v: add $res compile time function to get returned value in defer block

doc/docs.md

@@ -2119,6 +2119,42 @@ fn main() {
 }
 ```
 
+To access the result of the function inside a `defer` block the `$res()` expression can be used.
+`$res()` is only used when a single value is returned, while on multi-return the `$res(idx)`
+is parameterized.
+
+```v
+fn (mut app App) auth_middleware() bool {
+	defer {
+		if !$res() {
+			app.response.status_code = 401
+			app.response.body = 'Unauthorized'
+		}
+	}
+	header := app.get_header('Authorization')
+	if header == '' {
+		return false
+	}
+	return true
+}
+
+fn (mut app App) auth_with_user_middleware() (bool, string) {
+	defer {
+		if !$res(0) {
+			app.response.status_code = 401
+			app.response.body = 'Unauthorized'
+		} else {
+			app.user = $res(1)
+		}
+	}
+	header := app.get_header('Authorization')
+	if header == '' {
+		return false, ''
+	}
+	return true, 'TestUser'
+}
+```
+
 ### Goto
 
 V allows unconditionally jumping to a label with `goto`. The label name must be contained

vlib/v/checker/checker.v

@@ -115,6 +115,7 @@ mut:
 	comptime_call_pos                int // needed for correctly checking use before decl for templates
 	goto_labels                      map[string]ast.GotoLabel // to check for unused goto labels
 	enum_data_type                   ast.Type
+	fn_return_type                   ast.Type
 }
 
 pub fn new_checker(table &ast.Table, pref_ &pref.Preferences) &Checker {

vlib/v/checker/comptime.v

@@ -144,6 +144,40 @@ fn (mut c Checker) comptime_call(mut node ast.ComptimeCall) ast.Type {
 		// assume string for now
 		return ast.string_type
 	}
+	if node.method_name == 'res' {
+		if !c.inside_defer {
+			c.error('`res` can only be used in defer blocks', node.pos)
+			return ast.void_type
+		}
+
+		if c.fn_return_type == ast.void_type {
+			c.error('`res` can only be used in functions that returns something', node.pos)
+			return ast.void_type
+		}
+
+		sym := c.table.sym(c.fn_return_type)
+
+		if c.fn_return_type.has_flag(.result) {
+			c.error('`res` cannot be used in functions that returns a Result', node.pos)
+			return ast.void_type
+		}
+
+		if sym.info is ast.MultiReturn {
+			if node.args_var == '' {
+				c.error('`res` requires an index of the returned value', node.pos)
+				return ast.void_type
+			}
+			idx := node.args_var.int()
+			if idx < 0 || idx >= sym.info.types.len {
+				c.error('index ${idx} out of range of ${sym.info.types.len} return types',
+					node.pos)
+				return ast.void_type
+			}
+			return sym.info.types[idx]
+		}
+
+		return c.fn_return_type
+	}
 	if node.is_vweb {
 		return ast.string_type
 	}

vlib/v/checker/fn.v

@@ -102,6 +102,7 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 			}
 		}
 	}
+	c.fn_return_type = node.return_type
 	if node.return_type != ast.void_type {
 		if ct_attr_idx := node.attrs.find_comptime_define() {
 			sexpr := node.attrs[ct_attr_idx].ct_expr.str()

vlib/v/checker/tests/defer_use_multi_return_value_with_index_out_of_bounds.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/defer_use_multi_return_value_with_index_out_of_bounds.vv:3:11: error: index 2 out of range of 2 return types
+    1 | fn test() (string, string) {
+    2 |     defer {
+    3 |         println($res(2))
+      |                 ~~~~~~~
+    4 |     }
+    5 |     return 'test', 'test2'

vlib/v/checker/tests/defer_use_multi_return_value_with_index_out_of_bounds.vv

@@ -0,0 +1,6 @@
+fn test() (string, string) {
+	defer {
+		println($res(2))
+	}
+	return 'test', 'test2'
+}

vlib/v/checker/tests/defer_use_multi_return_value_without_index.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/defer_use_multi_return_value_without_index.vv:3:11: error: `res` requires an index of the returned value
+    1 | fn test() (string, string) {
