checker: fix cgen error on sliced references (#13736)

vlib/strings/builder.c.v

@@ -106,7 +106,7 @@ pub fn (mut b Builder) go_back(n int) {
 // cut_last cuts the last `n` bytes from the buffer and returns them
 pub fn (mut b Builder) cut_last(n int) string {
 	cut_pos := b.len - n
-	x := unsafe { (&[]byte(b))[cut_pos..] }
+	x := unsafe { (*&[]byte(b))[cut_pos..] }
 	res := x.bytestr()
 	b.trim(cut_pos)
 	return res
@@ -147,7 +147,7 @@ pub fn (b &Builder) last_n(n int) string {
 	if n > b.len {
 		return ''
 	}
-	x := unsafe { (&[]byte(b))[b.len - n..] }
+	x := unsafe { (*&[]byte(b))[b.len - n..] }
 	return x.bytestr()
 }
 
@@ -157,7 +157,7 @@ pub fn (b &Builder) after(n int) string {
 	if n >= b.len {
 		return ''
 	}
-	x := unsafe { (&[]byte(b))[n..] }
+	x := unsafe { (*&[]byte(b))[n..] }
 	return x.bytestr()
 }
 

vlib/v/checker/checker.v

@@ -3770,8 +3770,8 @@ pub fn (mut c Checker) index_expr(mut node ast.IndexExpr) ast.Type {
 			'(note, that variables may be mutable but string values are always immutable, like in Go and Java)',
 			node.pos)
 	}
-	if !c.inside_unsafe && ((typ.is_ptr() && !typ.has_flag(.shared_f)
+	if (typ.is_ptr() && !typ.has_flag(.shared_f) && !node.left.is_auto_deref_var())
-		&& !node.left.is_auto_deref_var()) || typ.is_pointer()) {
+		|| typ.is_pointer() {
 		mut is_ok := false
 		if mut node.left is ast.Ident {
 			if mut node.left.obj is ast.Var {
@@ -3779,7 +3779,10 @@ pub fn (mut c Checker) index_expr(mut node ast.IndexExpr) ast.Type {
 				is_ok = node.left.obj.is_mut && node.left.obj.is_arg && !typ.deref().is_ptr()
 			}
 		}
-		if !is_ok && !c.pref.translated && !c.file.is_translated {
+		if !is_ok && node.index is ast.RangeExpr {
+			s := c.table.type_to_str(typ)
+			c.error('type `$s` does not support slicing', node.pos)
+		} else if !c.inside_unsafe && !is_ok && !c.pref.translated && !c.file.is_translated {
 			c.warn('pointer indexing is only allowed in `unsafe` blocks', node.pos)
 		}
 	}

vlib/v/checker/tests/ptr_slice.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/ptr_slice.vv:9:17: error: type `&Foo` does not support slicing
+    7 | 
+    8 | fn main() {
+    9 |     fs := jeje()[1..]
+      |                 ~~~~~
+   10 |     println(fs)
+   11 |     vs := byteptr(0)[..3]
+vlib/v/checker/tests/ptr_slice.vv:11:21: error: type `byteptr` does not support slicing
+    9 |     fs := jeje()[1..]
+   10 |     println(fs)
+   11 |     vs := byteptr(0)[..3]
+      |                     ~~~~~
+   12 |     println(vs)
+   13 | }

vlib/v/checker/tests/ptr_slice.vv

@@ -0,0 +1,13 @@
+struct Foo {
+}
+
+fn jeje() &Foo {
+    return &Foo{}
+}
+
+fn main() {
+    fs := jeje()[1..]
+    println(fs)
+    vs := byteptr(0)[..3]
+    println(vs)
+}