Detect when move of !Copy value occurs within loop and should likely not be cloned

When encountering a move error on a value within a loop of any kind,
identify if the moved value belongs to a call expression that should not
be cloned and avoid the semantically incorrect suggestion. Also try to
suggest moving the call expression outside of the loop instead.

```
error[E0382]: use of moved value: `vec`
  --> $DIR/recreating-value-in-loop-condition.rs:6:33
   |
LL |     let vec = vec!["one", "two", "three"];
   |         --- move occurs because `vec` has type `Vec<&str>`, which does not implement the `Copy` trait
LL |     while let Some(item) = iter(vec).next() {
   |     ----------------------------^^^--------
   |     |                           |
   |     |                           value moved here, in previous iteration of loop
   |     inside of this loop
   |
note: consider changing this parameter type in function `iter` to borrow instead if owning the value isn't necessary
  --> $DIR/recreating-value-in-loop-condition.rs:1:17
   |
LL | fn iter<T>(vec: Vec<T>) -> impl Iterator<Item = T> {
   |    ----         ^^^^^^ this parameter takes ownership of the value
   |    |
   |    in this function
help: consider moving the expression out of the loop so it is only moved once
   |
LL ~     let mut value = iter(vec);
LL ~     while let Some(item) = value.next() {
   |
```

We use the presence of a `break` in the loop that would be affected by
the moved value as a heuristic for "shouldn't be cloned".

Fix #121466.

Author: estebank

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -443,8 +443,8 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     err.span_note(
                         span,
                         format!(
-                            "consider changing this parameter type in {descr} `{ident}` to \
-                                 borrow instead if owning the value isn't necessary",
+                            "consider changing this parameter type in {descr} `{ident}` to borrow \
+                             instead if owning the value isn't necessary",
                         ),
                     );
                 }
@@ -738,6 +738,163 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         true
     }
 
