compiler: fix generation of default .str() methods in interpolation

vlib/compiler/comptime.v

@@ -357,6 +357,41 @@ fn (p mut Parser) comptime_method_call(typ Type) {
 	}
 }
 
+fn (p mut Parser) gen_default_str_method_if_missing(typename string) (bool, string) {
+	// NB: string_type_name can be != typename, if the base typename has str()
+	mut string_type_name := typename
+	typ := p.table.find_type(typename)
+	is_varg := typename.starts_with('varg_')
+	is_array := typename.starts_with('array_')
+	is_struct := typ.cat == .struct_
+	mut has_str_method := p.table.type_has_method(typ, 'str')
+	if !has_str_method {
+		if is_varg {
+			p.gen_varg_str(typ)
+			has_str_method = true
+		}
+		else if is_array {
+			p.gen_array_str(typ)
+			has_str_method = true
+		}
+		else if is_struct {
+			p.gen_struct_str(typ)
+			has_str_method = true
+		}
+		else {
+			btypename := p.base_type(typ.name)
+			if btypename != typ.name {
+				base_type := p.find_type(btypename)
+				if base_type.has_method('str'){
+					string_type_name = base_type.name
+					has_str_method = true
+				}
+			}
+		}
+	}
+	return has_str_method, string_type_name
+}
+
 fn (p mut Parser) gen_array_str(typ Type) {
 	if typ.has_method('str') {
 		return
@@ -379,7 +414,10 @@ fn (p mut Parser) gen_array_str(typ Type) {
 		p.gen_array_str(elm_type2)
 	}
 	else if p.typ_to_fmt(elm_type, 0) == '' && !p.table.type_has_method(elm_type2, 'str') {
-		p.error('cant print ${elm_type}[], unhandled print of ${elm_type}')
+		has_str_method, _ := p.gen_default_str_method_if_missing( elm_type )
+		if !has_str_method {
+			p.error('cant print []${elm_type}, unhandled print of ${elm_type}')
+		}
 	}
 	p.v.vgen_buf.writeln('
 pub fn (a $typ.name) str() string {
@@ -398,7 +436,7 @@ pub fn (a $typ.name) str() string {
 	p.cgen.fns << 'string ${typ.name}_str();'
 }
 
-// `Foo { bar: 3, baz: 'hi' }` => '{ bar: 3, baz: "hi" }'
+// `Foo { bar: 3, baz: 'hi' }` => interpolated to string 'Foo { bar: 3, baz: "hi" }'
 fn (p mut Parser) gen_struct_str(typ Type) {
 	p.add_method(typ.name, Fn{
 		name: 'str'
@@ -413,7 +451,8 @@ fn (p mut Parser) gen_struct_str(typ Type) {
 	})
 	mut sb := strings.new_builder(typ.fields.len * 20)
 	sb.writeln('pub fn (a $typ.name) str() string {\nreturn')
-	sb.writeln("'{")
+	short_struct_name := typ.name.all_after('__')
+	sb.writeln("'$short_struct_name {")
 	for field in typ.fields {
 		sb.writeln('\t$field.name: $' + 'a.${field.name}')
 	}

vlib/compiler/string_expression.v

@@ -114,15 +114,10 @@ fn (p mut Parser) string_expr() {
 		else {
 			f := p.typ_to_fmt(typ, 0)
 			if f == '' {
-				is_array := typ.starts_with('array_')
+				has_str_method, styp := p.gen_default_str_method_if_missing( typ )
-				typ2 := p.table.find_type(typ)
+				if has_str_method {
-				has_str_method := p.table.type_has_method(typ2, 'str')
-				if is_array || has_str_method {
-					if is_array && !has_str_method {
-						p.gen_array_str(typ2)
-					}
 					tmp_var := p.get_tmp()
-					p.cgen.insert_before('string $tmp_var = ${typ}_str(${val});')
+					p.cgen.insert_before('string $tmp_var = ${styp}_str(${val});')
 					args = args.all_before_last(val) + '${tmp_var}.len, ${tmp_var}.str'
 					format += '%.*s '
 				}
@@ -165,4 +160,3 @@ fn (p mut Parser) string_expr() {
 		p.gen('_STR($format$args)')
 	}
 }
-

vlib/compiler/tests/prod/assoc.prod.v.expected.txt

@@ -1,3 +1,3 @@
-{
+MyStruct {
 	s: 6
 }

vlib/compiler/tests/string_interpolation_array_of_structs_test.v

@@ -0,0 +1,34 @@
+// This file tests whether v can generate default string methods for both:
+// a) an array of custom structs,
+// b) also for the custom struct itself (when the .str() for it is missing).
+//
+// NB: this is very simillar to string_interpolation_struct_test.v
+// but they should NOT be merged into 1 file.  If you merge it with
+// string_interpolation_struct_test.v, which tests whether the compiler
+// can generate the default method for a struct, then the b) case of
+// this test will be done by *that* test, and so the testing will
+// be incomplete.
+
+struct Man {
+	name string
+	age int
+	interests []string
+}
+
+fn test_default_struct_array_of_structs_interpolation(){
+	people := [
+		Man{'Superman', 30, ['flying','fighting evil','being nice']},
+		Man{'Bilbo Baggins', 111, ['exploring', 'hiding']},
+	]
+	s := '$people' // the compiler should generate code for both a) and b)
+	assert s.contains('Man {')
+	assert s.contains('name: Superman')
+	assert s.contains('age: 30')
+	assert s.contains('"being nice"')
+	assert s.contains('}, Man {')
+	assert s.contains('name: Bilbo Baggins')
+	assert s.contains('age: 111')
+	assert s.contains('interests: ["exploring", "hiding"]')
+	assert s.contains('}]')
+	//println(s)
+}

vlib/compiler/tests/string_interpolation_struct_test.v

@@ -0,0 +1,21 @@
+// This file tests whether V can generate a convenience default .str() method
+// for a custom struct, when the developer has not defined one himself.
+// The .str() methods are used for string interpolation and for println() calls.
+
+struct Man {
+	name string
+	age int
+	interests []string
+}
+
+fn test_default_struct_string_interpolation(){
+	superman := Man{'Superman', 30, ['flying','fighting evil','being nice']}
+	s := '$superman'
+	assert s.contains('Man {')
+	assert s.contains('name: Superman')
+	assert s.contains('age: 30')
+	assert s.contains('interests: [')
+	assert s.contains('"being nice"')
+	assert s.contains('}')
+	//println(s)
+}

vlib/compiler/tests/string_interpolation_variadic_test.v

@@ -0,0 +1,34 @@
+// This file tests whether V can generate a convenience default .str() method
+// for var args of a custom type, when the developer has NOT defined one.
+// Although similar to string_interpolation_struct_test.v, they should not be
+// merged.
+
+struct Man {
+	name string
+	age int
+	interests []string
+}
+
+fn my_variadic_function(x ...Man) string {
+	return '$x' // this interpolation should generate .str() methods for Man
+}
+
+fn test_vargs_string_interpolation(){
+	man := Man{'Me', 38, ['programming', 'reading', 'hiking']}
+	superman := Man{'Superman', 30, ['flying','fighting evil','being nice']}
+
+	results := my_variadic_function(superman, man)
+
+	assert results.contains('Man {')
+	//
+	assert results.contains('name: Superman')
+	assert results.contains('age: 30')
+	assert results.contains('}, Man {')
+	//
+	assert results.contains('interests: ["programming"')
+	assert results.contains('name: Me')
+	//
+	assert results.contains('}]')
+	//
+	println(results)
+}