checker: test for unnecessary default struct field values

vlib/strconv/f32_str.v

@@ -22,14 +22,14 @@ module strconv
 // dec32 is a floating decimal type representing m * 10^e.
 struct Dec32 {
 mut:
-	m u32 = 0
-	e int = 0
+	m u32
+	e int
 }
 
 // support union for convert f32 to u32
 union Uf32 {
 mut:
-	f f32 = 0
+	f f32
 	u u32
 }
 

vlib/strconv/f64_str.v

@@ -21,21 +21,21 @@ module strconv
 
 struct Uint128 {
 mut:
-	lo u64 = u64(0)
-	hi u64 = u64(0)
+	lo u64
+	hi u64
 }
 
 // dec64 is a floating decimal type representing m * 10^e.
 struct Dec64 {
 mut:
-	m u64 = 0
-	e int = 0
+	m u64
+	e int
 }
 
 // support union for convert f64 to u64
 union Uf64 {
 mut:
-	f f64 = 0
+	f f64
 	u u64
 }
 

vlib/v/checker/checker.v

@@ -364,6 +364,18 @@ pub fn (mut c Checker) struct_decl(decl ast.StructDecl) {
 					'has type `$field_expr_type_sym.source_name`, but should be `$field_type_sym.source_name`',
 					field.default_expr.position())
 			}
+			// Check for unnecessary inits like ` = 0` and ` = ''`
+			if field.default_expr is ast.IntegerLiteral as x {
+				if x.val == '0' {
+					c.error('unnecessary default value of `0`: struct fields are zeroed by default',
+						field.pos)
+				}
+			} else if field.default_expr is ast.StringLiteral as x {
+				if x.val == "''" {
+					c.error("unnecessary default value of '': struct fields are zeroed by default",
+						field.pos)
+				}
+			}
 		}
 	}
 }

vlib/v/checker/tests/struct_unneeded_default.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/struct_unneeded_default.vv:2:2: error: unnecessary default value of `0`: struct fields are zeroed by default
+    1 | struct Test {
+    2 |     n int = 0
+      |     ~~~~~~
+    3 |     s string = ''
+    4 | }

vlib/v/checker/tests/struct_unneeded_default.vv

@@ -0,0 +1,7 @@
+struct Test {
+	n int = 0
+	s string = ''
+}
+
+fn main() {
+}