Auto merge of #12734 - y21:issue12733, r=Manishearth

suppress `readonly_write_lock` for underscore-prefixed bindings

Fixes #12733

Unsure if there's a better way to prevent this kind of false positive but this is the one that made most sense to me.
In my experience, prefixing bindings with an underscore is the usual way to name variables that aren't used and that exist purely for executing drop code at the end of the scope.

-------

changelog: suppress [`readonly_write_lock`] for underscore-prefixed bindings

clippy_lints/src/methods/readonly_write_lock.rs

@@ -4,7 +4,7 @@
 use clippy_utils::source::snippet;
 use clippy_utils::ty::is_type_diagnostic_item;
 use rustc_errors::Applicability;
-use rustc_hir::{Expr, ExprKind, Node};
+use rustc_hir::{Expr, ExprKind, Node, PatKind};
 use rustc_lint::LateContext;
 use rustc_middle::mir::{Location, START_BLOCK};
 use rustc_span::sym;
@@ -25,6 +25,11 @@ pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, receiver
         && is_unwrap_call(cx, unwrap_call_expr)
         && let parent = cx.tcx.parent_hir_node(unwrap_call_expr.hir_id)
         && let Node::LetStmt(local) = parent
+        && let PatKind::Binding(.., ident, _) = local.pat.kind
+        // if the binding is prefixed with `_`, it typically means
+        // that this guard only exists to protect a section of code
+        // rather than the contained data
+        && !ident.as_str().starts_with('_')
         && let Some(mir) = enclosing_mir(cx.tcx, expr.hir_id)
         && let Some((local, _)) = mir
             .local_decls

tests/ui/readonly_write_lock.fixed

@@ -43,3 +43,7 @@ fn main() {
         *writer1 = *writer2;
     }
 }
+
+fn issue12733(rw: &RwLock<()>) {
+    let _write_guard = rw.write().unwrap();
+}

tests/ui/readonly_write_lock.rs

@@ -43,3 +43,7 @@ fn main() {
         *writer1 = *writer2;
     }
 }
+
+fn issue12733(rw: &RwLock<()>) {
+    let _write_guard = rw.write().unwrap();
+}