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 emit an `abort` anywhere that would have tried to return an
uninhabited value.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/librustc_codegen_ssa/mir/block.rs b/src/librustc_codegen_ssa/mir/block.rs
index 53e8f7ed88b..aaa0be20412 100644
--- a/src/librustc_codegen_ssa/mir/block.rs
+++ b/src/librustc_codegen_ssa/mir/block.rs
@@ -238,6 +238,13 @@ impl<'a, 'tcx: 'a, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 }
             }
         }
+        if self.fn_ty.ret.layout.abi.is_uninhabited() {
+            // Functions with uninhabited return values are marked `noreturn`,
+            // so we should make sure that we never actually do.
+            bx.abort();
+            bx.unreachable();
+            return;
+        }
         let llval = match self.fn_ty.ret.mode {
             PassMode::Ignore(IgnoreMode::Zst) | PassMode::Indirect(..) => {
                 bx.ret_void();