Improve let_underscore_lock

- lint if the lock was in a nested pattern
- lint if the lock is inside a `Result<Lock, _>`

compiler/rustc_lint/messages.ftl

@@ -345,6 +345,9 @@ lint_multiple_supertrait_upcastable = `{$ident}` is object-safe and has multiple
 lint_node_source = `forbid` level set here
     .note = {$reason}
 
+lint_non_binding_let_multi_drop_fn =
+    consider immediately dropping the value using `drop(..)` after the `let` statement
+
 lint_non_binding_let_multi_suggestion =
     consider immediately dropping the value
 

compiler/rustc_lint/src/let_underscore.rs

@@ -5,7 +5,7 @@ use crate::{
 use rustc_errors::MultiSpan;
 use rustc_hir as hir;
 use rustc_middle::ty;
-use rustc_span::Symbol;
+use rustc_span::{sym, Symbol};
 
 declare_lint! {
     /// The `let_underscore_drop` lint checks for statements which don't bind
@@ -105,46 +105,65 @@ const SYNC_GUARD_SYMBOLS: [Symbol; 3] = [
 
 impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
     fn check_local(&mut self, cx: &LateContext<'_>, local: &hir::Local<'_>) {
-        if !matches!(local.pat.kind, hir::PatKind::Wild) {
-            return;
-        }
-        if let Some(init) = local.init {
-            let init_ty = cx.typeck_results().expr_ty(init);
+        let mut top_level = true;
+
+        // We recursively walk through all patterns, so that we can catch cases where the lock is nested in a pattern.
+        // For the basic `let_underscore_drop` lint, we only look at the top level, since there are many legitimate reasons
+        // to bind a sub-pattern to an `_`, if we're only interested in the rest.
+        // But with locks, we prefer having the chance of "false positives" over missing cases, since the effects can be
+        // quite catastrophic.
+        local.pat.walk_always(|pat| {
+            let is_top_level = top_level;
+            top_level = false;
+
+            if !matches!(pat.kind, hir::PatKind::Wild) {
+                return;
+            }
+
+            let ty = cx.typeck_results().pat_ty(pat);
+
             // If the type has a trivial Drop implementation, then it doesn't
             // matter that we drop the value immediately.
-            if !init_ty.needs_drop(cx.tcx, cx.param_env) {
+            if !ty.needs_drop(cx.tcx, cx.param_env) {
                 return;
             }
-            let is_sync_lock = match init_ty.kind() {
+            // Lint for patterns like `mutex.lock()`, which returns `Result<MutexGuard, _>` as well.
+            let potential_lock_type = match ty.kind() {
+                ty::Adt(adt, args) if cx.tcx.is_diagnostic_item(sym::Result, adt.did()) => {
+                    args.type_at(0)
+                }
+                _ => ty,
+            };
+            let is_sync_lock = match potential_lock_type.kind() {
                 ty::Adt(adt, _) => SYNC_GUARD_SYMBOLS
                     .iter()
                     .any(|guard_symbol| cx.tcx.is_diagnostic_item(*guard_symbol, adt.did())),
                 _ => false,
             };
 
+            let can_use_init = is_top_level.then_some(local.init).flatten();
+
             let sub = NonBindingLetSub {
-                suggestion: local.pat.span,
-                multi_suggestion_start: local.span.until(init.span),
-                multi_suggestion_end: init.span.shrink_to_hi(),
+                suggestion: pat.span,
+                // We can't suggest `drop()` when we're on the top level.
+                drop_fn_start_end: can_use_init
+                    .map(|init| (local.span.until(init.span), init.span.shrink_to_hi())),
             };
             if is_sync_lock {
-                let mut span = MultiSpan::from_spans(vec![local.pat.span, init.span]);
+                let mut span = MultiSpan::from_span(pat.span);
                 span.push_span_label(
-                    local.pat.span,
+                    pat.span,
                     "this lock is not assigned to a binding and is immediately dropped".to_string(),
                 );
-                span.push_span_label(
-                    init.span,
-                    "this binding will immediately drop the value assigned to it".to_string(),
-                );
                 cx.emit_spanned_lint(LET_UNDERSCORE_LOCK, span, NonBindingLet::SyncLock { sub });
-            } else {
+            // Only emit let_underscore_drop for top-level `_` patterns.
+            } else if can_use_init.is_some() {
                 cx.emit_spanned_lint(
                     LET_UNDERSCORE_DROP,
                     local.span,
                     NonBindingLet::DropType { sub },
                 );
             }
-        }
+        });
     }
 }

compiler/rustc_lint/src/lints.rs

@@ -930,8 +930,7 @@ pub enum NonBindingLet {
 
 pub struct NonBindingLetSub {
     pub suggestion: Span,
-    pub multi_suggestion_start: Span,
-    pub multi_suggestion_end: Span,
+    pub drop_fn_start_end: Option<(Span, Span)>,
 }
 
 impl AddToDiagnostic for NonBindingLetSub {
@@ -945,14 +944,18 @@ impl AddToDiagnostic for NonBindingLetSub {
             "_unused",
             Applicability::MachineApplicable,
         );
-        diag.multipart_suggestion(
-            fluent::lint_non_binding_let_multi_suggestion,
-            vec![
-                (self.multi_suggestion_start, "drop(".to_string()),
-                (self.multi_suggestion_end, ")".to_string()),
-            ],
-            Applicability::MachineApplicable,
-        );
+        if let Some(drop_fn_start_end) = self.drop_fn_start_end {
+            diag.multipart_suggestion(
+                fluent::lint_non_binding_let_multi_suggestion,
+                vec![
+                    (drop_fn_start_end.0, "drop(".to_string()),
+                    (drop_fn_start_end.1, ")".to_string()),
+                ],
+                Applicability::MachineApplicable,
+            );
+        } else {
+            diag.help(fluent::lint_non_binding_let_multi_drop_fn);
+        }
     }
 }
 

