src/tools/clippy/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
  14     /// Checks for transmute calls which would receive a null pointer.
  15     ///
  16     /// ### Why is this bad?
  17     /// Transmuting a null pointer is undefined behavior.
  18     ///
  19     /// ### Known problems
  20     /// Not all cases can be detected at the moment of this writing.
  21     /// For example, variables which hold a null pointer and are then fed to a `transmute`
  22     /// call, aren't detectable yet.
  23     ///
  24     /// ### Example
  25     /// ```rust
  26     /// let null_ref: &u64 = unsafe { std::mem::transmute(0 as *const u64) };
  27     /// ```
  28     pub TRANSMUTING_NULL,
  29     correctness,
  30     "transmutes from a null pointer to a reference, which is undefined behavior"
  31 }
  32
  33 declare_lint_pass!(TransmutingNull => [TRANSMUTING_NULL]);
  34
  35 const LINT_MSG: &str = "transmuting a known null pointer into a reference";
  36
  37 impl<'tcx> LateLintPass<'tcx> for TransmutingNull {
  38     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  39         if in_external_macro(cx.sess(), expr.span) {
  40             return;
  41         }
  42
  43         if_chain! {
  44             if let ExprKind::Call(func, [arg]) = expr.kind;
  45             if is_expr_diagnostic_item(cx, func, sym::transmute);
  46
  47             then {
  48                 // Catching transmute over constants that resolve to `null`.
  49                 let mut const_eval_context = constant_context(cx, cx.typeck_results());
  50                 if_chain! {
  51                     if let ExprKind::Path(ref _qpath) = arg.kind;
  52                     if let Some(Constant::RawPtr(x)) = const_eval_context.expr(arg);
  53                     if x == 0;
  54                     then {
  55                         span_lint(cx, TRANSMUTING_NULL, expr.span, LINT_MSG)
  56                     }
  57                 }
  58
  59                 // Catching:
  60                 // `std::mem::transmute(0 as *const i32)`
  61                 if_chain! {
  62                     if let ExprKind::Cast(inner_expr, _cast_ty) = arg.kind;
  63                     if let ExprKind::Lit(ref lit) = inner_expr.kind;
  64                     if let LitKind::Int(0, _) = lit.node;
  65                     then {
  66                         span_lint(cx, TRANSMUTING_NULL, expr.span, LINT_MSG)
  67                     }
  68                 }
  69
  70                 // Catching:
  71                 // `std::mem::transmute(std::ptr::null::<i32>())`
  72                 if_chain! {
  73                     if let ExprKind::Call(func1, []) = arg.kind;
  74                     if is_expr_diagnostic_item(cx, func1, sym::ptr_null);
  75                     then {
  76                         span_lint(cx, TRANSMUTING_NULL, expr.span, LINT_MSG)
  77                     }
  78                 }
  79
  80                 // FIXME:
  81                 // Also catch transmutations of variables which are known nulls.
  82                 // To do this, MIR const propagation seems to be the better tool.
  83                 // Whenever MIR const prop routines are more developed, this will
  84                 // become available. As of this writing (25/03/19) it is not yet.
  85             }
  86         }
  87     }
  88 }