checker,gen: allow using methods as function pointers (#14407)

vlib/v/checker/checker.v

@@ -1850,6 +1850,27 @@ pub fn (mut c Checker) selector_expr(mut node ast.SelectorExpr) ast.Type {
 		node.typ = field.typ
 		return field.typ
 	}
+	if mut method := c.table.find_method(sym, field_name) {
+		receiver := method.params[0].typ
+		if receiver.nr_muls() > 0 {
+			if !c.inside_unsafe {
+				rec_sym := c.table.sym(receiver.set_nr_muls(0))
+				if !rec_sym.is_heap() {
+					suggestion := if rec_sym.kind == .struct_ {
+						'declaring `$rec_sym.name` as `[heap]`'
+					} else {
+						'wrapping the `$rec_sym.name` object in a `struct` declared as `[heap]`'
+					}
+					c.error('method `${c.table.type_to_str(receiver.idx())}.$method.name` cannot be used as a variable outside `unsafe` blocks as its receiver might refer to an object stored on stack. Consider ${suggestion}.',
+						node.expr.pos().extend(node.pos))
+				}
+			}
+		}
+		method.params = method.params[1..]
+		fn_type := ast.new_type(c.table.find_or_register_fn_type(c.mod, method, false,
+			true))
+		return fn_type
+	}
 	if sym.kind !in [.struct_, .aggregate, .interface_, .sum_type] {
 		if sym.kind != .placeholder {
 			unwrapped_sym := c.table.sym(c.unwrap_generic(typ))

vlib/v/checker/tests/unsafe_method_as_field.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/unsafe_method_as_field.vv:23:7: error: method `Foo.ref` cannot be used as a variable outside `unsafe` blocks as its receiver might refer to an object stored on stack. Consider declaring `Foo` as `[heap]`.
+   21 |     f := Foo{}
+   22 |     _ := f.no_ref // no error
+   23 |     _ := f.ref // error
+      |          ~~~~~
+   24 |
+   25 |     b := Bar{}

vlib/v/checker/tests/unsafe_method_as_field.vv

@@ -0,0 +1,27 @@
+struct Foo {
+}
+
+fn (f Foo) no_ref() int {
+	return 1
+}
+
+fn (f &Foo) ref() int {
+	return 1
+}
+
+[heap]
+struct Bar {
+}
+
+fn (f &Bar) ref() int {
+	return 1
+}
+
+fn main() {
+	f := Foo{}
+	_ := f.no_ref // no error
+	_ := f.ref // error
+
+	b := Bar{}
+	_ := b.ref // no error
+}

vlib/v/gen/c/assign.v

@@ -211,7 +211,11 @@ fn (mut g Gen) gen_assign_stmt(node_ ast.AssignStmt) {
 			} else if g.inside_for_c_stmt {
 				g.expr(val)
 			} else {
-				g.write('{$styp _ = ')
+				if left_sym.kind == .function {
+					g.write('{void* _ = ')
+				} else {
+					g.write('{$styp _ = ')
+				}
 				g.expr(val)
 				g.writeln(';}')
 			}

vlib/v/gen/c/cgen.v

@@ -3339,6 +3339,64 @@ fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 				}
 			}
 		}
+	} else if m := g.table.find_method(sym, node.field_name) {
+		mut has_embeds := false
+		if sym.info in [ast.Struct, ast.Aggregate] {
+			if node.from_embed_types.len > 0 {
+				has_embeds = true
+			}
+		}
+		if !has_embeds {
+			receiver := m.params[0]
+			expr_styp := g.typ(node.expr_type.idx())
+			data_styp := g.typ(receiver.typ.idx())
+			mut sb := strings.new_builder(256)
+			name := '_V_closure_${expr_styp}_${m.name}_$node.pos.pos'
+			sb.write_string('${g.typ(m.return_type)} ${name}(')
+			for i in 1 .. m.params.len {
+				param := m.params[i]
+				if i != 1 {
+					sb.write_string(', ')
+				}
+				sb.write_string('${g.typ(param.typ)} a$i')
+			}
+			sb.writeln(') {')
+			sb.writeln('\t$data_styp* a0 = *($data_styp**)(__RETURN_ADDRESS() - __CLOSURE_DATA_OFFSET);')
+			if m.return_type != ast.void_type {
+				sb.write_string('\treturn ')
+			} else {
+				sb.write_string('\t')
+			}
+			sb.write_string('${expr_styp}_${m.name}(')
+			if !receiver.typ.is_ptr() {
+				sb.write_string('*')
+			}
+			for i in 0 .. m.params.len {
+				if i != 0 {
+					sb.write_string(', ')
+				}
+				sb.write_string('a$i')
+			}
+			sb.writeln(');')
+			sb.writeln('}')
+
+			g.anon_fn_definitions << sb.str()
+			g.nr_closures++
+
+			g.write('__closure_create($name, ')
+			if !receiver.typ.is_ptr() {
+				g.write('memdup(')
+			}
+			if !node.expr_type.is_ptr() {
+				g.write('&')
+			}
+			g.expr(node.expr)
+			if !receiver.typ.is_ptr() {
+				g.write(', sizeof($expr_styp))')
+			}
+			g.write(')')
+			return
+		}
 	}
 	n_ptr := node.expr_type.nr_muls() - 1
 	if n_ptr > 0 {

