v.checker: fix `unused variable` warning on `*p = val`

vlib/v/checker/checker.v

@@ -121,14 +121,13 @@ pub fn (mut c Checker) check(ast_file &ast.File) {
 }
 
 pub fn (mut c Checker) check_scope_vars(sc &ast.Scope) {
+	if !c.pref.is_repl && !c.file.is_test {
 		for _, obj in sc.objects {
 			match obj {
 				ast.Var {
-				if !c.pref.is_repl {
-					if !obj.is_used && obj.name[0] != `_` && !c.file.is_test {
+					if !obj.is_used && obj.name[0] != `_` {
 						c.warn('unused variable: `$obj.name`', obj.pos)
 					}
-				}
 					if obj.is_mut && !obj.is_changed && !c.is_builtin_mod && obj.name != 'it' {
 						// if obj.is_mut && !obj.is_changed && !c.is_builtin {  //TODO C error bad field not checked
 						// c.warn('`$obj.name` is declared as mutable, but it was never changed',
@@ -138,6 +137,7 @@ pub fn (mut c Checker) check_scope_vars(sc &ast.Scope) {
 				else {}
 			}
 		}
+	}
 	for child in sc.children {
 		c.check_scope_vars(child)
 	}
@@ -3323,9 +3323,24 @@ pub fn (mut c Checker) assign_stmt(mut assign_stmt ast.AssignStmt) {
 			}
 			ast.PrefixExpr {
 				// Do now allow `*x = y` outside `unsafe`
-				if left.op == .mul && !c.inside_unsafe {
+				if left.op == .mul {
+					if !c.inside_unsafe {
 						c.error('modifying variables via dereferencing can only be done in `unsafe` blocks',
 							assign_stmt.pos)
+					} else {
+						// mark `p` in `*p = val` as used:
+						match mut left.right {
+							ast.Ident {
+								match mut left.right.obj {
+									ast.Var {
+										left.right.obj.is_used = true
+									}
+									else {}
+								}
+							}
+							else {}
+						}
+					}
 				}
 				if is_decl {
 					c.error('non-name on the left side of `:=`', left.pos)

vlib/v/tests/unsafe_test.v

@@ -72,3 +72,32 @@ fn test_unsafe_if_stmt() {
 	x := unsafe_if_stmt()
 	assert x == 4
 }
+
+const fixedbytes = [100]byte{}
+
+fn test_unsafe_pointers() {
+	fsize := fixedbytes.len
+	src := &fixedbytes[0]
+	//
+	b := []byte{}
+	eprintln('b.data before: $b.data')
+	eprintln('b.len before: $b.len')
+	eprintln('b.cap before: $b.cap')
+	assert b.len == 0
+	unsafe {
+		// here b will be setup to work with the mmaped region
+		mut pdata := &b.data
+		mut plen := &b.len
+		mut pcap := &b.cap
+		// note that pdata, plen, pcap are used here:
+		*pdata = src
+		*plen = int(fsize)
+		*pcap = int(fsize)
+	}
+	assert b.len == 100
+	assert b.cap == 100
+	assert b.data == src
+	eprintln('b.data after: $b.data')
+	eprintln('b.len after: $b.len')
+	eprintln('b.cap after: $b.cap')
+}