src/tools/clippy/clippy_lints/src/mem_replace.rs

use crate::utils::{
    in_macro, match_def_path, match_qpath, paths, snippet, snippet_with_applicability, span_lint_and_help,
    span_lint_and_sugg, span_lint_and_then,
};
use if_chain::if_chain;
use rustc::lint::in_external_macro;
use rustc_errors::Applicability;
use rustc_hir::{BorrowKind, Expr, ExprKind, Mutability, QPath};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::source_map::Span;

declare_clippy_lint! {
    /// **What it does:** Checks for `mem::replace()` on an `Option` with
    /// `None`.
    ///
    /// **Why is this bad?** `Option` already has the method `take()` for
    /// taking its current value (Some(..) or None) and replacing it with
    /// `None`.
    ///
    /// **Known problems:** None.
    ///
    /// **Example:**
    /// ```rust
    /// use std::mem;
    ///
    /// let mut an_option = Some(0);
    /// let replaced = mem::replace(&mut an_option, None);
    /// ```
    /// Is better expressed with:
    /// ```rust
    /// let mut an_option = Some(0);
    /// let taken = an_option.take();
    /// ```
    pub MEM_REPLACE_OPTION_WITH_NONE,
    style,
    "replacing an `Option` with `None` instead of `take()`"
}

declare_clippy_lint! {
    /// **What it does:** Checks for `mem::replace(&mut _, mem::uninitialized())`
    /// and `mem::replace(&mut _, mem::zeroed())`.
    ///
    /// **Why is this bad?** This will lead to undefined behavior even if the
    /// value is overwritten later, because the uninitialized value may be
    /// observed in the case of a panic.
    ///
    /// **Known problems:** None.
    ///
    /// **Example:**
    ///
    /// ```
    /// use std::mem;
    ///# fn may_panic(v: Vec<i32>) -> Vec<i32> { v }
    ///
    /// #[allow(deprecated, invalid_value)]
    /// fn myfunc (v: &mut Vec<i32>) {
    ///     let taken_v = unsafe { mem::replace(v, mem::uninitialized()) };
    ///     let new_v = may_panic(taken_v); // undefined behavior on panic
    ///     mem::forget(mem::replace(v, new_v));
    /// }
    /// ```
    ///
    /// The [take_mut](https://docs.rs/take_mut) crate offers a sound solution,
    /// at the cost of either lazily creating a replacement value or aborting
    /// on panic, to ensure that the uninitialized value cannot be observed.
    pub MEM_REPLACE_WITH_UNINIT,
    correctness,
    "`mem::replace(&mut _, mem::uninitialized())` or `mem::replace(&mut _, mem::zeroed())`"
}

declare_clippy_lint! {
    /// **What it does:** Checks for `std::mem::replace` on a value of type
    /// `T` with `T::default()`.
    ///
    /// **Why is this bad?** `std::mem` module already has the method `take` to
    /// take the current value and replace it with the default value of that type.
    ///
    /// **Known problems:** None.
    ///
    /// **Example:**
    /// ```rust
    /// let mut text = String::from("foo");
    /// let replaced = std::mem::replace(&mut text, String::default());
    /// ```
    /// Is better expressed with:
    /// ```rust
    /// let mut text = String::from("foo");
    /// let taken = std::mem::take(&mut text);
    /// ```
    pub MEM_REPLACE_WITH_DEFAULT,
    style,
    "replacing a value of type `T` with `T::default()` instead of using `std::mem::take`"
}

declare_lint_pass!(MemReplace =>
    [MEM_REPLACE_OPTION_WITH_NONE, MEM_REPLACE_WITH_UNINIT, MEM_REPLACE_WITH_DEFAULT]);

fn check_replace_option_with_none(cx: &LateContext<'_, '_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
    if let ExprKind::Path(ref replacement_qpath) = src.kind {
        // Check that second argument is `Option::None`
        if match_qpath(replacement_qpath, &paths::OPTION_NONE) {
            // Since this is a late pass (already type-checked),
            // and we already know that the second argument is an
            // `Option`, we do not need to check the first
            // argument's type. All that's left is to get
            // replacee's path.
            let replaced_path = match dest.kind {
                ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, ref replaced) => {
                    if let ExprKind::Path(QPath::Resolved(None, ref replaced_path)) = replaced.kind {
                        replaced_path
                    } else {
                        return;
                    }
                },
                ExprKind::Path(QPath::Resolved(None, ref replaced_path)) => replaced_path,
                _ => return,
            };

            let mut applicability = Applicability::MachineApplicable;
            span_lint_and_sugg(
                cx,
                MEM_REPLACE_OPTION_WITH_NONE,
                expr_span,
                "replacing an `Option` with `None`",
                "consider `Option::take()` instead",
                format!(
                    "{}.take()",
                    snippet_with_applicability(cx, replaced_path.span, "", &mut applicability)
                ),
                applicability,
            );
        }
    }
}

fn check_replace_with_uninit(cx: &LateContext<'_, '_>, src: &Expr<'_>, expr_span: Span) {
    if let ExprKind::Call(ref repl_func, ref repl_args) = src.kind {
        if_chain! {
            if repl_args.is_empty();
            if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
            if let Some(repl_def_id) = cx.tables.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
            then {
                if match_def_path(cx, repl_def_id, &paths::MEM_UNINITIALIZED) {
                    span_lint_and_help(
                        cx,
                        MEM_REPLACE_WITH_UNINIT,
                        expr_span,
                        "replacing with `mem::uninitialized()`",
                        "consider using the `take_mut` crate instead",
                    );
                } else if match_def_path(cx, repl_def_id, &paths::MEM_ZEROED) &&
                        !cx.tables.expr_ty(src).is_primitive() {
                    span_lint_and_help(
                        cx,
                        MEM_REPLACE_WITH_UNINIT,
                        expr_span,
                        "replacing with `mem::zeroed()`",
                        "consider using a default value or the `take_mut` crate instead",
                    );
                }
            }
        }
    }
}

fn check_replace_with_default(cx: &LateContext<'_, '_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
    if let ExprKind::Call(ref repl_func, _) = src.kind {
        if_chain! {
            if !in_external_macro(cx.tcx.sess, expr_span);
            if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
            if let Some(repl_def_id) = cx.tables.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
            if match_def_path(cx, repl_def_id, &paths::DEFAULT_TRAIT_METHOD);
            then {
                span_lint_and_then(
                    cx,
                    MEM_REPLACE_WITH_DEFAULT,
                    expr_span,
                    "replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`",
                    |db| {
                        if !in_macro(expr_span) {
                            let suggestion = format!("std::mem::take({})", snippet(cx, dest.span, ""));

                            db.span_suggestion(
                                expr_span,
                                "consider using",
                                suggestion,
                                Applicability::MachineApplicable
                            );
                        }
                    }
                );
            }
        }
    }
}

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MemReplace {
    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr<'_>) {
        if_chain! {
            // Check that `expr` is a call to `mem::replace()`
            if let ExprKind::Call(ref func, ref func_args) = expr.kind;
            if let ExprKind::Path(ref func_qpath) = func.kind;
            if let Some(def_id) = cx.tables.qpath_res(func_qpath, func.hir_id).opt_def_id();
            if match_def_path(cx, def_id, &paths::MEM_REPLACE);
            if let [dest, src] = &**func_args;
            then {
                check_replace_option_with_none(cx, src, dest, expr.span);
                check_replace_with_uninit(cx, src, expr.span);
                check_replace_with_default(cx, src, dest, expr.span);
            }
        }
    }
}