Auto merge of #10368 - c410-f3r:lock-1, r=xFrednet

[significant_drop_tightening] Evaluate the return expression of a block

For whatever reason, the return expression of a block is not contained inside the slice of statements and because of that the lint wasn't evaluating things that could potentially block the release of a lock.

```rust
pub fn example() -> i32 {
    let mutex = Mutex::new(1);
    let lock = mutex.lock().unwrap();
    let _ = *lock;
    let _ = *lock;
   do_heavy_computation_that_takes_time_and_returns_i32()
}
```

---

changelog: none
<!-- changelog_checked -->

Author: bors

clippy_lints/src/significant_drop_tightening.rs

@@ -61,10 +61,26 @@ pub struct SignificantDropTightening<'tcx> {
 }
 
 impl<'tcx> SignificantDropTightening<'tcx> {
+    /// Unifies the statements of a block with its return expression.
+    fn all_block_stmts<'ret, 'rslt, 'stmts>(
+        block_stmts: &'stmts [hir::Stmt<'tcx>],
+        dummy_ret_stmt: Option<&'ret hir::Stmt<'tcx>>,
+    ) -> impl Iterator<Item = &'rslt hir::Stmt<'tcx>>
+    where
+        'ret: 'rslt,
+        'stmts: 'rslt,
+    {
+        block_stmts.iter().chain(dummy_ret_stmt)
+    }
+
     /// Searches for at least one statement that could slow down the release of a significant drop.
-    fn at_least_one_stmt_is_expensive(stmts: &[hir::Stmt<'_>]) -> bool {
+    fn at_least_one_stmt_is_expensive<'stmt>(stmts: impl Iterator<Item = &'stmt hir::Stmt<'tcx>>) -> bool
+    where
+        'tcx: 'stmt,
+    {
         for stmt in stmts {
             match stmt.kind {
+                hir::StmtKind::Expr(expr) if let hir::ExprKind::Path(_) = expr.kind => {}
                 hir::StmtKind::Local(local) if let Some(expr) = local.init
                     && let hir::ExprKind::Path(_) = expr.kind => {},
                 _ => return true
@@ -99,7 +115,7 @@ impl<'tcx> SignificantDropTightening<'tcx> {
         expr: &'tcx hir::Expr<'_>,
         idx: usize,
         sdap: &mut SigDropAuxParams,
-        stmt: &'tcx hir::Stmt<'_>,
+        stmt: &hir::Stmt<'_>,
         cb: impl Fn(&mut SigDropAuxParams),
     ) {
         let mut sig_drop_finder = SigDropFinder::new(cx, &mut self.seen_types);
@@ -117,7 +133,7 @@ impl<'tcx> SignificantDropTightening<'tcx> {
         }
     }
 
-    /// Shows a generic overall message as well as specialized messages depending on the usage.
+    /// Shows generic overall messages as well as specialized messages depending on the usage.
     fn set_suggestions(cx: &LateContext<'tcx>, block_span: Span, diag: &mut Diagnostic, sdap: &SigDropAuxParams) {
         match sdap.number_of_stmts {
             0 | 1 => {},
@@ -172,8 +188,13 @@ impl<'tcx> SignificantDropTightening<'tcx> {
 
 impl<'tcx> LateLintPass<'tcx> for SignificantDropTightening<'tcx> {
     fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx hir::Block<'_>) {
+        let dummy_ret_stmt = block.expr.map(|expr| hir::Stmt {
+            hir_id: hir::HirId::INVALID,
+            kind: hir::StmtKind::Expr(expr),
+            span: DUMMY_SP,
+        });
         let mut sdap = SigDropAuxParams::default();
-        for (idx, stmt) in block.stmts.iter().enumerate() {
+        for (idx, stmt) in Self::all_block_stmts(block.stmts, dummy_ret_stmt.as_ref()).enumerate() {
             match stmt.kind {
                 hir::StmtKind::Expr(expr) => self.modify_sdap_if_sig_drop_exists(
                     cx,
@@ -213,11 +234,9 @@ impl<'tcx> LateLintPass<'tcx> for SignificantDropTightening<'tcx> {
                 _ => {}
             };
         }
-        let stmts_after_last_use = sdap
-            .last_use_stmt_idx
-            .checked_add(1)
-            .and_then(|idx| block.stmts.get(idx..))
-            .unwrap_or_default();
+
+        let idx = sdap.last_use_stmt_idx.wrapping_add(1);
+        let stmts_after_last_use = Self::all_block_stmts(block.stmts, dummy_ret_stmt.as_ref()).skip(idx);
         if sdap.number_of_stmts > 1 && Self::at_least_one_stmt_is_expensive(stmts_after_last_use) {
             span_lint_and_then(
                 cx,

tests/ui/significant_drop_tightening.fixed

@@ -4,6 +4,26 @@
 
 use std::sync::Mutex;
 
