checker: add more `string(x)` checks, with more detailed replacement suggestions

vlib/v/checker/checker.v

@@ -5690,25 +5690,6 @@ pub fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 		c.error('expression does not return a value so it cannot be cast', node.expr.position())
 	}
 	//
-	if to_type == ast.string_type {
-		if from_type in [ast.byte_type, ast.bool_type] {
-			c.error('cannot cast type `$from_type_sym.name` to string, use `${node.expr.str()}.str()` instead.',
-				node.pos)
-		}
-		if from_type.is_real_pointer() {
-			c.error('cannot cast pointer type `$from_type_sym.name` to string, use `&byte($node.expr.str()).vstring()` instead.',
-				node.pos)
-		}
-		if from_type.is_number() {
-			c.error('cannot cast number to string, use `${node.expr.str()}.str()` instead.',
-				node.pos)
-		}
-		if from_type_sym.kind == .alias && from_type_sym_final.name != 'string' {
-			c.error('cannot cast type `$from_type_sym.name` to string, use `x.str()` instead',
-				node.pos)
-		}
-	}
-	//
 	if to_type_sym.kind == .sum_type {
 		if from_type in [ast.int_literal_type, ast.float_literal_type] {
 			xx := if from_type == ast.int_literal_type { ast.int_type } else { ast.f64_type }
@@ -5723,15 +5704,6 @@ pub fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 			c.error('cannot convert type `$from_type_sym.name` to `$to_type_sym.name` (alias to `$to_type_sym_final.name`)',
 				node.pos)
 		}
-	} else if to_type != ast.string_type && from_type == ast.string_type
-		&& (!(to_type_sym.kind == .alias && to_type_sym_final.name == 'string')) {
-		mut error_msg := 'cannot cast a string to a type `$to_type_sym_final.name`, that is not an alias of string'
-		if mut node.expr is ast.StringLiteral {
-			if node.expr.val.len == 1 {
-				error_msg += ", for denoting characters use `$node.expr.val` instead of '$node.expr.val'"
-			}
-		}
-		c.error(error_msg, node.pos)
 	} else if to_type_sym.kind == .byte && from_type != ast.voidptr_type
 		&& from_type_sym.kind != .enum_ && !from_type.is_int() && !from_type.is_float()
 		&& from_type != ast.bool_type && !from_type.is_ptr() && from_type_sym.kind == .alias
@@ -5770,7 +5742,9 @@ pub fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 		if (to_type.is_ptr() || to_type_sym.kind !in [.sum_type, .interface_]) && !c.is_builtin_mod {
 			from_type_name := c.table.type_to_str(from_type)
 			type_name := c.table.type_to_str(to_type)
-			c.error('cannot cast struct `$from_type_name` to `$type_name`', node.pos)
+			snexpr := node.expr.str()
+			c.error('cannot cast struct `$from_type_name` to `$type_name`, use `${snexpr}.str()` instead.',
+				node.pos)
 		}
 	} else if from_type.has_flag(.optional) || from_type.has_flag(.variadic) {
 		// variadic case can happen when arrays are converted into variadic
@@ -5784,6 +5758,53 @@ pub fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 	} else if from_type_sym.kind == .array_fixed && !from_type.is_ptr() {
 		c.warn('cannot cast a fixed array (use e.g. `&arr[0]` instead)', node.pos)
 	}
+
+	if to_type == ast.string_type {
+		if from_type in [ast.byte_type, ast.bool_type] {
+			snexpr := node.expr.str()
+			c.error('cannot cast type `$from_type_sym.name` to string, use `${snexpr}.str()` instead.',
+				node.pos)
+		} else if from_type.is_real_pointer() {
+			snexpr := node.expr.str()
+			c.error('cannot cast pointer type `$from_type_sym.name` to string, use `&byte($snexpr).vstring()` or `cstring_to_vstring($snexpr)` instead.',
+				node.pos)
+		} else if from_type.is_number() {
+			snexpr := node.expr.str()
+			c.error('cannot cast number to string, use `${snexpr}.str()` instead.', node.pos)
+		} else if from_type_sym.kind == .alias && from_type_sym_final.name != 'string' {
+			c.error('cannot cast type `$from_type_sym.name` to string, use `x.str()` instead.',
+				node.pos)
+		} else if from_type_sym_final.kind == .array {
+			snexpr := node.expr.str()
+			if from_type_sym_final.name == '[]byte' {
+				c.error('cannot cast []byte to string, use `${snexpr}.bytestr()` or `${snexpr}.str()` instead.',
+					node.pos)
+			} else {
+				first_elem_idx := '[0]'
+				c.error('cannot cast array to string, use `$snexpr${first_elem_idx}.str()` instead.',
+					node.pos)
+			}
+		} else if from_type_sym_final.kind == .enum_ {
+			snexpr := node.expr.str()
+			c.error('cannot cast enum to string, use ${snexpr}.str() instead.', node.pos)
+		} else if from_type_sym_final.kind == .map {
+			c.error('cannot cast map to string.', node.pos)
+		} else if from_type_sym_final.kind == .sum_type {
+			snexpr := node.expr.str()
+			c.error('cannot cast sumtype `$from_type_sym.name` to string, use `${snexpr}.str()` instead.',
+				node.pos)
+		} else if to_type != ast.string_type && from_type == ast.string_type
+			&& (!(to_type_sym.kind == .alias && to_type_sym_final.name == 'string')) {
+			mut error_msg := 'cannot cast a string to a type `$to_type_sym_final.name`, that is not an alias of string'
+			if mut node.expr is ast.StringLiteral {
+				if node.expr.val.len == 1 {
+					error_msg += ", for denoting characters use `$node.expr.val` instead of '$node.expr.val'"
+				}
+			}
+			c.error(error_msg, node.pos)
+		}
+	}
+
 	if node.has_arg {
 		c.expr(node.arg)
 	}

