Remove overlap between rustc and clippy `let_underscore_lock` lint

4 files changed, 30 insertions, 82 deletions

diff --git a/clippy_lints/src/let_underscore.rs b/clippy_lints/src/let_underscore.rs
index 448688a3028..cfc1a21ea87 100644
--- a/clippy_lints/src/let_underscore.rs
+++ b/clippy_lints/src/let_underscore.rs
@@ -1,12 +1,11 @@
 use clippy_utils::diagnostics::span_lint_and_help;
-use clippy_utils::ty::{is_must_use_ty, is_type_diagnostic_item, match_type};
+use clippy_utils::ty::{is_must_use_ty, match_type};
 use clippy_utils::{is_must_use_func_call, paths};
 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 rustc_span::{sym, Symbol};
 
 declare_clippy_lint! {
     /// ### What it does
@@ -34,8 +33,9 @@ declare_clippy_lint! {
 
 declare_clippy_lint! {
     /// ### What it does
-    /// Checks for `let _ = sync_lock`.
-    /// This supports `mutex` and `rwlock` in `std::sync` and `parking_lot`.
+    /// Checks for `let _ = sync_lock`. This supports `mutex` and `rwlock` in
+    /// `parking_lot`. For `std` locks see the `rustc` lint
+    /// [`let_underscore_lock`](https://doc.rust-lang.org/nightly/rustc/lints/listing/deny-by-default.html#let-underscore-lock)
     ///
     /// ### Why is this bad?
     /// This statement immediately drops the lock instead of
@@ -61,8 +61,6 @@ declare_clippy_lint! {
 
 declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE, LET_UNDERSCORE_LOCK]);
 
-const SYNC_GUARD_SYMS: [Symbol; 3] = [sym::MutexGuard, sym::RwLockReadGuard, sym::RwLockWriteGuard];
-
 const SYNC_GUARD_PATHS: [&[&str]; 3] = [
     &paths::PARKING_LOT_MUTEX_GUARD,
     &paths::PARKING_LOT_RWLOCK_READ_GUARD,
@@ -77,13 +75,7 @@ impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
         {
             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_SYMS
-                        .iter()
-                        .any(|&sym| is_type_diagnostic_item(cx, inner_ty, sym))
-                        || SYNC_GUARD_PATHS.iter().any(|path| match_type(cx, inner_ty, path))
-                },
-
+                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 {
diff --git a/src/docs/let_underscore_lock.txt b/src/docs/let_underscore_lock.txt
index bd8217fb58b..88394a0a267 100644
--- a/src/docs/let_underscore_lock.txt
+++ b/src/docs/let_underscore_lock.txt
@@ -1,6 +1,7 @@
 ### What it does
-Checks for `let _ = sync_lock`.
-This supports `mutex` and `rwlock` in `std::sync` and `parking_lot`.
+Checks for `let _ = sync_lock`. This supports `mutex` and `rwlock` in
+`parking_lot`. For `std` locks see the `rustc` lint
+[`let_underscore_lock`](https://doc.rust-lang.org/nightly/rustc/lints/listing/deny-by-default.html#let-underscore-lock)
 
 ### Why is this bad?
 This statement immediately drops the lock instead of
diff --git a/tests/ui/let_underscore_lock.rs b/tests/ui/let_underscore_lock.rs
index 7a7c4e92499..4dff4d766bc 100644
--- a/tests/ui/let_underscore_lock.rs
+++ b/tests/ui/let_underscore_lock.rs
@@ -3,20 +3,6 @@
 extern crate parking_lot;
 
 fn main() {
-    let m = std::sync::Mutex::new(());
-    let rw = std::sync::RwLock::new(());
-
-    let _ = m.lock();
-    let _ = rw.read();
-    let _ = rw.write();
-    let _ = m.try_lock();
-    let _ = rw.try_read();
-    let _ = rw.try_write();
-
-    // These shouldn't throw an error.
-    let _ = m;
-    let _ = rw;
-
     use parking_lot::{lock_api::RawMutex, Mutex, RwLock};
 
