From cf3dd7a51ff98a01ff14637b12c5632cfb147172 Mon Sep 17 00:00:00 2001
From: Delyan Angelov <delian66@gmail.com>
Date: Wed, 12 May 2021 18:03:01 +0300
Subject: [PATCH] v.checker: fix `unused variable` warning on `*p = val`

---
 vlib/v/checker/checker.v   | 43 +++++++++++++++++++++++++-------------
 vlib/v/tests/unsafe_test.v | 29 +++++++++++++++++++++++++
 2 files changed, 58 insertions(+), 14 deletions(-)

diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 35c07cb793..b9984dfb89 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -121,21 +121,21 @@ pub fn (mut c Checker) check(ast_file &ast.File) {
 }
 
 pub fn (mut c Checker) check_scope_vars(sc &ast.Scope) {
-	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 !c.pref.is_repl && !c.file.is_test {
+		for _, obj in sc.objects {
+			match obj {
+				ast.Var {
+					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',
+						// 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',
-					// obj.pos)
-				}
+				else {}
 			}
-			else {}
 		}
 	}
 	for child in sc.children {
@@ -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 {
-					c.error('modifying variables via dereferencing can only be done in `unsafe` blocks',
-						assign_stmt.pos)
+				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)
diff --git a/vlib/v/tests/unsafe_test.v b/vlib/v/tests/unsafe_test.v
index 3e7f705f2b..44ce679433 100644
--- a/vlib/v/tests/unsafe_test.v
+++ b/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')
+}