checker/cgen: support lock expressions `x := rlock s { s.get() }` (#8540)

vlib/v/checker/checker.v

@@ -4589,8 +4589,19 @@ pub fn (mut c Checker) lock_expr(mut node ast.LockExpr) table.Type {
 	c.stmts(node.stmts)
 	c.rlocked_names = []
 	c.locked_names = []
-	// void for now... maybe sometime `x := lock a { a.getval() }`
-	return table.void_type
+	// handle `x := rlock a { a.getval() }`
+	mut ret_type := table.void_type
+	if node.stmts.len > 0 {
+		last_stmt := node.stmts[node.stmts.len - 1]
+		if last_stmt is ast.ExprStmt {
+			ret_type = last_stmt.typ
+		}
+	}
+	if ret_type != table.void_type {
+		node.is_expr = true
+	}
+	node.typ = ret_type
+	return ret_type
 }
 
 pub fn (mut c Checker) unsafe_expr(mut node ast.UnsafeExpr) table.Type {

vlib/v/gen/c/cgen.v

@@ -876,7 +876,7 @@ fn (mut g Gen) stmts_with_tmp_var(stmts []ast.Stmt, tmp_var string) {
 			// Handle if expressions, set the value of the last expression to the temp var.
 			g.stmt_path_pos << g.out.len
 			g.skip_stmt_pos = true
-			g.writeln('$tmp_var = /* if expr set */')
+			g.write('$tmp_var = ')
 		}
 		g.stmt(stmt)
 		g.skip_stmt_pos = false
@@ -3407,6 +3407,13 @@ fn (mut g Gen) infix_expr(node ast.InfixExpr) {
 }
 
 fn (mut g Gen) lock_expr(node ast.LockExpr) {
+	tmp_result := if node.is_expr { g.new_tmp_var() } else { '' }
+	mut cur_line := ''
+	if node.is_expr {
+		styp := g.typ(node.typ)
+		cur_line = g.go_before_stmt(0)
+		g.writeln('$styp $tmp_result;')
+	}
 	mut mtxs := ''
 	if node.lockeds.len == 0 {
 		// this should not happen
@@ -3449,7 +3456,7 @@ fn (mut g Gen) lock_expr(node ast.LockExpr) {
 		g.writeln('}')
 	}
 	g.writeln('/*lock*/ {')
-	g.stmts(node.stmts)
+	g.stmts_with_tmp_var(node.stmts, tmp_result)
 	g.writeln('}')
 	if node.lockeds.len == 0 {
 		// this should not happen
@@ -3469,6 +3476,11 @@ fn (mut g Gen) lock_expr(node ast.LockExpr) {
 		g.writeln('\t\tsync__RwMutex_unlock((sync__RwMutex*)_arr_$mtxs[$mtxs]);')
 		g.write('}')
 	}
+	if node.is_expr {
+		g.writeln('')
+		g.write(cur_line)
+		g.write('$tmp_result')
+	}
 }
 
 fn (mut g Gen) match_expr(node ast.MatchExpr) {

vlib/v/tests/shared_lock_expr_test.v

@@ -0,0 +1,14 @@
+struct St {
+mut:
+	i int
+}
+
+fn test_lock_expr() {
+	shared xx := St{ i: 173 }
+	shared y := St{ i: -57 }
+	mut m := 0
+	m = lock y { y.i }
+	n := rlock xx { xx.i }
+	assert m == -57
+	assert n == 173
+}