Add lint for &mut Mutex::lock

CHANGELOG.md

@@ -1721,6 +1721,7 @@ Released 2018-09-13
 [`must_use_unit`]: https://rust-lang.github.io/rust-clippy/master/index.html#must_use_unit
 [`mut_from_ref`]: https://rust-lang.github.io/rust-clippy/master/index.html#mut_from_ref
 [`mut_mut`]: https://rust-lang.github.io/rust-clippy/master/index.html#mut_mut
+[`mut_mutex_lock`]: https://rust-lang.github.io/rust-clippy/master/index.html#mut_mutex_lock
 [`mut_range_bound`]: https://rust-lang.github.io/rust-clippy/master/index.html#mut_range_bound
 [`mutable_key_type`]: https://rust-lang.github.io/rust-clippy/master/index.html#mutable_key_type
 [`mutex_atomic`]: https://rust-lang.github.io/rust-clippy/master/index.html#mutex_atomic

clippy_lints/src/lib.rs

@@ -254,6 +254,7 @@ mod modulo_arithmetic;
 mod multiple_crate_versions;
 mod mut_key;
 mod mut_mut;
+mod mut_mutex_lock;
 mod mut_reference;
 mod mutable_debug_assertion;
 mod mutex_atomic;
@@ -736,6 +737,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &multiple_crate_versions::MULTIPLE_CRATE_VERSIONS,
         &mut_key::MUTABLE_KEY_TYPE,
         &mut_mut::MUT_MUT,
+        &mut_mutex_lock::MUT_MUTEX_LOCK,
         &mut_reference::UNNECESSARY_MUT_PASSED,
         &mutable_debug_assertion::DEBUG_ASSERT_WITH_MUT_CALL,
         &mutex_atomic::MUTEX_ATOMIC,
@@ -1101,6 +1103,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| box future_not_send::FutureNotSend);
     store.register_late_pass(|| box utils::internal_lints::CollapsibleCalls);
     store.register_late_pass(|| box if_let_mutex::IfLetMutex);
+    store.register_late_pass(|| box mut_mutex_lock::MutMutexLock);
     store.register_late_pass(|| box match_on_vec_items::MatchOnVecItems);
     store.register_early_pass(|| box manual_non_exhaustive::ManualNonExhaustive);
     store.register_late_pass(|| box manual_async_fn::ManualAsyncFn);
@@ -1430,6 +1433,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&misc_early::UNNEEDED_WILDCARD_PATTERN),
         LintId::of(&misc_early::ZERO_PREFIXED_LITERAL),
         LintId::of(&mut_key::MUTABLE_KEY_TYPE),
+        LintId::of(&mut_mutex_lock::MUT_MUTEX_LOCK),
         LintId::of(&mut_reference::UNNECESSARY_MUT_PASSED),
         LintId::of(&mutex_atomic::MUTEX_ATOMIC),
         LintId::of(&needless_arbitrary_self_type::NEEDLESS_ARBITRARY_SELF_TYPE),
@@ -1760,6 +1764,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&misc::FLOAT_CMP),
         LintId::of(&misc::MODULO_ONE),
         LintId::of(&mut_key::MUTABLE_KEY_TYPE),
+        LintId::of(&mut_mutex_lock::MUT_MUTEX_LOCK),
         LintId::of(&non_copy_const::BORROW_INTERIOR_MUTABLE_CONST),
         LintId::of(&non_copy_const::DECLARE_INTERIOR_MUTABLE_CONST),
         LintId::of(&open_options::NONSENSICAL_OPEN_OPTIONS),

