src/tools/clippy/clippy_lints/src/transmuting_null.rs

   1 use crate::consts::{constant_context, Constant};
   2 use clippy_utils::diagnostics::span_lint;
   3 use clippy_utils::{match_qpath, paths};
   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
  11 declare_clippy_lint! {
  12     /// **What it does:** Checks for transmute calls which would receive a null pointer.
  13     ///
  14     /// **Why is this bad?** Transmuting a null pointer is undefined behavior.
  15     ///
  16     /// **Known problems:** Not all cases can be detected at the moment of this writing.
  17     /// For example, variables which hold a null pointer and are then fed to a `transmute`
  18     /// call, aren't detectable yet.
  19     ///
  20     /// **Example:**
  21     /// ```rust
  22     /// let null_ref: &u64 = unsafe { std::mem::transmute(0 as *const u64) };
  23     /// ```
  24     pub TRANSMUTING_NULL,
  25     correctness,
  26     "transmutes from a null pointer to a reference, which is undefined behavior"
  27 }
  28
  29 declare_lint_pass!(TransmutingNull => [TRANSMUTING_NULL]);
  30
  31 const LINT_MSG: &str = "transmuting a known null pointer into a reference";
  32
  33 impl<'tcx> LateLintPass<'tcx> for TransmutingNull {
  34     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  35         if in_external_macro(cx.sess(), expr.span) {
  36             return;
  37         }
  38
  39         if_chain! {
  40             if let ExprKind::Call(ref func, ref args) = expr.kind;
  41             if let ExprKind::Path(ref path) = func.kind;
  42             if match_qpath(path, &paths::STD_MEM_TRANSMUTE);
  43             if args.len() == 1;
  44
  45             then {
  46
  47                 // Catching transmute over constants that resolve to `null`.
  48                 let mut const_eval_context = constant_context(cx, cx.typeck_results());
  49                 if_chain! {
  50                     if let ExprKind::Path(ref _qpath) = args[0].kind;
  51                     let x = const_eval_context.expr(&args[0]);
  52                     if let Some(Constant::RawPtr(0)) = x;
  53                     then {
  54                         span_lint(cx, TRANSMUTING_NULL, expr.span, LINT_MSG)
  55                     }
  56                 }
  57
  58                 // Catching:
  59                 // `std::mem::transmute(0 as *const i32)`
  60                 if_chain! {
  61                     if let ExprKind::Cast(ref inner_expr, ref _cast_ty) = args[0].kind;
  62                     if let ExprKind::Lit(ref lit) = inner_expr.kind;
  63                     if let LitKind::Int(0, _) = lit.node;
  64                     then {
  65                         span_lint(cx, TRANSMUTING_NULL, expr.span, LINT_MSG)
  66                     }
  67                 }
  68
  69                 // Catching:
  70                 // `std::mem::transmute(std::ptr::null::<i32>())`
  71                 if_chain! {
  72                     if let ExprKind::Call(ref func1, ref args1) = args[0].kind;
  73                     if let ExprKind::Path(ref path1) = func1.kind;
  74                     if match_qpath(path1, &paths::STD_PTR_NULL);
  75                     if args1.is_empty();
  76                     then {
  77                         span_lint(cx, TRANSMUTING_NULL, expr.span, LINT_MSG)
  78                     }
  79                 }
  80
  81                 // FIXME:
  82                 // Also catch transmutations of variables which are known nulls.
  83                 // To do this, MIR const propagation seems to be the better tool.
  84                 // Whenever MIR const prop routines are more developed, this will
  85                 // become available. As of this writing (25/03/19) it is not yet.
  86             }
  87         }
  88     }
  89 }