+pub fn complex_return_triggers_the_lint() -> i32 {
+    fn foo() -> i32 {
+        1
+    }
+    let mutex = Mutex::new(1);
+    let lock = mutex.lock().unwrap();
+    let _ = *lock;
+    let _ = *lock;
+    drop(lock);
+    foo()
+}
+
+pub fn path_return_can_be_ignored() -> i32 {
+    let mutex = Mutex::new(1);
+    let lock = mutex.lock().unwrap();
+    let rslt = *lock;
+    let _ = *lock;
+    rslt
+}
+
 pub fn post_bindings_can_be_ignored() {
     let mutex = Mutex::new(1);
     let lock = mutex.lock().unwrap();

tests/ui/significant_drop_tightening.rs

@@ -4,6 +4,25 @@
 
 use std::sync::Mutex;
 
+pub fn complex_return_triggers_the_lint() -> i32 {
+    fn foo() -> i32 {
+        1
+    }
+    let mutex = Mutex::new(1);
+    let lock = mutex.lock().unwrap();
+    let _ = *lock;
+    let _ = *lock;
+    foo()
+}
+
+pub fn path_return_can_be_ignored() -> i32 {
+    let mutex = Mutex::new(1);
+    let lock = mutex.lock().unwrap();
+    let rslt = *lock;
+    let _ = *lock;
+    rslt
+}
+
 pub fn post_bindings_can_be_ignored() {
     let mutex = Mutex::new(1);
     let lock = mutex.lock().unwrap();

tests/ui/significant_drop_tightening.stderr

@@ -1,5 +1,29 @@
 error: temporary with significant `Drop` can be early dropped
-  --> $DIR/significant_drop_tightening.rs:25:13
+  --> $DIR/significant_drop_tightening.rs:12:9
+   |
+LL |   pub fn complex_return_triggers_the_lint() -> i32 {
+   |  __________________________________________________-
+LL | |     fn foo() -> i32 {
+LL | |         1
+LL | |     }
+LL | |     let mutex = Mutex::new(1);
+LL | |     let lock = mutex.lock().unwrap();
+   | |         ^^^^
+...  |
+LL | |     foo()
+LL | | }
+   | |_- temporary `lock` is currently being dropped at the end of its contained scope
+   |
+   = note: this might lead to unnecessary resource contention
+   = note: `-D clippy::significant-drop-tightening` implied by `-D warnings`
+help: drop the temporary after the end of its last usage
+   |
+LL ~     let _ = *lock;
+LL +     drop(lock);
+   |
+
+error: temporary with significant `Drop` can be early dropped
+  --> $DIR/significant_drop_tightening.rs:44:13
    |
 LL | /     {
 LL | |         let mutex = Mutex::new(1i32);
@@ -12,15 +36,14 @@ LL | |     }
    | |_____- temporary `lock` is currently being dropped at the end of its contained scope
    |
    = note: this might lead to unnecessary resource contention
-   = note: `-D clippy::significant-drop-tightening` implied by `-D warnings`
 help: drop the temporary after the end of its last usage
    |
 LL ~         let rslt1 = lock.is_positive();
 LL +         drop(lock);
    |
 
 error: temporary with significant `Drop` can be early dropped
-  --> $DIR/significant_drop_tightening.rs:46:13
+  --> $DIR/significant_drop_tightening.rs:65:13
    |
 LL | /     {
 LL | |         let mutex = Mutex::new(1i32);
@@ -44,7 +67,7 @@ LL +
    |
 
 error: temporary with significant `Drop` can be early dropped
-  --> $DIR/significant_drop_tightening.rs:52:17
+  --> $DIR/significant_drop_tightening.rs:71:17
    |
 LL | /     {
 LL | |         let mutex = Mutex::new(vec![1i32]);
@@ -67,5 +90,5 @@ LL -         lock.clear();
 LL +         
    |
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors