checker: allow *x = y only inside unsafe blocks

vlib/rand/rand.v

@@ -18,6 +18,8 @@ pub fn next(max int) int {
 // writes a result value to the seed argument.
 pub fn rand_r(seed &int) int {
 	ns := *seed * 1103515245 + 12345
-	(*seed) = ns
+	unsafe {
+		(*seed) = ns
+	}
 	return ns & 0x7fffffff
 }

vlib/v/checker/checker.v

@@ -38,6 +38,7 @@ pub mut:
 	scope_returns  bool
 	mod            string // current module name
 	is_builtin_mod bool // are we in `builtin`?
+	inside_unsafe  bool
 }
 
 pub fn new_checker(table &table.Table, pref &pref.Preferences) Checker {
@@ -571,6 +572,14 @@ fn (mut c Checker) assign_expr(mut assign_expr ast.AssignExpr) {
 	}
 	// Make sure the variable is mutable
 	c.fail_if_immutable(assign_expr.left)
+	// Do now allow `*x = y` outside `unsafe`
+	if assign_expr.left is ast.PrefixExpr {
+		p := assign_expr.left as ast.PrefixExpr
+		if p.op == .mul && !c.inside_unsafe {
+			c.error('modifying variables via deferencing can only be done in `unsafe` blocks',
+				assign_expr.pos)
+		}
+	}
 	// Single side check
 	match assign_expr.op {
 		.assign {} // No need to do single side check for =. But here put it first for speed.
@@ -1550,7 +1559,9 @@ fn (mut c Checker) stmt(node ast.Stmt) {
 			c.type_decl(it)
 		}
 		ast.UnsafeStmt {
+			c.inside_unsafe = true
 			c.stmts(it.stmts)
+			c.inside_unsafe = false
 		}
 		else {
 			// println('checker.stmt(): unhandled node')

vlib/v/parser/parser.v

@@ -44,6 +44,7 @@ mut:
 	returns           bool
 	inside_match      bool // to separate `match A { }` from `Struct{}`
 	inside_match_case bool // to separate `match_expr { }` from `Struct{}`
+	inside_unsafe     bool
 	is_stmt_ident     bool // true while the beginning of a statement is an ident/selector
 	expecting_type    bool // `is Type`, expecting type
 	errors            []errors.Error
@@ -84,8 +85,8 @@ pub fn parse_file(path string, b_table &table.Table, comments_mode scanner.Comme
 			start_pos: 0
 			parent: 0
 		}
-		errors: []errors.Error{},
-		warnings: []errors.Warning{},
+		errors: []errors.Error{}
+		warnings: []errors.Warning{}
 		global_scope: global_scope
 	}
 	// comments_mode: comments_mode
@@ -128,8 +129,8 @@ pub fn parse_file(path string, b_table &table.Table, comments_mode scanner.Comme
 		imports: p.ast_imports
 		stmts: stmts
 		scope: p.scope
-		global_scope: p.global_scope,
-		errors: p.errors,
+		global_scope: p.global_scope
+		errors: p.errors
 		warnings: p.warnings
 	}
 }
@@ -326,7 +327,8 @@ pub fn (mut p Parser) top_stmt() ast.Stmt {
 			return p.interface_decl()
 		}
 		.key_import {
-			p.error_with_pos('`import x` can only be declared at the beginning of the file', p.tok.position())
+			p.error_with_pos('`import x` can only be declared at the beginning of the file',
+				p.tok.position())
 			return p.import_stmt()
 		}
 		.key_global {
@@ -433,7 +435,9 @@ pub fn (mut p Parser) stmt() ast.Stmt {
 		}
 		.key_unsafe {
 			p.next()
+			p.inside_unsafe = true
 			stmts := p.parse_block()
+			p.inside_unsafe = false
 			return ast.UnsafeStmt{
 				stmts: stmts
 			}
@@ -580,9 +584,9 @@ pub fn (mut p Parser) error_with_pos(s string, pos token.Position) {
 		exit(1)
 	} else {
 		p.errors << errors.Error{
-			file_path: p.file_name,
-			pos: pos,
-			reporter: .parser,
+			file_path: p.file_name
+			pos: pos
+			reporter: .parser
 			message: s
 		}
 	}
@@ -594,9 +598,9 @@ pub fn (mut p Parser) warn_with_pos(s string, pos token.Position) {
 		eprintln(ferror)
 	} else {
 		p.warnings << errors.Warning{
-			file_path: p.file_name,
-			pos: pos,
-			reporter: .parser,
+			file_path: p.file_name
+			pos: pos
+			reporter: .parser
 			message: s
 		}
 	}

vlib/v/parser/pratt.v

@@ -212,6 +212,9 @@ fn (mut p Parser) prefix_expr() ast.PrefixExpr {
 	if op == .amp {
 		p.is_amp = true
 	}
+	// if op == .mul && !p.inside_unsafe {
+	// p.warn('unsafe')
+	// }
 	p.next()
 	right := p.expr(token.Precedence.prefix)
 	p.is_amp = false