rust-clippy/clippy_lints/src/mem_replace.rs

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use crate::utils::{
in_macro, match_def_path, match_qpath, meets_msrv, paths, snippet, snippet_with_applicability, span_lint_and_help,
span_lint_and_sugg, span_lint_and_then,
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};
use if_chain::if_chain;
use rustc_errors::Applicability;
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use rustc_hir::{BorrowKind, Expr, ExprKind, Mutability, QPath};
use rustc_lint::{LateContext, LateLintPass, LintContext};
use rustc_middle::lint::in_external_macro;
use rustc_semver::RustcVersion;
use rustc_session::{declare_tool_lint, impl_lint_pass};
use rustc_span::source_map::Span;
use rustc_span::symbol::sym;
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
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/// 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()`"
}
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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`"
}
impl_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) {
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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.
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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,
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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<'_>, dest: &Expr<'_>, expr_span: Span) {
if_chain! {
// check if replacement is mem::MaybeUninit::uninit().assume_init()
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if let Some(method_def_id) = cx.typeck_results().type_dependent_def_id(src.hir_id);
if cx.tcx.is_diagnostic_item(sym::assume_init, method_def_id);
then {
let mut applicability = Applicability::MachineApplicable;
span_lint_and_sugg(
cx,
MEM_REPLACE_WITH_UNINIT,
expr_span,
"replacing with `mem::MaybeUninit::uninit().assume_init()`",
"consider using",
format!(
"std::ptr::read({})",
snippet_with_applicability(cx, dest.span, "", &mut applicability)
),
applicability,
);
return;
}
}
if_chain! {
if let ExprKind::Call(ref repl_func, ref repl_args) = src.kind;
if repl_args.is_empty();
if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
if let Some(repl_def_id) = cx.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
then {
if cx.tcx.is_diagnostic_item(sym::mem_uninitialized, repl_def_id) {
let mut applicability = Applicability::MachineApplicable;
span_lint_and_sugg(
cx,
MEM_REPLACE_WITH_UNINIT,
expr_span,
"replacing with `mem::uninitialized()`",
"consider using",
format!(
"std::ptr::read({})",
snippet_with_applicability(cx, dest.span, "", &mut applicability)
),
applicability,
);
} else if cx.tcx.is_diagnostic_item(sym::mem_zeroed, repl_def_id) &&
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!cx.typeck_results().expr_ty(src).is_primitive() {
span_lint_and_help(
cx,
MEM_REPLACE_WITH_UNINIT,
expr_span,
"replacing with `mem::zeroed()`",
None,
"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) {
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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.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,
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expr_span,
"replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`",
|diag| {
if !in_macro(expr_span) {
let suggestion = format!("std::mem::take({})", snippet(cx, dest.span, ""));
diag.span_suggestion(
expr_span,
"consider using",
suggestion,
Applicability::MachineApplicable
);
}
}
);
}
}
}
}
const MEM_REPLACE_WITH_DEFAULT_MSRV: RustcVersion = RustcVersion::new(1, 40, 0);
pub struct MemReplace {
msrv: Option<RustcVersion>,
}
impl MemReplace {
#[must_use]
pub fn new(msrv: Option<RustcVersion>) -> Self {
Self { msrv }
}
}
impl<'tcx> LateLintPass<'tcx> for MemReplace {
fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
if_chain! {
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// Check that `expr` is a call to `mem::replace()`
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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.qpath_res(func_qpath, func.hir_id).opt_def_id();
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if match_def_path(cx, def_id, &paths::MEM_REPLACE);
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if let [dest, src] = &**func_args;
then {
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check_replace_option_with_none(cx, src, dest, expr.span);
check_replace_with_uninit(cx, src, dest, expr.span);
if meets_msrv(self.msrv.as_ref(), &MEM_REPLACE_WITH_DEFAULT_MSRV) {
check_replace_with_default(cx, src, dest, expr.span);
}
}
}
}
extract_msrv_attr!(LateContext);
}