vlib/v/checker/tests/cast_string_err.out

@@ -1,6 +1,42 @@
-vlib/v/checker/tests/cast_string_err.vv:2:7: error: cannot cast number to string, use `1.str()` instead.
+vlib/v/checker/tests/cast_string_err.vv:14:9: error: cannot cast enum to string, use ev.str() instead.
-    1 | fn main() {
+   12 | fn main() {
-    2 |     a := string(1)
+   13 |     ev := MyEnum.val1
-      |          ~~~~~~~~~
+   14 |     sev := string(ev)
-    3 |     println(a)
+      |            ~~~~~~~~~~
-    4 | }
+   15 |     println(sev)
+   16 |     //
+vlib/v/checker/tests/cast_string_err.vv:18:9: error: cannot cast sumtype `Sumtype` to string, use `st.str()` instead.
+   16 |     //
+   17 |     st := Sumtype(int(456))
+   18 |     sst := string(st)
+      |            ~~~~~~~~~~
+   19 |     println(sst)
+   20 |     //
+vlib/v/checker/tests/cast_string_err.vv:22:10: error: cannot cast struct `Abc` to `string`, use `abc.str()` instead.
+   20 |     //
+   21 |     abc := Abc{}
+   22 |     sabc := string(abc)
+      |             ~~~~~~~~~~~
+   23 |     println(sabc)
+   24 |     //
+vlib/v/checker/tests/cast_string_err.vv:26:8: error: cannot cast map to string.
+   24 |     //
+   25 |     mm := map[string]int{}
+   26 |     sm := string(mm)
+      |           ~~~~~~~~~~
+   27 |     println(sm)
+   28 |     //
+vlib/v/checker/tests/cast_string_err.vv:30:8: error: cannot cast []byte to string, use `arr.bytestr()` or `arr.str()` instead.
+   28 |     //
+   29 |     arr := []byte{}
+   30 |     sa := string(arr)
+      |           ~~~~~~~~~~~
+   31 |     println(sa)
+   32 |     //
+vlib/v/checker/tests/cast_string_err.vv:34:8: error: cannot cast number to string, use `ii.str()` instead.
+   32 |     //
+   33 |     ii := 1
+   34 |     si := string(ii)
+      |           ~~~~~~~~~~
+   35 |     println(si)
+   36 | }

vlib/v/checker/tests/cast_string_err.vv

@@ -1,4 +1,36 @@
-fn main() {
+struct Abc {
-	a := string(1)
+	x int
-	println(a)
+}
+
+type Sumtype = int | u64
+
+enum MyEnum {
+	val1
+	val2
+}
+
+fn main() {
+	ev := MyEnum.val1
+	sev := string(ev)
+	println(sev)
+	//
+	st := Sumtype(int(456))
+	sst := string(st)
+	println(sst)
+	//
+	abc := Abc{}
+	sabc := string(abc)
+	println(sabc)
+	//
+	mm := map[string]int{}
+	sm := string(mm)
+	println(sm)
+	//
+	arr := []byte{}
+	sa := string(arr)
+	println(sa)
+	//
+	ii := 1
+	si := string(ii)
+	println(si)
 }

