use if_chain::if_chain;
-use rustc::lint::in_external_macro;
-use rustc_hir::*;
+use rustc_hir::{Local, PatKind};
use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::lint::in_external_macro;
+use rustc_middle::ty::subst::GenericArgKind;
use rustc_session::{declare_lint_pass, declare_tool_lint};
-use crate::utils::{is_must_use_func_call, is_must_use_ty, span_help_and_lint};
+use crate::utils::{is_must_use_func_call, is_must_use_ty, match_type, paths, span_lint_and_help};
declare_clippy_lint! {
/// **What it does:** Checks for `let _ = <expr>`
"non-binding let on a `#[must_use]` expression"
}
-declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE]);
+declare_clippy_lint! {
+ /// **What it does:** Checks for `let _ = sync_lock`
+ ///
+ /// **Why is this bad?** This statement immediately drops the lock instead of
+ /// extending its lifetime to the end of the scope, which is often not intended.
+ /// To extend lock lifetime to the end of the scope, use an underscore-prefixed
+ /// name instead (i.e. _lock). If you want to explicitly drop the lock,
+ /// `std::mem::drop` conveys your intention better and is less error-prone.
+ ///
+ /// **Known problems:** None.
+ ///
+ /// **Example:**
+ ///
+ /// Bad:
+ /// ```rust,ignore
+ /// let _ = mutex.lock();
+ /// ```
+ ///
+ /// Good:
+ /// ```rust,ignore
+ /// let _lock = mutex.lock();
+ /// ```
+ pub LET_UNDERSCORE_LOCK,
+ correctness,
+ "non-binding let on a synchronization lock"
+}
+
+declare_clippy_lint! {
+ /// **What it does:** Checks for `let _ = <expr>`
+ /// where expr has a type that implements `Drop`
+ ///
+ /// **Why is this bad?** This statement immediately drops the initializer
+ /// expression instead of extending its lifetime to the end of the scope, which
+ /// is often not intended. To extend the expression's lifetime to the end of the
+ /// scope, use an underscore-prefixed name instead (i.e. _var). If you want to
+ /// explicitly drop the expression, `std::mem::drop` conveys your intention
+ /// better and is less error-prone.
+ ///
+ /// **Known problems:** None.
+ ///
+ /// **Example:**
+ ///
+ /// Bad:
+ /// ```rust,ignore
+ /// struct Droppable;
+ /// impl Drop for Droppable {
+ /// fn drop(&mut self) {}
+ /// }
+ /// {
+ /// let _ = Droppable;
+ /// // ^ dropped here
+ /// /* more code */
+ /// }
+ /// ```
+ ///
+ /// Good:
+ /// ```rust,ignore
+ /// {
+ /// let _droppable = Droppable;
+ /// /* more code */
+ /// // dropped at end of scope
+ /// }
+ /// ```
+ pub LET_UNDERSCORE_DROP,
+ pedantic,
+ "non-binding let on a type that implements `Drop`"
+}
+
+declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE, LET_UNDERSCORE_LOCK, LET_UNDERSCORE_DROP]);
-impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LetUnderscore {
- fn check_stmt(&mut self, cx: &LateContext<'_, '_>, stmt: &Stmt<'_>) {
- if in_external_macro(cx.tcx.sess, stmt.span) {
+const SYNC_GUARD_PATHS: [&[&str]; 3] = [
+ &paths::MUTEX_GUARD,
+ &paths::RWLOCK_READ_GUARD,
+ &paths::RWLOCK_WRITE_GUARD,
+];
+
+impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
+ fn check_local(&mut self, cx: &LateContext<'_>, local: &Local<'_>) {
+ if in_external_macro(cx.tcx.sess, local.span) {
return;
}
if_chain! {
- if let StmtKind::Local(ref local) = stmt.kind;
if let PatKind::Wild = local.pat.kind;
if let Some(ref init) = local.init;
then {
- if is_must_use_ty(cx, cx.tables.expr_ty(init)) {
- span_help_and_lint(
+ let init_ty = cx.typeck_results().expr_ty(init);
+ let contains_sync_guard = init_ty.walk().any(|inner| match inner.unpack() {
+ GenericArgKind::Type(inner_ty) => {
+ SYNC_GUARD_PATHS.iter().any(|path| match_type(cx, inner_ty, path))
+ },
+
+ GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => false,
+ });
+ if contains_sync_guard {
+ span_lint_and_help(
+ cx,
+ LET_UNDERSCORE_LOCK,
+ local.span,
+ "non-binding let on a synchronization lock",
+ None,
+ "consider using an underscore-prefixed named \
+ binding or dropping explicitly with `std::mem::drop`"
+ )
+ } else if init_ty.needs_drop(cx.tcx, cx.param_env) {
+ span_lint_and_help(
+ cx,
+ LET_UNDERSCORE_DROP,
+ local.span,
+ "non-binding `let` on a type that implements `Drop`",
+ None,
+ "consider using an underscore-prefixed named \
+ binding or dropping explicitly with `std::mem::drop`"
+ )
+ } else if is_must_use_ty(cx, cx.typeck_results().expr_ty(init)) {
+ span_lint_and_help(
cx,
LET_UNDERSCORE_MUST_USE,
- stmt.span,
+ local.span,
"non-binding let on an expression with `#[must_use]` type",
+ None,
"consider explicitly using expression value"
)
} else if is_must_use_func_call(cx, init) {
- span_help_and_lint(
+ span_lint_and_help(
cx,
LET_UNDERSCORE_MUST_USE,
- stmt.span,
+ local.span,
"non-binding let on a result of a `#[must_use]` function",
+ None,
"consider explicitly using function result"
)
}