vlib/v/markused/walker.v

@@ -378,6 +378,11 @@ fn (mut w Walker) expr(node_ ast.Expr) {
 		}
 		ast.SelectorExpr {
 			w.expr(node.expr)
+			if node.expr_type != 0 {
+				if method := w.table.find_method(w.table.sym(node.expr_type), node.field_name) {
+					w.fn_by_name(method.fkey())
+				}
+			}
 		}
 		ast.SqlExpr {
 			w.expr(node.db_expr)

vlib/v/tests/methods_as_fields_test.v

@@ -0,0 +1,97 @@
+struct Foo {
+	s string
+mut:
+	i int
+}
+
+fn (f Foo) get_s() string {
+	return f.s
+}
+
+fn (f &Foo) get_s_ref() string {
+	return f.s
+}
+
+fn (f Foo) add(a int) int {
+	return a + f.i
+}
+
+fn (f &Foo) add_ref(a int) int {
+	return a + f.i
+}
+
+fn (mut f Foo) set(a int) {
+	f.i = a
+}
+
+fn (f_ Foo) set_val(a int) int {
+	mut f := unsafe { &f_ }
+	old := f.i
+	f.i = a
+	return old
+}
+
+fn test_methods_as_fields() {
+	mut f := Foo{
+		s: 'hello'
+		i: 1
+	}
+
+	get_s := f.get_s
+	get_s_ref := unsafe { f.get_s_ref }
+	add := f.add
+	add_ref := unsafe { f.add_ref }
+	set := unsafe { f.set }
+	set_val := f.set_val
+
+	assert typeof(get_s).str() == 'fn () string'
+	assert typeof(get_s_ref).str() == 'fn () string'
+	assert typeof(add).str() == 'fn (int) int'
+	assert typeof(add_ref).str() == 'fn (int) int'
+
+	assert get_s() == 'hello'
+	assert get_s_ref() == 'hello'
+	assert add(2) == 3
+	assert add_ref(2) == 3
+
+	assert f.i == 1
+	set(2)
+	assert f.i == 2
+	old := set_val(3)
+	assert f.i == 2
+	new := set_val(5)
+	assert old == new && old == 1
+}
+
+// the difference between these two tests is that here `f` is &Foo
+fn test_methods_as_fields_ref() {
+	mut f := &Foo{
+		s: 'hello'
+		i: 1
+	}
+
+	get_s := f.get_s
+	get_s_ref := unsafe { f.get_s_ref }
+	add := f.add
+	add_ref := unsafe { f.add_ref }
+	set := unsafe { f.set }
+	set_val := f.set_val
+
+	assert typeof(get_s).str() == 'fn () string'
+	assert typeof(get_s_ref).str() == 'fn () string'
+	assert typeof(add).str() == 'fn (int) int'
+	assert typeof(add_ref).str() == 'fn (int) int'
+
+	assert get_s() == 'hello'
+	assert get_s_ref() == 'hello'
+	assert add(2) == 3
+	assert add_ref(2) == 3
+
+	assert f.i == 1
+	set(2)
+	assert f.i == 2
+	old := set_val(3)
+	assert f.i == 2
+	new := set_val(5)
+	assert old == new && old == 1
+}

vlib/v/tests/skip_unused/method_as_fn_pointer.vv

@@ -0,0 +1,12 @@
+struct Foo {
+	x int
+}
+
+fn (f Foo) hi() int {
+	return f.x
+}
+
+fn main() {
+	f := Foo{123}
+	_ = f.hi
+}