[significant_drop_tightening] Ignore inexpensive statements

Author: c410-f3r

clippy_lints/src/significant_drop_tightening.rs

@@ -61,6 +61,17 @@ pub struct SignificantDropTightening<'tcx> {
 }
 
 impl<'tcx> SignificantDropTightening<'tcx> {
+    fn at_least_one_stmt_is_expensive(stmts: &[hir::Stmt<'_>]) -> bool {
+        for stmt in stmts {
+            match stmt.kind {
+                hir::StmtKind::Local(local) if let Some(expr) = local.init
+                    && let hir::ExprKind::Path(_) = expr.kind => {},
+                _ => return true
+            };
+        }
+        false
+    }
+
     /// Verifies if the expression is of type `drop(some_lock_path)` to assert that the temporary
     /// is already being dropped before the end of its scope.
     fn has_drop(expr: &'tcx hir::Expr<'_>, init_bind_ident: Ident) -> bool {
@@ -201,10 +212,12 @@ impl<'tcx> LateLintPass<'tcx> for SignificantDropTightening<'tcx> {
                 _ => continue
             };
         }
-        if sdap.number_of_stmts > 1 && {
-            let last_stmts_idx = block.stmts.len().wrapping_sub(1);
-            sdap.last_use_stmt_idx != last_stmts_idx
-        } {
+        let stmts_after_last_use = sdap
+            .last_use_stmt_idx
+            .checked_add(1)
+            .and_then(|idx| block.stmts.get(idx..))
+            .unwrap_or_default();
+        if sdap.number_of_stmts > 1 && Self::at_least_one_stmt_is_expensive(stmts_after_last_use) {
             span_lint_and_then(
                 cx,
                 SIGNIFICANT_DROP_TIGHTENING,

tests/ui/significant_drop_tightening.fixed

@@ -4,6 +4,14 @@
 
 use std::sync::Mutex;
 
+pub fn post_bindings_can_be_ignored() {
+    let mutex = Mutex::new(1);
+    let lock = mutex.lock().unwrap();
+    let rslt = *lock;
+    let another = rslt;
+    let _ = another;
+}
+
 pub fn unnecessary_contention_with_multiple_owned_results() {
     {
         let mutex = Mutex::new(1i32);

tests/ui/significant_drop_tightening.rs

@@ -4,6 +4,14 @@
 
 use std::sync::Mutex;
 
+pub fn post_bindings_can_be_ignored() {
+    let mutex = Mutex::new(1);
+    let lock = mutex.lock().unwrap();
+    let rslt = *lock;
+    let another = rslt;
+    let _ = another;
+}
+
 pub fn unnecessary_contention_with_multiple_owned_results() {
     {
         let mutex = Mutex::new(1i32);

tests/ui/significant_drop_tightening.stderr

@@ -1,5 +1,5 @@
 error: temporary with significant `Drop` can be early dropped
-  --> $DIR/significant_drop_tightening.rs:17:13
+  --> $DIR/significant_drop_tightening.rs:25:13
    |
 LL | /     {
 LL | |         let mutex = Mutex::new(1i32);
@@ -20,7 +20,7 @@ LL +         drop(lock);
    |
 
 error: temporary with significant `Drop` can be early dropped
-  --> $DIR/significant_drop_tightening.rs:38:13
+  --> $DIR/significant_drop_tightening.rs:46:13
    |
 LL | /     {
 LL | |         let mutex = Mutex::new(1i32);
@@ -44,7 +44,7 @@ LL +
    |
 
 error: temporary with significant `Drop` can be early dropped
-  --> $DIR/significant_drop_tightening.rs:44:17
+  --> $DIR/significant_drop_tightening.rs:52:17
    |
 LL | /     {
 LL | |         let mutex = Mutex::new(vec![1i32]);