When setting suggestion, add suggestion for MoveAndClone for non-ref

When trying to set the current suggestion, if the type of the expression
is not a reference and it is not trivially pure clone copy, we should still
trigger and emit a lint message. Since this fix may require cloning an
expensive-to-clone type, do not attempt to offer a suggested fix.

This change means that matches generated from TryDesugar and AwaitDesugar
would normally trigger a lint, but they are out of scope for this lint,
so we will explicitly ignore matches with sources of TryDesugar or
AwaitDesugar.

changelog: Update for [`significant_drop_in_scrutinee`] to correctly
emit lint messages for cases where the type is not a reference and
not trivially pure clone copy.

Author: PrestonFrom

clippy_lints/src/significant_drop_in_scrutinee.rs

@@ -4,7 +4,7 @@ use clippy_utils::get_attr;
 use clippy_utils::source::{indent_of, snippet};
 use rustc_errors::{Applicability, Diagnostic};
 use rustc_hir::intravisit::{walk_expr, Visitor};
-use rustc_hir::{Expr, ExprKind};
+use rustc_hir::{Expr, ExprKind, MatchSource};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::{Ty, TypeAndMut};
@@ -154,9 +154,17 @@ fn has_significant_drop_in_scrutinee<'tcx, 'a>(
     cx: &'a LateContext<'tcx>,
     expr: &'tcx Expr<'tcx>,
 ) -> Option<Vec<FoundSigDrop>> {
-    let mut helper = SigDropHelper::new(cx);
     match expr.kind {
-        ExprKind::Match(match_expr, _, _) => helper.find_sig_drop(match_expr),
+        ExprKind::Match(match_expr, _, source) => {
+            match source {
+                MatchSource::Normal | MatchSource::ForLoopDesugar => {
+                    let mut helper = SigDropHelper::new(cx);
+                    helper.find_sig_drop(match_expr)
+                },
+                // MatchSource of TryDesugar or AwaitDesugar is out of scope for this lint
+                MatchSource::TryDesugar | MatchSource::AwaitDesugar => None,
+            }
+        },
         _ => None,
     }
 }
@@ -213,6 +221,19 @@ impl<'a, 'tcx> SigDropHelper<'a, 'tcx> {
         self.sig_drop_spans.take()
     }
 
+    fn replace_current_sig_drop(
+        &mut self,
+        found_span: Span,
+        is_unit_return_val: bool,
+        lint_suggestion: LintSuggestion,
+    ) {
+        self.current_sig_drop.replace(FoundSigDrop {
+            found_span,
+            is_unit_return_val,
+            lint_suggestion,
+        });
+    }
+
     /// This will try to set the current suggestion (so it can be moved into the suggestions vec
     /// later). If `allow_move_and_clone` is false, the suggestion *won't* be set -- this gives us
     /// an opportunity to look for another type in the chain that will be trivially copyable.
@@ -229,25 +250,15 @@ impl<'a, 'tcx> SigDropHelper<'a, 'tcx> {
             // but let's avoid any chance of an ICE
             if let Some(TypeAndMut { ty, .. }) = ty.builtin_deref(true) {
                 if ty.is_trivially_pure_clone_copy() {
-                    self.current_sig_drop.replace(FoundSigDrop {
-                        found_span: expr.span,
-                        is_unit_return_val: false,
-                        lint_suggestion: LintSuggestion::MoveAndDerefToCopy,
-                    });
+                    self.replace_current_sig_drop(expr.span, false, LintSuggestion::MoveAndDerefToCopy);
                 } else if allow_move_and_clone {
-                    self.current_sig_drop.replace(FoundSigDrop {
-                        found_span: expr.span,
-                        is_unit_return_val: false,
-                        lint_suggestion: LintSuggestion::MoveAndClone,
-                    });
+                    self.replace_current_sig_drop(expr.span, false, LintSuggestion::MoveAndClone);
                 }
             }
         } else if ty.is_trivially_pure_clone_copy() {
-            self.current_sig_drop.replace(FoundSigDrop {
-                found_span: expr.span,
-                is_unit_return_val: false,
-                lint_suggestion: LintSuggestion::MoveOnly,
-            });
+            self.replace_current_sig_drop(expr.span, false, LintSuggestion::MoveOnly);
+        } else if allow_move_and_clone {
+            self.replace_current_sig_drop(expr.span, false, LintSuggestion::MoveAndClone);
         }
     }
 
@@ -279,11 +290,7 @@ impl<'a, 'tcx> SigDropHelper<'a, 'tcx> {
         // If either side had a significant drop, suggest moving the entire scrutinee to avoid
         // unnecessary copies and to simplify cases where both sides have significant drops.
         if self.has_significant_drop {
-            self.current_sig_drop.replace(FoundSigDrop {
-                found_span: span,
-                is_unit_return_val,
-                lint_suggestion: LintSuggestion::MoveOnly,
-            });
+            self.replace_current_sig_drop(span, is_unit_return_val, LintSuggestion::MoveOnly);
         }
 
