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``

3 files changed, 11 insertions, 13 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/machine.rs b/compiler/rustc_const_eval/src/interpret/machine.rs
index ee5de44a651..c798f1ce018 100644
--- a/compiler/rustc_const_eval/src/interpret/machine.rs
+++ b/compiler/rustc_const_eval/src/interpret/machine.rs
@@ -374,11 +374,17 @@ pub trait Machine<'mir, 'tcx: 'mir>: Sized {
         kind: Option<MemoryKind<Self::MemoryKind>>,
     ) -> InterpResult<'tcx, Cow<'b, Allocation<Self::Provenance, Self::AllocExtra, Self::Bytes>>>;
 
+    /// Evaluate the inline assembly.
+    ///
+    /// This should take care of jumping to the next block (one of `targets`) when asm goto
+    /// is triggered, `targets[0]` when the assembly falls through, or diverge in case of
+    /// `InlineAsmOptions::NORETURN` being set.
     fn eval_inline_asm(
         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
         _template: &'tcx [InlineAsmTemplatePiece],
         _operands: &[mir::InlineAsmOperand<'tcx>],
         _options: InlineAsmOptions,
+        _targets: &[mir::BasicBlock],
     ) -> InterpResult<'tcx> {
         throw_unsup_format!("inline assembly is not supported")
     }
diff --git a/compiler/rustc_const_eval/src/interpret/terminator.rs b/compiler/rustc_const_eval/src/interpret/terminator.rs
index 55f85573f38..f2b1ec42567 100644
--- a/compiler/rustc_const_eval/src/interpret/terminator.rs
+++ b/compiler/rustc_const_eval/src/interpret/terminator.rs
@@ -1,6 +1,5 @@
 use std::borrow::Cow;
 
-use rustc_ast::ast::InlineAsmOptions;
 use rustc_middle::{
     mir,
     ty::{
@@ -224,15 +223,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 terminator.kind
             ),
 
-            InlineAsm { template, ref operands, options, destination, .. } => {
-                M::eval_inline_asm(self, template, operands, options)?;
-                if options.contains(InlineAsmOptions::NORETURN) {
-                    throw_ub_custom!(fluent::const_eval_noreturn_asm_returned);
-                }
-                self.go_to_block(
-                    destination
-                        .expect("InlineAsm terminators without noreturn must have a destination"),
-                )
+            InlineAsm { template, ref operands, options, ref targets, .. } => {
+                M::eval_inline_asm(self, template, operands, options, targets)?;
             }
         }
 
diff --git a/compiler/rustc_const_eval/src/transform/validate.rs b/compiler/rustc_const_eval/src/transform/validate.rs
index cc49e8ea247..74ba2f6039e 100644
--- a/compiler/rustc_const_eval/src/transform/validate.rs
+++ b/compiler/rustc_const_eval/src/transform/validate.rs
@@ -471,9 +471,9 @@ impl<'a, 'tcx> Visitor<'tcx> for CfgChecker<'a, 'tcx> {
                 self.check_edge(location, *real_target, EdgeKind::Normal);
                 self.check_unwind_edge(location, *unwind);
             }
-            TerminatorKind::InlineAsm { destination, unwind, .. } => {
-                if let Some(destination) = destination {
-                    self.check_edge(location, *destination, EdgeKind::Normal);
+            TerminatorKind::InlineAsm { targets, unwind, .. } => {
+                for &target in targets {
+                    self.check_edge(location, target, EdgeKind::Normal);
                 }
                 self.check_unwind_edge(location, *unwind);
             }