+    /// In a move error that occurs on a call wihtin a loop, we try to identify cases where cloning
+    /// the value would lead to a logic error. We infer these cases by seeing if the moved value is
+    /// part of the logic to break the loop, either through an explicit `break` or if the expression
+    /// is part of a `while let`.
+    fn suggest_hoisting_call_outside_loop(
+        &self,
+        err: &mut DiagnosticBuilder<'_>,
+        expr: &hir::Expr<'_>,
+    ) -> bool {
+        let tcx = self.infcx.tcx;
+        let mut can_suggest_clone = true;
+
+        // If the moved value is a locally declared binding, we'll look upwards on the expression
+        // tree until the scope where it is defined, and no further, as suggesting to move the
+        // expression beyond that point would be illogical.
+        let local_hir_id = if let hir::ExprKind::Path(hir::QPath::Resolved(
+            _,
+            hir::Path { res: hir::def::Res::Local(local_hir_id), .. },
+        )) = expr.kind
+        {
+            Some(local_hir_id)
+        } else {
+            // This case would be if the moved value comes from an argument binding, we'll just
+            // look within the entire item, that's fine.
+            None
+        };
+
+        /// This will allow us to look for a specific `HirId`, in our case `local_hir_id` where the
+        /// binding was declared, within any other expression. We'll use it to search for the
+        /// binding declaration within every scope we inspect.
+        struct Finder {
+            hir_id: hir::HirId,
+            found: bool,
+        }
+        impl<'hir> Visitor<'hir> for Finder {
+            fn visit_pat(&mut self, pat: &'hir hir::Pat<'hir>) {
+                if pat.hir_id == self.hir_id {
+                    self.found = true;
+                }
+                hir::intravisit::walk_pat(self, pat);
+            }
+            fn visit_expr(&mut self, ex: &'hir hir::Expr<'hir>) {
+                if ex.hir_id == self.hir_id {
+                    self.found = true;
+                }
+                hir::intravisit::walk_expr(self, ex);
+            }
+        }
+        // The immediate HIR parent of the moved expression. We'll look for it to be a call.
+        let mut parent = None;
+        // The top-most loop where the moved expression could be moved to a new binding.
+        let mut outer_most_loop: Option<&hir::Expr<'_>> = None;
+        for (_, node) in tcx.hir().parent_iter(expr.hir_id) {
+            let e = match node {
+                hir::Node::Expr(e) => e,
+                hir::Node::Local(hir::Local { els: Some(els), .. }) => {
+                    let mut finder = BreakFinder { found_break: false };
+                    finder.visit_block(els);
+                    if finder.found_break {
+                        // Don't suggest clone as it could be will likely end in an infinite
+                        // loop.
+                        // let Some(_) = foo(non_copy.clone()) else { break; }
+                        // ---                       ^^^^^^^^         -----
+                        can_suggest_clone = false;
+                    }
+                    continue;
+                }
+                _ => continue,
+            };
+            if let Some(&hir_id) = local_hir_id {
+                let mut finder = Finder { hir_id, found: false };
+                finder.visit_expr(e);
+                if finder.found {
+                    // The current scope includes the declaration of the binding we're accessing, we
+                    // can't look up any further for loops.
+                    break;
+                }
+            }
+            if parent.is_none() {
+                parent = Some(e);
+            }
+            match e.kind {
+                hir::ExprKind::Let(_) => {
+                    match tcx.parent_hir_node(e.hir_id) {
+                        hir::Node::Expr(hir::Expr {
+                            kind: hir::ExprKind::If(cond, ..), ..
+                        }) => {
+                            let mut finder = Finder { hir_id: expr.hir_id, found: false };
+                            finder.visit_expr(cond);
+                            if finder.found {
+                                // The expression where the move error happened is in a `while let`
+                                // condition Don't suggest clone as it will likely end in an
+                                // infinite loop.
+                                // while let Some(_) = foo(non_copy.clone()) { }
+                                // ---------                       ^^^^^^^^
+                                can_suggest_clone = false;
+                            }
+                        }
+                        _ => {}
+                    }
+                }
+                hir::ExprKind::Loop(..) => {
+                    outer_most_loop = Some(e);
+                }
+                _ => {}
+            }
+        }
+
+        /// Look for `break` expressions within any arbitrary expressions. We'll do this to infer
+        /// whether this is a case where the moved value would affect the exit of a loop, making it
+        /// unsuitable for a `.clone()` suggestion.
+        struct BreakFinder {
+            found_break: bool,
+        }
+        impl<'hir> Visitor<'hir> for BreakFinder {
+            fn visit_expr(&mut self, ex: &'hir hir::Expr<'hir>) {
+                if let hir::ExprKind::Break(..) = ex.kind {
+                    self.found_break = true;
+                }
+                hir::intravisit::walk_expr(self, ex);
+            }
+        }
+        if let Some(in_loop) = outer_most_loop
+            && let Some(parent) = parent
+            && let hir::ExprKind::MethodCall(..) | hir::ExprKind::Call(..) = parent.kind
+        {
+            // FIXME: We could check that the call's *parent* takes `&mut val` to make the
+            // suggestion more targeted to the `mk_iter(val).next()` case. Maybe do that only to
+            // check for wheter to suggest `let value` or `let mut value`.
+
+            let span = in_loop.span;
+            let mut finder = BreakFinder { found_break: false };
+            finder.visit_expr(in_loop);
+            let sm = tcx.sess.source_map();
+            if (finder.found_break || true)
+                && let Ok(value) = sm.span_to_snippet(parent.span)
+            {
+                // We know with high certainty that this move would affect the early return of a
+                // loop, so we suggest moving the expression with the move out of the loop.
+                let indent = if let Some(indent) = sm.indentation_before(span) {
+                    format!("\n{indent}")
+                } else {
+                    " ".to_string()
+                };
+                err.multipart_suggestion(
+                    "consider moving the expression out of the loop so it is only moved once",
+                    vec![
+                        (parent.span, "value".to_string()),
+                        (span.shrink_to_lo(), format!("let mut value = {value};{indent}")),
+                    ],
+                    Applicability::MaybeIncorrect,
+                );
+            }
+        }
+        can_suggest_clone
+    }
+
     fn suggest_cloning(
         &self,
         err: &mut DiagnosticBuilder<'_>,
@@ -746,6 +903,11 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         span: Span,
     ) {
         let tcx = self.infcx.tcx;
+        if !self.suggest_hoisting_call_outside_loop(err, expr) {
+            // The place where the the type moves would be misleading to suggest clone. (#121466)
+            return;
+        }
+
         // Try to find predicates on *generic params* that would allow copying `ty`
         let suggestion =
             if let Some(symbol) = tcx.hir().maybe_get_struct_pattern_shorthand_field(expr) {

compiler/rustc_hir/src/hir.rs

@@ -2049,7 +2049,7 @@ impl LoopSource {
     }
 }
 
-#[derive(Copy, Clone, Debug, HashStable_Generic)]
+#[derive(Copy, Clone, Debug, PartialEq, HashStable_Generic)]
 pub enum LoopIdError {
     OutsideLoopScope,
     UnlabeledCfInWhileCondition,

