Rollup merge of rust-lang#59639 - cuviper:ignore-uninhabited, r=sanxiyn

Never return uninhabited values at all

Functions with uninhabited return values are already marked `noreturn`,
but we were still generating return instructions for this. When running
with `C passes=lint`, LLVM prints:

    Unusual: Return statement in function with noreturn attribute

The LLVM manual makes a stronger statement about `noreturn` though:

> This produces undefined behavior at runtime if the function ever does
dynamically return.

We now mark such return values with a new `IgnoreMode::Uninhabited`, and
emit an `abort` anywhere that would have returned.

Fixes rust-lang#48227
cc rust-lang#7463 rust-lang#48229

r? @eddyb

src/librustc_codegen_llvm/abi.rs

@@ -518,7 +518,11 @@ impl<'tcx> FnTypeExt<'tcx> for FnType<'tcx, Ty<'tcx>> {
         let arg_of = |ty: Ty<'tcx>, arg_idx: Option<usize>| {
             let is_return = arg_idx.is_none();
             let mut arg = mk_arg_type(ty, arg_idx);
-            if arg.layout.is_zst() {
+            if is_return && arg.layout.abi.is_uninhabited() {
+                // Functions with uninhabited return values are marked `noreturn`,
+                // so we don't actually need to do anything with the value.
+                arg.mode = PassMode::Ignore(IgnoreMode::Uninhabited);
+            } else if arg.layout.is_zst() {
                 // For some forsaken reason, x86_64-pc-windows-gnu
                 // doesn't ignore zero-sized struct arguments.
                 // The same is true for s390x-unknown-linux-gnu
@@ -677,7 +681,8 @@ impl<'tcx> FnTypeExt<'tcx> for FnType<'tcx, Ty<'tcx>> {
         );
 
         let llreturn_ty = match self.ret.mode {
-            PassMode::Ignore(IgnoreMode::Zst) => cx.type_void(),
+            PassMode::Ignore(IgnoreMode::Zst)
+            | PassMode::Ignore(IgnoreMode::Uninhabited) => cx.type_void(),
             PassMode::Ignore(IgnoreMode::CVarArgs) =>
                 bug!("`va_list` should never be a return type"),
             PassMode::Direct(_) | PassMode::Pair(..) => {

src/librustc_codegen_ssa/mir/block.rs

@@ -244,6 +244,12 @@ impl<'a, 'tcx: 'a, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 return;
             }
 
+            PassMode::Ignore(IgnoreMode::Uninhabited) => {
+                bx.abort();
+                bx.unreachable();
+                return;
+            }
+
             PassMode::Ignore(IgnoreMode::CVarArgs) => {
                 bug!("C-variadic arguments should never be the return type");
             }

src/librustc_codegen_ssa/mir/mod.rs

@@ -521,7 +521,8 @@ fn arg_local_refs<'a, 'tcx: 'a, Bx: BuilderMethods<'a, 'tcx>>(
                 PassMode::Ignore(IgnoreMode::Zst) => {
                     return local(OperandRef::new_zst(bx, arg.layout));
                 }
-                PassMode::Ignore(IgnoreMode::CVarArgs) => {}
+                PassMode::Ignore(IgnoreMode::CVarArgs)
+                | PassMode::Ignore(IgnoreMode::Uninhabited) => {}
                 PassMode::Direct(_) => {
                     let llarg = bx.get_param(llarg_idx);
                     bx.set_value_name(llarg, &name);

src/librustc_target/abi/call/mod.rs

@@ -29,6 +29,8 @@ pub enum IgnoreMode {
     CVarArgs,
     /// A zero-sized type.
     Zst,
+    /// An uninhabited type.
+    Uninhabited,
 }
 
 #[derive(Clone, Copy, PartialEq, Eq, Debug)]

src/test/codegen/noreturn-uninhabited.rs

@@ -0,0 +1,32 @@
+// compile-flags: -g -C no-prepopulate-passes
+// ignore-tidy-linelength
+
+#![crate_type = "lib"]
+
+#[derive(Clone, Copy)]
+pub enum EmptyEnum {}
+
+#[no_mangle]
+pub fn empty(x: &EmptyEnum) -> EmptyEnum {
+    // CHECK: @empty({{.*}}) unnamed_addr #0
+    // CHECK-NOT: ret void
+    // CHECK: call void @llvm.trap()
+    // CHECK: unreachable
+    *x
+}
+
+pub struct Foo(String, EmptyEnum);
+
+#[no_mangle]
+pub fn foo(x: String, y: &EmptyEnum) -> Foo {
+    // CHECK: @foo({{.*}}) unnamed_addr #0
+    // CHECK-NOT: ret %Foo
+    // CHECK: call void @llvm.trap()
+    // CHECK: unreachable
+    Foo(x, *y)
+}
+
+// CHECK: attributes #0 = {{{.*}} noreturn {{.*}}}
+
+// CHECK: DISubprogram(name: "empty", {{.*}} DIFlagNoReturn
+// CHECK: DISubprogram(name: "foo", {{.*}} DIFlagNoReturn