Add codegen support for goto (Cpp2Rust#163)

Depends on Cpp2Rust#161. See Cpp2Rust#161 for a more detailed description of how goto is
implemented.

On the codegen part, this PR emits the goto_block and goto macros when
it hits functions that use labels and gotos internally. It also hoists
local variables at the start of the function so that all goto_block arms
can see and use the local variables.

Author: lucic71

cpp2rust/converter/converter.cpp

@@ -363,7 +363,59 @@ bool Converter::VisitFunctionDecl(clang::FunctionDecl *decl) {
   return false;
 }
 
+void Converter::EmitHoistedDecls(clang::CompoundStmt *body) {
+  for (auto *child : body->body()) {
+    if (auto *decl_stmt = clang::dyn_cast<clang::DeclStmt>(child)) {
+      for (auto *decl : decl_stmt->decls()) {
+        if (auto *var = clang::dyn_cast<clang::VarDecl>(decl);
+            var && var->isLocalVarDecl() && !IsGlobalVar(var)) {
+          hoisted_decls_.insert(var);
+          if (ConvertVarDeclSkipInit(var)) {
+            StrCat(token::kAssign, ConvertVarDefaultInit(var->getType()),
+                   token::kSemiColon);
+          }
+        }
+      }
+    }
+  }
+}
+
+void Converter::ConvertGotoBlock(clang::CompoundStmt *body) {
+  PushHoistedDecls push(hoisted_decls_);
+  EmitHoistedDecls(body);
+
+  StrCat("goto_block!");
+  {
+    PushParen paren(*this);
+    PushBrace outer(*this);
+    StrCat("'__entry: ");
+    std::optional<PushBrace> arm;
+    arm.emplace(*this);
+    for (auto *child : body->body()) {
+      if (auto *label = clang::dyn_cast<clang::LabelStmt>(child)) {
+        arm.reset();
+        StrCat(std::format("'{}: ", label->getDecl()->getName().str()));
+        arm.emplace(*this);
+        Convert(label->getSubStmt());
+      } else {
+        Convert(child);
+      }
+    }
+  }
+  StrCat(token::kSemiColon);
+}
+
 void Converter::ConvertFunctionBody(clang::FunctionDecl *decl) {
+  if (auto compound = clang::dyn_cast<clang::CompoundStmt>(decl->getBody())) {
+    if (CompoundHasTopLevelLabel(compound)) {
+      ConvertGotoBlock(compound);
+      if (!decl->getReturnType()->isVoidType()) {
+        StrCat(R"(panic!("ub: non-void function does not return a value"))");
+      }
+      return;
+    }
+  }
+
   Convert(decl->getBody());
   if (!decl->getReturnType()->isVoidType()) {
     if (auto compound = clang::dyn_cast<clang::CompoundStmt>(decl->getBody())) {
@@ -466,7 +518,20 @@ void Converter::ConvertVarDeclInitializer(clang::VarDecl *decl) {
   }
 }
 
+void Converter::EmitHoistedInArmAssignment(clang::VarDecl *decl) {
+  if (!decl->hasInit()) {
+    return;
+  }
+  StrCat(GetNamedDeclAsString(decl), token::kAssign);
+  ConvertVarInit(decl->getType(), decl->getInit());
+  StrCat(token::kSemiColon);
+}
+
 void Converter::ConvertVarDecl(clang::VarDecl *decl) {
+  if (hoisted_decls_.contains(decl)) {
+    EmitHoistedInArmAssignment(decl);
+    return;
+  }
   if (!ConvertVarDeclSkipInit(decl)) {
     // Skip global variables declared extern
     return;
@@ -984,6 +1049,10 @@ bool Converter::Convert(clang::Stmt *stmt) {
 }
 
 bool Converter::VisitCompoundStmt(clang::CompoundStmt *stmt) {
+  if (CompoundHasTopLevelLabel(stmt)) {
+    ConvertGotoBlock(stmt);
+    return false;
+  }
   for (auto *child : stmt->body()) {
     Convert(child);
   }
@@ -1010,6 +1079,11 @@ bool Converter::VisitReturnStmt(clang::ReturnStmt *stmt) {
   return false;
 }
 
+bool Converter::VisitGotoStmt(clang::GotoStmt *stmt) {
+  StrCat(std::format("goto!('{})", stmt->getLabel()->getName().str()));
+  return false;
+}
+
 void Converter::ConvertCondition(clang::Expr *cond) {
   PushExprKind push(*this, ExprKind::RValue);
   Convert(NormalizeToBool(cond, ctx_));

tests/unit/out/refcount/goto_cleanup.rs

@@ -56,7 +56,7 @@ pub fn from_switch_2(n: i32) -> i32 {
             'switch: {
                 let __match_cond = (*n.borrow());
                 match __match_cond {
-                    __v if __v == 1 => {
+                    v if v == 1 => {
                         (*ret.borrow_mut()) = 10;
                         goto!('out);
                     }

tests/unit/out/refcount/goto_switch_fallthrough.rs

@@ -13,10 +13,10 @@ pub fn sm_0(n: i32) -> i32 {
         '__entry: {
             *ret.borrow_mut() = 0;
             switch!(match (*n.borrow()) {
-                __v if __v == 0 => {
+                v if v == 0 => {
                     (*ret.borrow_mut()) += 1;
                 }
-                __v if __v == 1 => {
+                v if v == 1 => {
                     (*ret.borrow_mut()) += 10;
                     goto!('out);
                 }

tests/unit/out/unsafe/goto_cleanup.rs

@@ -53,7 +53,7 @@ pub unsafe fn from_switch_2(mut n: i32) -> i32 {
             'switch: {
                 let __match_cond = n;
                 match __match_cond {
-                    __v if __v == 1 => {
+                    v if v == 1 => {
                         ret = 10;
                         goto!('out);
                     }

tests/unit/out/unsafe/goto_switch_fallthrough.rs

@@ -12,10 +12,10 @@ pub unsafe fn sm_0(mut n: i32) -> i32 {
         '__entry: {
             ret = 0;
             switch!(match n {
-                __v if __v == 0 => {
+                v if v == 0 => {
                     ret += 1;
                 }
-                __v if __v == 1 => {
+                v if v == 1 => {
                     ret += 10;
                     goto!('out);
                 }