tests/ui/lint/let_underscore/let_underscore_drop.rs

@@ -11,4 +11,6 @@ impl Drop for NontrivialDrop {
 
 fn main() {
     let _ = NontrivialDrop; //~WARNING non-binding let on a type that implements `Drop`
+
+    let (_, _) = (NontrivialDrop, NontrivialDrop); // This should be ignored.
 }

tests/ui/lint/let_underscore/let_underscore_lock.rs

@@ -1,7 +1,21 @@
-// check-fail
 use std::sync::{Arc, Mutex};
 
+struct Struct<T> {
+    a: T,
+}
+
 fn main() {
     let data = Arc::new(Mutex::new(0));
     let _ = data.lock().unwrap(); //~ERROR non-binding let on a synchronization lock
+
+    let _ = data.lock(); //~ERROR non-binding let on a synchronization lock
+
+    let (_, _) = (data.lock(), 1); //~ERROR non-binding let on a synchronization lock
+
+    let (_a, Struct { a: _ }) = (0, Struct { a: data.lock() }); //~ERROR non-binding let on a synchronization lock
+
+    (_ , _) = (data.lock(), 1); //~ERROR non-binding let on a synchronization lock
+
+    let _b;
+    (_b, Struct { a: _ }) = (0, Struct { a: data.lock() }); //~ERROR non-binding let on a synchronization lock
 }

tests/ui/lint/let_underscore/let_underscore_lock.stderr

@@ -1,10 +1,8 @@
 error: non-binding let on a synchronization lock
-  --> $DIR/let_underscore_lock.rs:6:9
+  --> $DIR/let_underscore_lock.rs:9:9
    |
 LL |     let _ = data.lock().unwrap();
-   |         ^   ^^^^^^^^^^^^^^^^^^^^ this binding will immediately drop the value assigned to it
-   |         |
-   |         this lock is not assigned to a binding and is immediately dropped
+   |         ^ this lock is not assigned to a binding and is immediately dropped
    |
    = note: `#[deny(let_underscore_lock)]` on by default
 help: consider binding to an unused variable to avoid immediately dropping the value
@@ -16,5 +14,68 @@ help: consider immediately dropping the value
 LL |     drop(data.lock().unwrap());
    |     ~~~~~                    +
 
-error: aborting due to 1 previous error
+error: non-binding let on a synchronization lock
+  --> $DIR/let_underscore_lock.rs:11:9
+   |
+LL |     let _ = data.lock();
+   |         ^ this lock is not assigned to a binding and is immediately dropped
+   |
+help: consider binding to an unused variable to avoid immediately dropping the value
+   |
+LL |     let _unused = data.lock();
+   |         ~~~~~~~
+help: consider immediately dropping the value
+   |
+LL |     drop(data.lock());
+   |     ~~~~~           +
+
+error: non-binding let on a synchronization lock
+  --> $DIR/let_underscore_lock.rs:13:10
+   |
+LL |     let (_, _) = (data.lock(), 1);
+   |          ^ this lock is not assigned to a binding and is immediately dropped
+   |
+   = help: consider immediately dropping the value using `drop(..)` after the `let` statement
+help: consider binding to an unused variable to avoid immediately dropping the value
+   |
+LL |     let (_unused, _) = (data.lock(), 1);
+   |          ~~~~~~~
+
+error: non-binding let on a synchronization lock
+  --> $DIR/let_underscore_lock.rs:15:26
+   |
+LL |     let (_a, Struct { a: _ }) = (0, Struct { a: data.lock() });
+   |                          ^ this lock is not assigned to a binding and is immediately dropped
+   |
+   = help: consider immediately dropping the value using `drop(..)` after the `let` statement
+help: consider binding to an unused variable to avoid immediately dropping the value
+   |
+LL |     let (_a, Struct { a: _unused }) = (0, Struct { a: data.lock() });
+   |                          ~~~~~~~
+
+error: non-binding let on a synchronization lock
+  --> $DIR/let_underscore_lock.rs:17:6
+   |
+LL |     (_ , _) = (data.lock(), 1);
+   |      ^ this lock is not assigned to a binding and is immediately dropped
+   |
+   = help: consider immediately dropping the value using `drop(..)` after the `let` statement
+help: consider binding to an unused variable to avoid immediately dropping the value
+   |
+LL |     (_unused , _) = (data.lock(), 1);
+   |      ~~~~~~~
+
+error: non-binding let on a synchronization lock
+  --> $DIR/let_underscore_lock.rs:20:22
+   |
+LL |     (_b, Struct { a: _ }) = (0, Struct { a: data.lock() });
+   |                      ^ this lock is not assigned to a binding and is immediately dropped
+   |
+   = help: consider immediately dropping the value using `drop(..)` after the `let` statement
+help: consider binding to an unused variable to avoid immediately dropping the value
+   |
+LL |     (_b, Struct { a: _unused }) = (0, Struct { a: data.lock() });
+   |                      ~~~~~~~
+
+error: aborting due to 6 previous errors