+    2 |     defer {
+    3 |         println($res())
+      |                 ~~~~~~
+    4 |     }
+    5 |     return 'test', 'test2'

vlib/v/checker/tests/defer_use_multi_return_value_without_index.vv

@@ -0,0 +1,6 @@
+fn test() (string, string) {
+	defer {
+		println($res())
+	}
+	return 'test', 'test2'
+}

vlib/v/checker/tests/defer_use_returned_value_when_nothing_is_returned.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/defer_use_returned_value_when_nothing_is_returned.vv:3:11: error: `res` can only be used in functions that returns something
+    1 | fn test() {
+    2 |     defer {
+    3 |         println($res())
+      |                 ~~~~~~
+    4 |     }
+    5 | }

vlib/v/checker/tests/defer_use_returned_value_when_nothing_is_returned.vv

@@ -0,0 +1,5 @@
+fn test() {
+	defer {
+		println($res())
+	}
+}

vlib/v/checker/tests/defer_use_returned_value_when_result_is_returned.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/defer_use_returned_value_when_result_is_returned.vv:3:11: error: `res` cannot be used in functions that returns a Result
+    1 | fn test() !string {
+    2 |     defer {
+    3 |         println($res())
+      |                 ~~~~~~
+    4 |     }
+    5 |     return 'test'

vlib/v/checker/tests/defer_use_returned_value_when_result_is_returned.vv

@@ -0,0 +1,6 @@
+fn test() !string {
+	defer {
+		println($res())
+	}
+	return 'test'
+}

vlib/v/checker/tests/res_use_outside_defer.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/res_use_outside_defer.vv:2:10: error: `res` can only be used in defer blocks
+    1 | fn test() string {
+    2 |     println($res())
+      |             ~~~~~~
+    3 |     return 'test'
+    4 | }

vlib/v/checker/tests/res_use_outside_defer.vv

@@ -0,0 +1,4 @@
+fn test() string {
+	println($res())
+	return 'test'
+}

vlib/v/fmt/fmt.v

@@ -1969,6 +1969,13 @@ pub fn (mut f Fmt) comptime_call(node ast.ComptimeCall) {
 			node.method_name in ['compile_error', 'compile_warn'] {
 				f.write("\$${node.method_name}('${node.args_var}')")
 			}
+			node.method_name == 'res' {
+				if node.args_var != '' {
+					f.write('\$res(${node.args_var})')
+				} else {
+					f.write('\$res()')
+				}
+			}
 			else {
 				inner_args := if node.args_var != '' {
 					node.args_var

vlib/v/gen/c/cgen.v

@@ -245,8 +245,9 @@ mut:
 	out_fn_start_pos []int  // for generating multiple .c files, stores locations of all fn positions in `out` string builder
 	static_modifier  string // for parallel_cc
 
-	has_reflection     bool
-	reflection_strings &map[string]int
+	has_reflection       bool
+	reflection_strings   &map[string]int
+	defer_return_tmp_var string
 }
 
 // global or const variable definition string
@@ -4695,6 +4696,7 @@ fn (mut g Gen) return_stmt(node ast.Return) {
 		return
 	}
 	tmpvar := g.new_tmp_var()
+	g.defer_return_tmp_var = tmpvar
 	mut ret_typ := g.typ(g.unwrap_generic(fn_ret_type))
 	if fn_ret_type.has_flag(.generic) && fn_return_is_fixed_array {
 		ret_typ = '_v_${ret_typ}'
@@ -4738,7 +4740,7 @@ fn (mut g Gen) return_stmt(node ast.Return) {
 					}
 				}
 				g.write_defer_stmts_when_needed()
-				g.writeln('return ${tmpvar};')
+				g.writeln('return ${tmpvar}; //test')
 			}
 			return
 		}
@@ -4766,7 +4768,7 @@ fn (mut g Gen) return_stmt(node ast.Return) {
 			g.writeln(';')
 			if use_tmp_var {
 				g.write_defer_stmts_when_needed()
-				g.writeln('return ${tmpvar};')
+				g.writeln('return ${tmpvar}; //test1')
 			}
 			return
 		}
@@ -4874,7 +4876,7 @@ fn (mut g Gen) return_stmt(node ast.Return) {
 				g.writeln(';')
 			}
 			g.write_defer_stmts_when_needed()
-			g.writeln('return ${tmpvar};')
+			g.writeln('return ${tmpvar}; //test2')
 			has_semicolon = true
 		}
 	} else if node.exprs.len >= 1 {
@@ -4913,7 +4915,7 @@ fn (mut g Gen) return_stmt(node ast.Return) {
