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

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)
+}