[compiler-v2] Do not pop non-droppables on stack flush

Fixes #14233

On stack flush we must not pop values which cannot be dropped. Rather they should be stored in temps. This faciltates scenarios where an `abort` happens before the end of a block. Without this, we pop the value before the abort, leading to a bytecode verification error.

There is still an issue with a missing warning, added #14320 for that.

third_party/move/move-compiler-v2/src/file_format_generator/function_generator.rs

@@ -1025,8 +1025,10 @@ impl<'a> FunctionGenerator<'a> {
             if before && ctx.is_alive_before(temp)
                 || !before && ctx.is_alive_after(temp, &[], false)
                 || self.pinned.contains(&temp)
+                || !ctx.fun_ctx.is_droppable(temp)
             {
-                // Only need to save to a local if the temp is still used afterwards
+                // Only need to save to a local if the temp is still used afterwards,
+                // or if it is not droppable
                 let local = self.temp_to_local(fun_ctx, Some(ctx.attr_id), temp);
                 self.emit(FF::Bytecode::StLoc(local));
             } else {
@@ -1147,6 +1149,14 @@ impl<'env> FunctionContext<'env> {
             .type_abilities(self.temp_type(temp), &self.type_parameters)
             .has_ability(FF::Ability::Copy)
     }
+
+    /// Returns true of the given temporary can/should be dropped when flushing the stack.
+    pub fn is_droppable(&self, temp: TempIndex) -> bool {
+        self.module
+            .env
+            .type_abilities(self.temp_type(temp), &self.type_parameters)
+            .has_ability(FF::Ability::Drop)
+    }
 }
 
 impl<'env> BytecodeContext<'env> {

third_party/move/move-compiler-v2/tests/ability-check/bug_14233_unused_non_droppable.exp

@@ -0,0 +1,2 @@
+
+============ bytecode verification succeeded ========

third_party/move/move-compiler-v2/tests/ability-check/bug_14233_unused_non_droppable.move

@@ -0,0 +1,9 @@
+module 0xCAFE::Module0 {
+    struct S {}
+
+    public fun f() {
+        let _x = S {};
+        abort 1;
+        let S {} = _x;
+    }
+}

third_party/move/move-compiler-v2/tests/ability-check/bug_14233_unused_non_droppable_no_abort.exp

@@ -0,0 +1,7 @@
+
+Diagnostics:
+error: local `_x` of type `Module0::S` does not have the `drop` ability
+  ┌─ tests/ability-check/bug_14233_unused_non_droppable_no_abort.move:5:18
+  │
+5 │         let _x = S {};
+  │                  ^^^^ implicitly dropped here since it is no longer used

third_party/move/move-compiler-v2/tests/ability-check/bug_14233_unused_non_droppable_no_abort.move

@@ -0,0 +1,9 @@
+module 0xCAFE::Module0 {
+    struct S {}
+
+    public fun f() {
+        let _x = S {};
+        //abort 1;
+        //let S {} = _x;
+    }
+}

third_party/move/move-compiler-v2/tests/testsuite.rs

@@ -283,11 +283,11 @@ const TEST_CONFIGS: Lazy<BTreeMap<&str, TestConfig>> = Lazy::new(|| {
             dump_bytecode: DumpLevel::None,
             dump_bytecode_filter:
             // For debugging (dump_bytecode set DumpLevel::AllStages)
-             Some(vec![
-               INITIAL_BYTECODE_STAGE,
-               "ReferenceSafetyProcessor",
-               "DeadStoreElimination",
-               FILE_FORMAT_STAGE,
+            Some(vec![
+                INITIAL_BYTECODE_STAGE,
+                "ReferenceSafetyProcessor",
+                "DeadStoreElimination",
+                FILE_FORMAT_STAGE,
             ]),
         },
         // Reference safety tests (with optimizations on)

third_party/move/move-compiler-v2/transactional-tests/tests/optimization/bug_14233_unused_non_droppable.exp

@@ -0,0 +1,22 @@
+comparison between v1 and v2 failed:
+- processed 2 tasks
++ processed 2 tasks
+= 
+- task 0 'publish'. lines 1-10:
+- warning[W09005]: dead or unreachable code
+-   ┌─ TEMPFILE:7:9
+-   │
+- 7 │         let S {} = _x;
+-   │         ^^^^^^^^^^^^^ Unreachable code. This statement (and any following statements) will not be executed.
+- 
+- 
+- 
+= task 1 'run'. lines 12-12:
+= Error: Function execution failed with VMError: {
+=     major_status: ABORTED,
+=     sub_status: Some(1),
+=     location: 0xcafe::m,
+=     indices: redacted,
+=     offsets: redacted,
+= }
+= 

third_party/move/move-compiler-v2/transactional-tests/tests/optimization/bug_14233_unused_non_droppable.move

@@ -0,0 +1,12 @@
+//# publish
+module 0xCAFE::m {
+    struct S {}
+
+    public fun f() {
+        let _x = S {};
+        abort 1;
+        let S {} = _x;
+    }
+}
+
+//# run 0xCAFE::m::f