clippy_lints/src/mut_mutex_lock.rs

@@ -0,0 +1,75 @@
+use crate::utils::{is_type_diagnostic_item, span_lint_and_help};
+use if_chain::if_chain;
+use rustc_hir::{Expr, ExprKind, Mutability};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for `&mut Mutex::lock` calls
+    ///
+    /// **Why is this bad?** `Mutex::lock` is less efficient than
+    /// calling `Mutex::get_mut`
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// use std::sync::{Arc, Mutex};
+    ///
+    /// let mut value_rc = Arc::new(Mutex::new(42_u8));
+    /// let value_mutex = Arc::get_mut(&mut value_rc).unwrap();
+    ///
+    /// let value = value_mutex.lock().unwrap();
+    /// do_stuff(value);
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// use std::sync::{Arc, Mutex};
+    ///
+    /// let mut value_rc = Arc::new(Mutex::new(42_u8));
+    /// let value_mutex = Arc::get_mut(&mut value_rc).unwrap();
+    ///
+    /// let value = value_mutex.get_mut().unwrap();
+    /// do_stuff(value);
+    /// ```
+    pub MUT_MUTEX_LOCK,
+    correctness,
+    "`&mut Mutex::lock` does unnecessary locking"
+}
+
+declare_lint_pass!(MutMutexLock => [MUT_MUTEX_LOCK]);
+
+impl<'tcx> LateLintPass<'tcx> for MutMutexLock {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, ex: &'tcx Expr<'tcx>) {
+        if_chain! {
+            if is_mut_mutex_lock_call(cx, ex).is_some();
+            then {
+                span_lint_and_help(
+                    cx,
+                    MUT_MUTEX_LOCK,
+                    ex.span,
+                    "calling `&mut Mutex::lock` unnecessarily locks an exclusive (mutable) reference",
+                    None,
+                    "use `&mut Mutex::get_mut` instead",
+                );
+            }
+        }
+    }
+}
+
+fn is_mut_mutex_lock_call<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> Option<&'tcx Expr<'tcx>> {
+    if_chain! {
+        if let ExprKind::MethodCall(path, _span, args, _) = &expr.kind;
+        if path.ident.name == sym!(lock);
+        let ty = cx.typeck_results().expr_ty(&args[0]);
+        if let ty::Ref(_, inner_ty, Mutability::Mut) = ty.kind();
+        if is_type_diagnostic_item(cx, inner_ty, sym!(mutex_type));
+        then {
+            Some(&args[0])
+        } else {
+            None
+        }
+    }
+}

src/lintlist/mod.rs

@@ -1480,6 +1480,13 @@ pub static ref ALL_LINTS: Vec<Lint> = vec![
         deprecation: None,
         module: "mut_mut",
     },
+    Lint {
+        name: "mut_mutex_lock",
+        group: "correctness",
+        desc: "`&mut Mutex::lock` does unnecessary locking",
+        deprecation: None,
+        module: "mut_mutex_lock",
+    },
     Lint {
         name: "mut_range_bound",
         group: "complexity",

tests/ui/mut_mutex_lock.rs

@@ -0,0 +1,19 @@
+#![warn(clippy::mut_mutex_lock)]
+
+use std::sync::{Arc, Mutex};
+
+fn mut_mutex_lock() {
+    let mut value_rc = Arc::new(Mutex::new(42_u8));
+    let value_mutex = Arc::get_mut(&mut value_rc).unwrap();
+
+    let value = value_mutex.lock().unwrap();
+    *value += 1;
+}
+
+fn no_owned_mutex_lock() {
+    let mut value_rc = Arc::new(Mutex::new(42_u8));
+    let value = value_rc.lock().unwrap();
+    *value += 1;
+}
+
+fn main() {}

tests/ui/mut_mutex_lock.stderr

@@ -0,0 +1,19 @@
+error[E0596]: cannot borrow `value` as mutable, as it is not declared as mutable
+  --> $DIR/mut_mutex_lock.rs:10:6
+   |
+LL |     let value = value_mutex.lock().unwrap();
+   |         ----- help: consider changing this to be mutable: `mut value`
+LL |     *value += 1;
+   |      ^^^^^ cannot borrow as mutable
+
+error[E0596]: cannot borrow `value` as mutable, as it is not declared as mutable
+  --> $DIR/mut_mutex_lock.rs:16:6
+   |
+LL |     let value = value_rc.lock().unwrap();
+   |         ----- help: consider changing this to be mutable: `mut value`
+LL |     *value += 1;
+   |      ^^^^^ cannot borrow as mutable
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0596`.