         self.special_handling_for_binary_op = false;
@@ -363,34 +370,34 @@ impl<'a, 'tcx> Visitor<'tcx> for SigDropHelper<'a, 'tcx> {
                 }
             }
             ExprKind::Box(..) |
-                ExprKind::Array(..) |
-                ExprKind::Call(..) |
-                ExprKind::Unary(..) |
-                ExprKind::If(..) |
-                ExprKind::Match(..) |
-                ExprKind::Field(..) |
-                ExprKind::Index(..) |
-                ExprKind::Ret(..) |
-                ExprKind::Repeat(..) |
-                ExprKind::Yield(..) |
-                ExprKind::MethodCall(..) => walk_expr(self, ex),
+            ExprKind::Array(..) |
+            ExprKind::Call(..) |
+            ExprKind::Unary(..) |
+            ExprKind::If(..) |
+            ExprKind::Match(..) |
+            ExprKind::Field(..) |
+            ExprKind::Index(..) |
+            ExprKind::Ret(..) |
+            ExprKind::Repeat(..) |
+            ExprKind::Yield(..) |
+            ExprKind::MethodCall(..) => walk_expr(self, ex),
             ExprKind::AddrOf(_, _, _) |
-                ExprKind::Block(_, _) |
-                ExprKind::Break(_, _) |
-                ExprKind::Cast(_, _) |
-                // Don't want to check the closure itself, only invocation, which is covered by MethodCall
-                ExprKind::Closure(_, _, _, _, _) |
-                ExprKind::ConstBlock(_) |
-                ExprKind::Continue(_) |
-                ExprKind::DropTemps(_) |
-                ExprKind::Err |
-                ExprKind::InlineAsm(_) |
-                ExprKind::Let(_) |
-                ExprKind::Lit(_) |
-                ExprKind::Loop(_, _, _, _) |
-                ExprKind::Path(_) |
-                ExprKind::Struct(_, _, _) |
-                ExprKind::Type(_, _) => {
+            ExprKind::Block(_, _) |
+            ExprKind::Break(_, _) |
+            ExprKind::Cast(_, _) |
+            // Don't want to check the closure itself, only invocation, which is covered by MethodCall
+            ExprKind::Closure(_, _, _, _, _) |
+            ExprKind::ConstBlock(_) |
+            ExprKind::Continue(_) |
+            ExprKind::DropTemps(_) |
+            ExprKind::Err |
+            ExprKind::InlineAsm(_) |
+            ExprKind::Let(_) |
+            ExprKind::Lit(_) |
+            ExprKind::Loop(_, _, _, _) |
+            ExprKind::Path(_) |
+            ExprKind::Struct(_, _, _) |
+            ExprKind::Type(_, _) => {
                 return;
             }
         }

clippy_utils/src/lib.rs

@@ -2073,7 +2073,8 @@ static TEST_ITEM_NAMES_CACHE: SyncOnceCell<Mutex<FxHashMap<LocalDefId, Vec<Symbo
 fn with_test_item_names<'tcx>(tcx: TyCtxt<'tcx>, module: LocalDefId, f: impl Fn(&[Symbol]) -> bool) -> bool {
     let cache = TEST_ITEM_NAMES_CACHE.get_or_init(|| Mutex::new(FxHashMap::default()));
     let mut map: MutexGuard<'_, FxHashMap<LocalDefId, Vec<Symbol>>> = cache.lock().unwrap();
-    match map.entry(module) {
+    let value = map.entry(module);
+    match value {
         Entry::Occupied(entry) => f(entry.get()),
         Entry::Vacant(entry) => {
             let mut names = Vec::new();

tests/ui/significant_drop_in_scrutinee.rs

@@ -7,8 +7,10 @@
 #![allow(unused_assignments)]
 #![allow(dead_code)]
 
+use std::num::ParseIntError;
 use std::ops::Deref;
 use std::sync::atomic::{AtomicU64, Ordering};
+use std::sync::RwLock;
 use std::sync::{Mutex, MutexGuard};
 
 struct State {}
@@ -552,4 +554,41 @@ fn should_not_cause_stack_overflow() {
     }
 }
 
+fn should_not_produce_lint_for_try_desugar() -> Result<u64, ParseIntError> {
+    // TryDesugar (i.e. using `?` for a Result type) will turn into a match but is out of scope
+    // for this lint
+    let rwlock = RwLock::new("1".to_string());
+    let result = rwlock.read().unwrap().parse::<u64>()?;
+    println!("{}", result);
+    rwlock.write().unwrap().push('2');
+    Ok(result)
+}
+
+struct ResultReturner {
+    s: String,
+}
+
+impl ResultReturner {
+    fn to_number(&self) -> Result<i64, ParseIntError> {
+        self.s.parse::<i64>()
+    }
+}
+
+fn should_trigger_lint_for_non_ref_move_and_clone_suggestion() {
+    let rwlock = RwLock::<ResultReturner>::new(ResultReturner { s: "1".to_string() });
+    match rwlock.read().unwrap().to_number() {
+        Ok(n) => println!("Converted to number: {}", n),
+        Err(e) => println!("Could not convert {} to number", e),
+    };
+}
+
+fn should_trigger_lint_for_read_write_lock_for_loop() {
+    // For-in loops desugar to match expressions and are prone to the type of deadlock this lint is
+    // designed to look for.
+    let rwlock = RwLock::<Vec<String>>::new(vec!["1".to_string()]);
+    for s in rwlock.read().unwrap().iter() {
+        println!("{}", s);
+    }
+}
+
 fn main() {}