]
+
+// CHECK: @rust_item_that_cannot_unwind() unnamed_addr #0 {
+#[no_mangle]
+pub extern "system" fn rust_item_that_cannot_unwind() {
+}
+
+// CHECK: @rust_item_that_can_unwind() unnamed_addr #1 {
+#[no_mangle]
+pub extern "system-unwind" fn rust_item_that_can_unwind() {
+}
+
+// Now, make some assertions that the LLVM attributes for these functions are correct.  First, make
+// sure that the first item is correctly marked with the `nounwind` attribute:
+//
+// CHECK: attributes #0 = { {{.*}}nounwind{{.*}} }
+//
+// Next, let's assert that the second item, which CAN unwind, does not have this attribute.
+//
+// CHECK: attributes #1 = {
+// CHECK-NOT: nounwind
+// CHECK: }
diff --git a/src/test/codegen/unwind-abis/thiscall-unwind-abi.rs b/src/test/codegen/unwind-abis/thiscall-unwind-abi.rs
new file mode 100644
index 00000000000..aaa63ae55c3
--- /dev/null
+++ b/src/test/codegen/unwind-abis/thiscall-unwind-abi.rs
@@ -0,0 +1,33 @@
+// compile-flags: -C opt-level=0
+// ignore-arm thiscall isn't supported
+// ignore-aarch64 thiscall isn't supported
+// ignore-riscv64 thiscall isn't supported
+
+// Test that `nounwind` atributes are correctly applied to exported `thiscall` and
+// `thiscall-unwind` extern functions. `thiscall-unwind` functions MUST NOT have this attribute. We
+// disable optimizations above to prevent LLVM from inferring the attribute.
+
+#![crate_type = "lib"]
+#![feature(abi_thiscall)]
+#![feature(c_unwind)]
+
+// CHECK: @rust_item_that_cannot_unwind() unnamed_addr #0 {
+#[no_mangle]
+pub extern "thiscall" fn rust_item_that_cannot_unwind() {
+}
+
+// CHECK: @rust_item_that_can_unwind() unnamed_addr #1 {
+#[no_mangle]
+pub extern "thiscall-unwind" fn rust_item_that_can_unwind() {
+}
+
+// Now, make some assertions that the LLVM attributes for these functions are correct.  First, make
+// sure that the first item is correctly marked with the `nounwind` attribute:
+//
+// CHECK: attributes #0 = { {{.*}}nounwind{{.*}} }
+//
+// Next, let's assert that the second item, which CAN unwind, does not have this attribute.
+//
+// CHECK: attributes #1 = {
+// CHECK-NOT: nounwind
+// CHECK: }