Rollup merge of #119365 - nbdd0121:asm-goto, r=Amanieu

Add asm goto support to `asm!`

Tracking issue: #119364

This PR implements asm-goto support, using the syntax described in "future possibilities" section of [RFC2873](https://rust-lang.github.io/rfcs/2873-inline-asm.html#asm-goto).

Currently I have only implemented the `label` part, not the `fallthrough` part (i.e. fallthrough is implicit). This doesn't reduce the expressive though, since you can use label-break to get arbitrary control flow or simply set a value and rely on jump threading optimisation to get the desired control flow. I can add that later if deemed necessary.

r? ``@Amanieu``
cc ``@ojeda``

compiler/rustc_mir_build/src/build/expr/into.rs

@@ -383,6 +383,14 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 line_spans,
             }) => {
                 use rustc_middle::{mir, thir};
+
+                let destination_block = this.cfg.start_new_block();
+                let mut targets = if options.contains(InlineAsmOptions::NORETURN) {
+                    vec![]
+                } else {
+                    vec![destination_block]
+                };
+
                 let operands = operands
                     .into_iter()
                     .map(|op| match *op {
@@ -438,14 +446,28 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         thir::InlineAsmOperand::SymStatic { def_id } => {
                             mir::InlineAsmOperand::SymStatic { def_id }
                         }
+                        thir::InlineAsmOperand::Label { block } => {
+                            let target = this.cfg.start_new_block();
+                            let target_index = targets.len();
+                            targets.push(target);
+
+                            let tmp = this.get_unit_temp();
+                            let target = unpack!(this.ast_block(tmp, target, block, source_info));
+                            this.cfg.terminate(
+                                target,
+                                source_info,
+                                TerminatorKind::Goto { target: destination_block },
+                            );
+
+                            mir::InlineAsmOperand::Label { target_index }
+                        }
                     })
                     .collect();
 
-                if !options.contains(InlineAsmOptions::NORETURN) {
+                if !expr.ty.is_never() {
                     this.cfg.push_assign_unit(block, source_info, destination, this.tcx);
                 }
 
-                let destination_block = this.cfg.start_new_block();
                 this.cfg.terminate(
                     block,
                     source_info,
@@ -454,11 +476,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         operands,
                         options,
                         line_spans,
-                        destination: if options.contains(InlineAsmOptions::NORETURN) {
-                            None
-                        } else {
-                            Some(destination_block)
-                        },
+                        targets,
                         unwind: if options.contains(InlineAsmOptions::MAY_UNWIND) {
                             UnwindAction::Continue
                         } else {

compiler/rustc_mir_build/src/lints.rs

@@ -199,9 +199,10 @@ impl<'mir, 'tcx, C: TerminatorClassifier<'tcx>> TriColorVisitor<BasicBlocks<'tcx
             | TerminatorKind::Unreachable
             | TerminatorKind::Yield { .. } => ControlFlow::Break(NonRecursive),
 
-            // A diverging InlineAsm is treated as non-recursing
-            TerminatorKind::InlineAsm { destination, .. } => {
-                if destination.is_some() {
+            // A InlineAsm without targets (diverging and contains no labels)
+            // is treated as non-recursing.
+            TerminatorKind::InlineAsm { ref targets, .. } => {
+                if !targets.is_empty() {
                     ControlFlow::Continue(())
                 } else {
                     ControlFlow::Break(NonRecursive)

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -656,6 +656,9 @@ impl<'tcx> Cx<'tcx> {
                         hir::InlineAsmOperand::SymStatic { path: _, def_id } => {
                             InlineAsmOperand::SymStatic { def_id }
                         }
+                        hir::InlineAsmOperand::Label { block } => {
+                            InlineAsmOperand::Label { block: self.mirror_block(block) }
+                        }
                     })
                     .collect(),
                 options: asm.options,

compiler/rustc_mir_build/src/thir/print.rs

@@ -889,6 +889,12 @@ impl<'a, 'tcx> ThirPrinter<'a, 'tcx> {
                 print_indented!(self, format!("def_id: {:?}", def_id), depth_lvl + 1);
                 print_indented!(self, "}", depth_lvl + 1);
             }
+            InlineAsmOperand::Label { block } => {
+                print_indented!(self, "InlineAsmOperand::Block {", depth_lvl);
+                print_indented!(self, "block:", depth_lvl + 1);
+                self.print_block(*block, depth_lvl + 2);
+                print_indented!(self, "}", depth_lvl + 1);
+            }
         }
     }
 }