 			g.writeln(' }, (${c.option_name}*)(&${tmpvar}), sizeof(${styp}));')
 			g.write_defer_stmts_when_needed()
 			g.autofree_scope_vars(node.pos.pos - 1, node.pos.line_nr, true)
-			g.writeln('return ${tmpvar};')
+			g.writeln('return ${tmpvar}; //test4')
 			return
 		}
 		expr_type_is_result := match expr0 {
@@ -4946,7 +4948,7 @@ fn (mut g Gen) return_stmt(node ast.Return) {
 			g.writeln(' }, (${c.result_name}*)(&${tmpvar}), sizeof(${styp}));')
 			g.write_defer_stmts_when_needed()
 			g.autofree_scope_vars(node.pos.pos - 1, node.pos.line_nr, true)
-			g.writeln('return ${tmpvar};')
+			g.writeln('return ${tmpvar}; //test 4')
 			return
 		}
 		// autofree before `return`
@@ -5024,7 +5026,7 @@ fn (mut g Gen) return_stmt(node ast.Return) {
 			if !g.is_builtin_mod {
 				g.autofree_scope_vars(node.pos.pos - 1, node.pos.line_nr, true)
 			}
-			g.write('return ${tmpvar}')
+			g.write('return ${tmpvar} /* test5 */')
 			has_semicolon = false
 		}
 	} else {

vlib/v/gen/c/comptime.v

@@ -80,6 +80,15 @@ fn (mut g Gen) comptime_call(mut node ast.ComptimeCall) {
 		g.write('_SLIT("${val}")')
 		return
 	}
+	if node.method_name == 'res' {
+		if node.args_var != '' {
+			g.write('${g.defer_return_tmp_var}.arg${node.args_var}')
+			return
+		}
+
+		g.write('${g.defer_return_tmp_var}')
+		return
+	}
 	if node.is_vweb {
 		is_html := node.method_name == 'html'
 		mut cur_line := ''

vlib/v/parser/comptime.v

@@ -10,7 +10,7 @@ import v.token
 
 const (
 	supported_comptime_calls = ['html', 'tmpl', 'env', 'embed_file', 'pkgconfig', 'compile_error',
-		'compile_warn']
+		'compile_warn', 'res']
 	comptime_types           = ['map', 'array', 'int', 'float', 'struct', 'interface', 'enum',
 		'sumtype', 'alias', 'function', 'option']
 )
@@ -97,7 +97,7 @@ fn (mut p Parser) comptime_call() ast.ComptimeCall {
 	}
 	start_pos := p.tok.pos()
 	p.check(.dollar)
-	error_msg := 'only `\$tmpl()`, `\$env()`, `\$embed_file()`, `\$pkgconfig()`, `\$vweb.html()`, `\$compile_error()` and `\$compile_warn()` comptime functions are supported right now'
+	error_msg := 'only `\$tmpl()`, `\$env()`, `\$embed_file()`, `\$pkgconfig()`, `\$vweb.html()`, `\$compile_error()`, `\$compile_warn()` and `\$res()` comptime functions are supported right now'
 	if p.peek_tok.kind == .dot {
 		name := p.check_name() // skip `vweb.html()` TODO
 		if name != 'vweb' {
@@ -129,6 +129,28 @@ fn (mut p Parser) comptime_call() ast.ComptimeCall {
 			env_pos: start_pos
 			pos: start_pos.extend(p.prev_tok.pos())
 		}
+	} else if method_name == 'res' {
+		mut has_args := false
+		mut type_index := ''
+		if p.tok.kind == .number {
+			has_args = true
+			type_index = p.tok.lit
+			p.check(.number)
+		}
+		p.check(.rpar)
+		if has_args {
+			return ast.ComptimeCall{
+				scope: 0
+				method_name: method_name
+				args_var: type_index
+				pos: start_pos.extend(p.prev_tok.pos())
+			}
+		}
+		return ast.ComptimeCall{
+			scope: 0
+			method_name: method_name
+			pos: start_pos.extend(p.prev_tok.pos())
+		}
 	}
 	mut literal_string_param := if is_html { '' } else { p.tok.lit }
 	if p.tok.kind == .name {

vlib/v/tests/defer_use_returned_value_test.v

@@ -0,0 +1,40 @@
+struct Test {
+mut:
+	a int
+	b string
+}
+
+fn (mut test Test) with_single_return() int {
+	defer {
+		test.a = $res()
+	}
+	return 41
+}
+
+fn (mut test Test) with_multi_return() (int, string) {
+	defer {
+		test.a = $res(0)
+		test.b = $res(1)
+	}
+	return 41, 'foo'
+}
+
+fn test_with_single_return() {
+	mut test := Test{
+		a: 0
+	}
+	assert test.with_single_return() == 41
+	assert test.a == 41
+}
+
+fn test_with_multi_return() {
+	mut test := Test{
+		a: 0
+		b: ''
+	}
+	a, b := test.with_multi_return()
+	assert a == 41
+	assert b == 'foo'
+	assert test.a == a
+	assert test.b == b
+}