Avoid code generation for ThinVec<Diagnostic>'s destructor in the query system

Cargo.lock

@@ -5353,9 +5353,9 @@ dependencies = [
 
 [[package]]
 name = "thin-vec"
-version = "0.2.12"
+version = "0.2.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "aac81b6fd6beb5884b0cf3321b8117e6e5d47ecb6fc89f414cfdcca8b2fe2dd8"
+checksum = "a38c90d48152c236a3ab59271da4f4ae63d678c5d7ad6b7714d7cb9760be5e4b"
 
 [[package]]
 name = "thiserror"

compiler/rustc_query_system/src/dep_graph/graph.rs

@@ -891,7 +891,7 @@ impl<D: Deps> DepGraphData<D> {
                       insertion for {dep_node:?}"
         );
 
-        if !side_effects.is_empty() {
+        if side_effects.maybe_any() {
             qcx.dep_context().dep_graph().with_query_deserialization(|| {
                 self.emit_side_effects(qcx, dep_node_index, side_effects)
             });

compiler/rustc_query_system/src/query/mod.rs

@@ -93,10 +93,13 @@ pub struct QuerySideEffects {
 }
 
 impl QuerySideEffects {
+    /// Returns true if there might be side effects.
     #[inline]
-    pub fn is_empty(&self) -> bool {
+    pub fn maybe_any(&self) -> bool {
         let QuerySideEffects { diagnostics } = self;
-        diagnostics.is_empty()
+        // Use `has_capacity` so that the destructor for `self.diagnostics` can be skipped
+        // if `maybe_any` is known to be false.
+        diagnostics.has_capacity()
     }
     pub fn append(&mut self, other: QuerySideEffects) {
         let QuerySideEffects { diagnostics } = self;

compiler/rustc_query_system/src/query/plumbing.rs

@@ -538,10 +538,9 @@ where
 
     prof_timer.finish_with_query_invocation_id(dep_node_index.into());
 
-    let diagnostics = diagnostics.into_inner();
-    let side_effects = QuerySideEffects { diagnostics };
+    let side_effects = QuerySideEffects { diagnostics: diagnostics.into_inner() };
 
-    if std::intrinsics::unlikely(!side_effects.is_empty()) {
+    if std::intrinsics::unlikely(side_effects.maybe_any()) {
         if query.anon() {
             qcx.store_side_effects_for_anon_node(dep_node_index, side_effects);
         } else {