tests/ui/borrowck/mut-borrow-in-loop-2.fixed

@@ -1,4 +1,3 @@
-//@ run-rustfix
 #![allow(dead_code)]
 
 struct Events<R>(R);

tests/ui/borrowck/mut-borrow-in-loop-2.rs

@@ -1,5 +1,5 @@
 error[E0382]: use of moved value: `value`
-  --> $DIR/mut-borrow-in-loop-2.rs:31:23
+  --> $DIR/mut-borrow-in-loop-2.rs:30:23
    |
 LL | fn this_does_not<'a, R>(value: &'a mut Events<R>) {
    |                         ----- move occurs because `value` has type `&mut Events<R>`, which does not implement the `Copy` trait
@@ -9,12 +9,18 @@ LL |         Other::handle(value);
    |                       ^^^^^ value moved here, in previous iteration of loop
    |
 note: consider changing this parameter type in function `handle` to borrow instead if owning the value isn't necessary
-  --> $DIR/mut-borrow-in-loop-2.rs:9:22
+  --> $DIR/mut-borrow-in-loop-2.rs:8:22
    |
 LL |     fn handle(value: T) -> Self;
    |        ------        ^ this parameter takes ownership of the value
    |        |
    |        in this function
+help: consider moving the expression out of the loop so it is only moved once
+   |
+LL ~     let mut value = Other::handle(value);
+LL ~     for _ in 0..3 {
+LL ~         value;
+   |
 help: consider creating a fresh reborrow of `value` here
    |
 LL |         Other::handle(&mut *value);

tests/ui/borrowck/mut-borrow-in-loop-2.stderr

@@ -16,6 +16,12 @@ LL | fn take(_x: Box<isize>) {}
    |    ----     ^^^^^^^^^^ this parameter takes ownership of the value
    |    |
    |    in this function
+help: consider moving the expression out of the loop so it is only moved once
+   |
+LL ~         let mut value = take(x);
+LL ~         loop {
+LL ~             value;
+   |
 help: consider cloning the value if the performance cost is acceptable
    |
 LL |             take(x.clone());

tests/ui/liveness/liveness-move-call-arg-2.stderr

@@ -16,6 +16,12 @@ LL | fn take(_x: Box<isize>) {}
    |    ----     ^^^^^^^^^^ this parameter takes ownership of the value
    |    |
    |    in this function
+help: consider moving the expression out of the loop so it is only moved once
+   |
+LL ~     let mut value = take(x);
+LL ~     loop {
+LL ~         value;
+   |
 help: consider cloning the value if the performance cost is acceptable
    |
 LL |         take(x.clone());

tests/ui/liveness/liveness-move-call-arg.stderr

@@ -1,5 +1,5 @@
 fn takes_fn(f: impl Fn()) {
-    loop {
+    loop { //~ HELP consider moving the expression out of the loop so it is only computed once
         takes_fnonce(f);
         //~^ ERROR use of moved value
         //~| HELP consider borrowing

tests/ui/moves/borrow-closures-instead-of-move.rs

@@ -15,6 +15,12 @@ LL | fn takes_fnonce(_: impl FnOnce()) {}
    |    ------------    ^^^^^^^^^^^^^ this parameter takes ownership of the value
    |    |
    |    in this function
+help: consider moving the expression out of the loop so it is only moved once
+   |
+LL ~     let mut value = takes_fnonce(f);
+LL ~     loop {
+LL ~         value;
+   |
 help: consider borrowing `f`
    |
 LL |         takes_fnonce(&f);

tests/ui/moves/borrow-closures-instead-of-move.stderr

@@ -0,0 +1,24 @@
+fn main() {
+    let foos = vec![String::new()];
+    let bars = vec![""];
+    let mut baz = vec![];
+    let mut qux = vec![];
+    for foo in foos {
+    //~^ NOTE this reinitialization might get skipped
+    //~| NOTE move occurs because `foo` has type `String`
+        for bar in &bars {
+        //~^ NOTE inside of this loop
+        //~| HELP consider moving the expression out of the loop
+        //~| NOTE in this expansion of desugaring of `for` loop
+            if foo == *bar {
+                baz.push(foo);
+                //~^ NOTE value moved here
+                //~| HELP consider cloning the value
+                continue;
+            }
+        }
+        qux.push(foo);
+        //~^ ERROR use of moved value
+        //~| NOTE value used here
+    }
+}