rust/clippy_lints/src/transmute.rs

use rustc::lint::*;
use rustc::ty::TypeVariants::{TyRawPtr, TyRef};
use rustc::ty;
use rustc::hir::*;
use utils::{match_def_path, paths, snippet_opt, span_lint, span_lint_and_then};

/// **What it does:** This lint checks for transmutes to the original type of the object.
///
/// **Why is this bad?** Readability. The code tricks people into thinking that the original value was of some other type.
///
/// **Known problems:** None.
///
/// **Example:** `core::intrinsics::transmute(t)` where the result type is the same as `t`'s.
declare_lint! {
    pub USELESS_TRANSMUTE,
    Warn,
    "transmutes that have the same to and from types"
}

/// **What it does:*** This lint checks for transmutes between a type `T` and `*T`.
///
/// **Why is this bad?** It's easy to mistakenly transmute between a type and a pointer to that type.
///
/// **Known problems:** None.
///
/// **Example:** `core::intrinsics::transmute(t)` where the result type is the same as `*t` or `&t`'s.
declare_lint! {
    pub CROSSPOINTER_TRANSMUTE,
    Warn,
    "transmutes that have to or from types that are a pointer to the other"
}

/// **What it does:*** This lint checks for transmutes from a pointer to a reference.
///
/// **Why is this bad?** This can always be rewritten with `&` and `*`.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// let _: &T = std::mem::transmute(p); // where p: *const T
/// // can be written:
/// let _: &T = &*p;
/// ```
declare_lint! {
    pub TRANSMUTE_PTR_TO_REF,
    Warn,
    "transmutes from a pointer to a reference type"
}

pub struct Transmute;

impl LintPass for Transmute {
    fn get_lints(&self) -> LintArray {
        lint_array![CROSSPOINTER_TRANSMUTE, TRANSMUTE_PTR_TO_REF, USELESS_TRANSMUTE]
    }
}

impl LateLintPass for Transmute {
    fn check_expr(&mut self, cx: &LateContext, e: &Expr) {
        if let ExprCall(ref path_expr, ref args) = e.node {
            if let ExprPath(None, _) = path_expr.node {
                let def_id = cx.tcx.def_map.borrow()[&path_expr.id].def_id();

                if match_def_path(cx, def_id, &paths::TRANSMUTE) {
                    let from_ty = cx.tcx.expr_ty(&args[0]);
                    let to_ty = cx.tcx.expr_ty(e);

                    if from_ty == to_ty {
                        span_lint(cx,
                                  USELESS_TRANSMUTE,
                                  e.span,
                                  &format!("transmute from a type (`{}`) to itself", from_ty));
                    } else if is_ptr_to(to_ty, from_ty) {
                        span_lint(cx,
                                  CROSSPOINTER_TRANSMUTE,
                                  e.span,
                                  &format!("transmute from a type (`{}`) to a pointer to that type (`{}`)",
                                           from_ty,
                                           to_ty));
                    } else if is_ptr_to(from_ty, to_ty) {
                        span_lint(cx,
                                  CROSSPOINTER_TRANSMUTE,
                                  e.span,
                                  &format!("transmute from a type (`{}`) to the type that it points to (`{}`)",
                                           from_ty,
                                           to_ty));
                    } else {
                        check_ptr_to_ref(cx, from_ty, to_ty, e, &args[0]);
                    }
                }
            }
        }
    }
}

fn is_ptr_to(from: ty::Ty, to: ty::Ty) -> bool {
    if let TyRawPtr(from_ptr) = from.sty {
        from_ptr.ty == to
    } else {
        false
    }
}

fn check_ptr_to_ref<'tcx>(cx: &LateContext, from_ty: ty::Ty<'tcx>, to_ty: ty::Ty<'tcx>, e: &Expr, arg: &Expr) {
    if let TyRawPtr(ref from_pty) = from_ty.sty {
        if let TyRef(_, ref to_rty) = to_ty.sty {
            let mess = format!("transmute from a pointer type (`{}`) to a reference type (`{}`)",
                               from_ty,
                               to_ty);
            span_lint_and_then(cx, TRANSMUTE_PTR_TO_REF, e.span, &mess, |db| {
                if let Some(arg) = snippet_opt(cx, arg.span) {
                    let (deref, cast) = if to_rty.mutbl == Mutability::MutMutable {
                        ("&mut *", "*mut")
                    } else {
                        ("&*", "*const")
                    };


                    let sugg = if from_pty.ty == to_rty.ty {
                        format!("{}{}", deref, arg)
                    } else {
                        format!("{}({} as {} {})", deref, arg, cast, to_rty.ty)
                    };

                    db.span_suggestion(e.span, "try", sugg);
                }
            });
        }
    }
}