Add lint for `&mut Mutex::lock`

6 files changed, 128 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1d995cc9701..ba080835f5c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1843,6 +1843,7 @@ 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
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 8d53b9799f0..3a108bcfe6a 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -255,6 +255,7 @@ 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;
@@ -744,6 +745,7 @@ 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,
@@ -1112,6 +1114,7 @@ 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);
@@ -1446,6 +1449,7 @@ 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),
@@ -1780,6 +1784,9 @@ 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),
         LintId::of(&option_env_unwrap::OPTION_ENV_UNWRAP),
         LintId::of(&ptr::MUT_FROM_REF),
diff --git a/clippy_lints/src/mut_mutex_lock.rs b/clippy_lints/src/mut_mutex_lock.rs
new file mode 100644
index 00000000000..4f3108355ca
--- /dev/null
+++ b/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
+        }
+    }
+}
diff --git a/src/lintlist/mod.rs b/src/lintlist/mod.rs
index 98ad994ea7b..5e48757a4c5 100644
--- a/src/lintlist/mod.rs
+++ b/src/lintlist/mod.rs
@@ -1503,6 +1503,13 @@ vec![
         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",
         desc: "for loop over a range where one of the bounds is a mutable variable",
diff --git a/tests/ui/mut_mutex_lock.rs b/tests/ui/mut_mutex_lock.rs
new file mode 100644
index 00000000000..516d44bb7a9
--- /dev/null
+++ b/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() {}
diff --git a/tests/ui/mut_mutex_lock.stderr b/tests/ui/mut_mutex_lock.stderr
new file mode 100644
index 00000000000..426e0240830
--- /dev/null
+++ b/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`.