     let p_m: Mutex<()> = Mutex::const_new(RawMutex::INIT, ());
@@ -34,3 +20,20 @@ fn main() {
     let _ = p_m1;
     let _ = p_rw;
 }
+
+fn uplifted() {
+    // shouldn't lint std locks as they were uplifted as rustc's `let_underscore_lock`
+
+    let m = std::sync::Mutex::new(());
+    let rw = std::sync::RwLock::new(());
+
+    let _ = m.lock();
+    let _ = rw.read();
+    let _ = rw.write();
+    let _ = m.try_lock();
+    let _ = rw.try_read();
+    let _ = rw.try_write();
+
+    let _ = m;
+    let _ = rw;
+}
diff --git a/tests/ui/let_underscore_lock.stderr b/tests/ui/let_underscore_lock.stderr
index d7779e7b6c4..89350b92e82 100644
--- a/tests/ui/let_underscore_lock.stderr
+++ b/tests/ui/let_underscore_lock.stderr
@@ -1,62 +1,14 @@
 error: non-binding let on a synchronization lock
   --> $DIR/let_underscore_lock.rs:9:5
    |
-LL |     let _ = m.lock();
-   |     ^^^^^^^^^^^^^^^^^
-   |
-   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
-   = note: `-D clippy::let-underscore-lock` implied by `-D warnings`
-
-error: non-binding let on a synchronization lock
-  --> $DIR/let_underscore_lock.rs:10:5
-   |
-LL |     let _ = rw.read();
-   |     ^^^^^^^^^^^^^^^^^^
-   |
-   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
-
-error: non-binding let on a synchronization lock
-  --> $DIR/let_underscore_lock.rs:11:5
-   |
-LL |     let _ = rw.write();
-   |     ^^^^^^^^^^^^^^^^^^^
-   |
-   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
-
-error: non-binding let on a synchronization lock
-  --> $DIR/let_underscore_lock.rs:12:5
-   |
-LL |     let _ = m.try_lock();
-   |     ^^^^^^^^^^^^^^^^^^^^^
-   |
-   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
-
-error: non-binding let on a synchronization lock
-  --> $DIR/let_underscore_lock.rs:13:5
-   |
-LL |     let _ = rw.try_read();
-   |     ^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
-
-error: non-binding let on a synchronization lock
-  --> $DIR/let_underscore_lock.rs:14:5
-   |
-LL |     let _ = rw.try_write();
-   |     ^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
-
-error: non-binding let on a synchronization lock
-  --> $DIR/let_underscore_lock.rs:23:5
-   |
 LL |     let _ = p_m.lock();
    |     ^^^^^^^^^^^^^^^^^^^
    |
    = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+   = note: `-D clippy::let-underscore-lock` implied by `-D warnings`
 
 error: non-binding let on a synchronization lock
-  --> $DIR/let_underscore_lock.rs:26:5
+  --> $DIR/let_underscore_lock.rs:12:5
    |
 LL |     let _ = p_m1.lock();
    |     ^^^^^^^^^^^^^^^^^^^^
@@ -64,7 +16,7 @@ LL |     let _ = p_m1.lock();
    = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
 
 error: non-binding let on a synchronization lock
-  --> $DIR/let_underscore_lock.rs:29:5
+  --> $DIR/let_underscore_lock.rs:15:5
    |
 LL |     let _ = p_rw.read();
    |     ^^^^^^^^^^^^^^^^^^^^
@@ -72,12 +24,12 @@ LL |     let _ = p_rw.read();
    = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
 
 error: non-binding let on a synchronization lock
-  --> $DIR/let_underscore_lock.rs:30:5
+  --> $DIR/let_underscore_lock.rs:16:5
    |
 LL |     let _ = p_rw.write();
    |     ^^^^^^^^^^^^^^^^^^^^^
    |
    = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
 
-error: aborting due to 10 previous errors
+error: aborting due to 4 previous errors