vlib/v/checker/tests/struct_type_cast_err.out

@@ -1,60 +1,60 @@
-vlib/v/checker/tests/struct_type_cast_err.vv:5:10: error: cannot cast struct `Foo` to `string`
+vlib/v/checker/tests/struct_type_cast_err.vv:5:10: error: cannot cast struct `Foo` to `string`, use `foo.str()` instead.
     3 | fn main() {
     4 |     foo := Foo{}
     5 |     _ := string(foo)
       |          ~~~~~~~~~~~
     6 |     _ := int(foo)
     7 |     _ := u64(foo)
-vlib/v/checker/tests/struct_type_cast_err.vv:6:10: error: cannot cast struct `Foo` to `int`
+vlib/v/checker/tests/struct_type_cast_err.vv:6:10: error: cannot cast struct `Foo` to `int`, use `foo.str()` instead.
     4 |     foo := Foo{}
     5 |     _ := string(foo)
     6 |     _ := int(foo)
       |          ~~~~~~~~
     7 |     _ := u64(foo)
     8 |     _ := u32(foo)
-vlib/v/checker/tests/struct_type_cast_err.vv:7:10: error: cannot cast struct `Foo` to `u64`
+vlib/v/checker/tests/struct_type_cast_err.vv:7:10: error: cannot cast struct `Foo` to `u64`, use `foo.str()` instead.
     5 |     _ := string(foo)
     6 |     _ := int(foo)
     7 |     _ := u64(foo)
       |          ~~~~~~~~
     8 |     _ := u32(foo)
     9 |     _ := rune(foo)
-vlib/v/checker/tests/struct_type_cast_err.vv:8:10: error: cannot cast struct `Foo` to `u32`
+vlib/v/checker/tests/struct_type_cast_err.vv:8:10: error: cannot cast struct `Foo` to `u32`, use `foo.str()` instead.
     6 |     _ := int(foo)
     7 |     _ := u64(foo)
     8 |     _ := u32(foo)
       |          ~~~~~~~~
     9 |     _ := rune(foo)
    10 |     _ := byte(foo)
-vlib/v/checker/tests/struct_type_cast_err.vv:9:10: error: cannot cast struct `Foo` to `rune`
+vlib/v/checker/tests/struct_type_cast_err.vv:9:10: error: cannot cast struct `Foo` to `rune`, use `foo.str()` instead.
     7 |     _ := u64(foo)
     8 |     _ := u32(foo)
     9 |     _ := rune(foo)
       |          ~~~~~~~~~
    10 |     _ := byte(foo)
    11 |     _ := i8(foo)
-vlib/v/checker/tests/struct_type_cast_err.vv:10:10: error: cannot cast struct `Foo` to `byte`
+vlib/v/checker/tests/struct_type_cast_err.vv:10:10: error: cannot cast struct `Foo` to `byte`, use `foo.str()` instead.
     8 |     _ := u32(foo)
     9 |     _ := rune(foo)
    10 |     _ := byte(foo)
       |          ~~~~~~~~~
    11 |     _ := i8(foo)
    12 |     _ := i64(foo)
-vlib/v/checker/tests/struct_type_cast_err.vv:11:10: error: cannot cast struct `Foo` to `i8`
+vlib/v/checker/tests/struct_type_cast_err.vv:11:10: error: cannot cast struct `Foo` to `i8`, use `foo.str()` instead.
     9 |     _ := rune(foo)
    10 |     _ := byte(foo)
    11 |     _ := i8(foo)
       |          ~~~~~~~
    12 |     _ := i64(foo)
    13 |     _ := int(foo)
-vlib/v/checker/tests/struct_type_cast_err.vv:12:10: error: cannot cast struct `Foo` to `i64`
+vlib/v/checker/tests/struct_type_cast_err.vv:12:10: error: cannot cast struct `Foo` to `i64`, use `foo.str()` instead.
    10 |     _ := byte(foo)
    11 |     _ := i8(foo)
    12 |     _ := i64(foo)
       |          ~~~~~~~~
    13 |     _ := int(foo)
    14 |     _ = &I1(foo)
-vlib/v/checker/tests/struct_type_cast_err.vv:13:10: error: cannot cast struct `Foo` to `int`
+vlib/v/checker/tests/struct_type_cast_err.vv:13:10: error: cannot cast struct `Foo` to `int`, use `foo.str()` instead.
    11 |     _ := i8(foo)
    12 |     _ := i64(foo)
    13 |     _ := int(foo)