Add assertions and clarify asm-goto with noreturn

compiler/rustc_codegen_llvm/src/asm.rs

@@ -465,6 +465,7 @@ pub(crate) fn inline_asm_call<'ll>(
             );
 
             let call = if !labels.is_empty() {
+                assert!(catch_funclet.is_none());
                 bx.callbr(fty, None, None, v, inputs, dest.unwrap(), labels, None)
             } else if let Some((catch, funclet)) = catch_funclet {
                 bx.invoke(fty, None, None, v, inputs, dest.unwrap(), catch, funclet)

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -265,6 +265,7 @@ impl<'a, 'tcx> TerminatorCodegenHelper<'tcx> {
         };
 
         if operands.iter().any(|x| matches!(x, InlineAsmOperandRef::Label { .. })) {
+            assert!(unwind_target.is_none());
             let ret_llbb = if let Some(target) = destination {
                 fx.llbb(target)
             } else {

src/doc/unstable-book/src/language-features/asm-goto.md

@@ -21,4 +21,10 @@ unsafe {
 }
 ```
 
-The block must have unit type.
+The block must have unit type or diverge.
+
+When `label <block>` is used together with `noreturn` option, it means that the
+assembly will not fallthrough. It's allowed to jump to a label within the
+assembly. In this case, the entire `asm!` expression will have an unit type as
+opposed to diverging, if not all label blocks diverge. The `asm!` expression
+still diverges if `noreturn` option is used and all label blocks diverge.