clippy_lints/src/transmuting_null.rs

   1 use clippy_utils::consts::{constant_context, Constant};
   2 use clippy_utils::diagnostics::span_lint;
   3 use clippy_utils::is_expr_diagnostic_item;
   4 use if_chain::if_chain;
   5 use rustc_ast::LitKind;
   6 use rustc_hir::{Expr, ExprKind};
   7 use rustc_lint::{LateContext, LateLintPass, LintContext};
   8 use rustc_middle::lint::in_external_macro;
   9 use rustc_session::{declare_lint_pass, declare_tool_lint};
  10 use rustc_span::symbol::sym;
  11
  12 declare_clippy_lint! {
  13     /// **What it does:** Checks for transmute calls which would receive a null pointer.
  14     ///
  15     /// **Why is this bad?** Transmuting a null pointer is undefined behavior.
  16     ///
  17     /// **Known problems:** Not all cases can be detected at the moment of this writing.
  18     /// For example, variables which hold a null pointer and are then fed to a `transmute`
  19     /// call, aren't detectable yet.
  20     ///
  21     /// **Example:**
  22     /// ```rust
  23     /// let null_ref: &u64 = unsafe { std::mem::transmute(0 as *const u64) };
  24     /// ```
  25     pub TRANSMUTING_NULL,
  26     correctness,
  27     "transmutes from a null pointer to a reference, which is undefined behavior"
  28 }
  29
  30 declare_lint_pass!(TransmutingNull => [TRANSMUTING_NULL]);
  31
  32 const LINT_MSG: &str = "transmuting a known null pointer into a reference";
  33
  34 impl<'tcx> LateLintPass<'tcx> for TransmutingNull {
  35     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  36         if in_external_macro(cx.sess(), expr.span) {
  37             return;
  38         }
  39
  40         if_chain! {
  41             if let ExprKind::Call(func, [arg]) = expr.kind;
  42             if is_expr_diagnostic_item(cx, func, sym::transmute);
  43
  44             then {
  45                 // Catching transmute over constants that resolve to `null`.
  46                 let mut const_eval_context = constant_context(cx, cx.typeck_results());
  47                 if_chain! {
  48                     if let ExprKind::Path(ref _qpath) = arg.kind;
  49                     let x = const_eval_context.expr(arg);
  50                     if let Some(Constant::RawPtr(0)) = x;
  51                     then {
  52                         span_lint(cx, TRANSMUTING_NULL, expr.span, LINT_MSG)
  53                     }
  54                 }
  55
  56                 // Catching:
  57                 // `std::mem::transmute(0 as *const i32)`
  58                 if_chain! {
  59                     if let ExprKind::Cast(inner_expr, _cast_ty) = arg.kind;
  60                     if let ExprKind::Lit(ref lit) = inner_expr.kind;
  61                     if let LitKind::Int(0, _) = lit.node;
  62                     then {
  63                         span_lint(cx, TRANSMUTING_NULL, expr.span, LINT_MSG)
  64                     }
  65                 }
  66
  67                 // Catching:
  68                 // `std::mem::transmute(std::ptr::null::<i32>())`
  69                 if_chain! {
  70                     if let ExprKind::Call(func1, []) = arg.kind;
  71                     if is_expr_diagnostic_item(cx, func1, sym::ptr_null);
  72                     then {
  73                         span_lint(cx, TRANSMUTING_NULL, expr.span, LINT_MSG)
  74                     }
  75                 }
  76
  77                 // FIXME:
  78                 // Also catch transmutations of variables which are known nulls.
  79                 // To do this, MIR const propagation seems to be the better tool.
  80                 // Whenever MIR const prop routines are more developed, this will
  81                 // become available. As of this writing (25/03/19) it is not yet.
  82             }
  83         }
  84     }
  85 }