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use rustc_hir::{Arm, Expr, ExprKind, Node};
use rustc_span::sym;
use crate::{
lints::{DropCopyDiag, DropRefDiag, ForgetCopyDiag, ForgetRefDiag},
LateContext, LateLintPass, LintContext,
};
declare_lint! {
/// The `drop_ref` lint checks for calls to `std::mem::drop` with a reference
/// instead of an owned value.
///
/// ### Example
///
/// ```rust
/// # fn operation_that_requires_mutex_to_be_unlocked() {} // just to make it compile
/// # let mutex = std::sync::Mutex::new(1); // just to make it compile
/// let mut lock_guard = mutex.lock();
/// std::mem::drop(&lock_guard); // Should have been drop(lock_guard), mutex
/// // still locked
/// operation_that_requires_mutex_to_be_unlocked();
/// ```
///
/// {{produces}}
///
/// ### Explanation
///
/// Calling `drop` on a reference will only drop the
/// reference itself, which is a no-op. It will not call the `drop` method (from
/// the `Drop` trait implementation) on the underlying referenced value, which
/// is likely what was intended.
pub DROP_REF,
Warn,
"calls to `std::mem::drop` with a reference instead of an owned value"
}
declare_lint! {
/// The `forget_ref` lint checks for calls to `std::mem::forget` with a reference
/// instead of an owned value.
///
/// ### Example
///
/// ```rust
/// let x = Box::new(1);
/// std::mem::forget(&x); // Should have been forget(x), x will still be dropped
/// ```
///
/// {{produces}}
///
/// ### Explanation
///
/// Calling `forget` on a reference will only forget the
/// reference itself, which is a no-op. It will not forget the underlying
/// referenced value, which is likely what was intended.
pub FORGET_REF,
Warn,
"calls to `std::mem::forget` with a reference instead of an owned value"
}
declare_lint! {
/// The `dropping_copy_types` lint checks for calls to `std::mem::drop` with a value
/// that derives the Copy trait.
///
/// ### Example
///
/// ```rust
/// let x: i32 = 42; // i32 implements Copy
/// std::mem::drop(x); // A copy of x is passed to the function, leaving the
/// // original unaffected
/// ```
///
/// {{produces}}
///
/// ### Explanation
///
/// Calling `std::mem::drop` [does nothing for types that
/// implement Copy](https://doc.rust-lang.org/std/mem/fn.drop.html), since the
/// value will be copied and moved into the function on invocation.
pub DROPPING_COPY_TYPES,
Warn,
"calls to `std::mem::drop` with a value that implements Copy"
}
declare_lint! {
/// The `forgetting_copy_types` lint checks for calls to `std::mem::forget` with a value
/// that derives the Copy trait.
///
/// ### Example
///
/// ```rust
/// let x: i32 = 42; // i32 implements Copy
/// std::mem::forget(x); // A copy of x is passed to the function, leaving the
/// // original unaffected
/// ```
///
/// {{produces}}
///
/// ### Explanation
///
/// Calling `std::mem::forget` [does nothing for types that
/// implement Copy](https://doc.rust-lang.org/std/mem/fn.drop.html) since the
/// value will be copied and moved into the function on invocation.
///
/// An alternative, but also valid, explanation is that Copy types do not
/// implement the Drop trait, which means they have no destructors. Without a
/// destructor, there is nothing for `std::mem::forget` to ignore.
pub FORGETTING_COPY_TYPES,
Warn,
"calls to `std::mem::forget` with a value that implements Copy"
}
declare_lint_pass!(DropForgetUseless => [DROP_REF, FORGET_REF, DROPPING_COPY_TYPES, FORGETTING_COPY_TYPES]);
impl<'tcx> LateLintPass<'tcx> for DropForgetUseless {
fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
if let ExprKind::Call(path, [arg]) = expr.kind
&& let ExprKind::Path(ref qpath) = path.kind
&& let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id()
&& let Some(fn_name) = cx.tcx.get_diagnostic_name(def_id)
{
let arg_ty = cx.typeck_results().expr_ty(arg);
let is_copy = arg_ty.is_copy_modulo_regions(cx.tcx, cx.param_env);
let drop_is_single_call_in_arm = is_single_call_in_arm(cx, arg, expr);
match fn_name {
sym::mem_drop if arg_ty.is_ref() && !drop_is_single_call_in_arm => {
cx.emit_spanned_lint(DROP_REF, expr.span, DropRefDiag { arg_ty, label: arg.span });
},
sym::mem_forget if arg_ty.is_ref() => {
cx.emit_spanned_lint(FORGET_REF, expr.span, ForgetRefDiag { arg_ty, label: arg.span });
},
sym::mem_drop if is_copy && !drop_is_single_call_in_arm => {
cx.emit_spanned_lint(DROPPING_COPY_TYPES, expr.span, DropCopyDiag { arg_ty, label: arg.span });
}
sym::mem_forget if is_copy => {
cx.emit_spanned_lint(FORGETTING_COPY_TYPES, expr.span, ForgetCopyDiag { arg_ty, label: arg.span });
}
_ => return,
};
}
}
}
// Dropping returned value of a function, as in the following snippet is considered idiomatic, see
// rust-lang/rust-clippy#9482 for examples.
//
// ```
// match <var> {
// <pat> => drop(fn_with_side_effect_and_returning_some_value()),
// ..
// }
// ```
fn is_single_call_in_arm<'tcx>(
cx: &LateContext<'tcx>,
arg: &'tcx Expr<'_>,
drop_expr: &'tcx Expr<'_>,
) -> bool {
if matches!(arg.kind, ExprKind::Call(..) | ExprKind::MethodCall(..)) {
let parent_node = cx.tcx.hir().find_parent(drop_expr.hir_id);
if let Some(Node::Arm(Arm { body, .. })) = &parent_node {
return body.hir_id == drop_expr.hir_id;
}
}
false
}
|
