diff --git a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
index a068f2d69268b..93eef4d5966b4 100644
--- a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
@@ -1,6 +1,5 @@
-use std::iter;
-
use either::Either;
+use hir::PatField;
use rustc_data_structures::captures::Captures;
use rustc_data_structures::fx::FxIndexSet;
use rustc_errors::{
@@ -28,6 +27,7 @@ use rustc_span::symbol::{kw, sym, Ident};
use rustc_span::{BytePos, Span, Symbol};
use rustc_trait_selection::infer::InferCtxtExt;
use rustc_trait_selection::traits::ObligationCtxt;
+use std::iter;
use crate::borrow_set::TwoPhaseActivation;
use crate::borrowck_errors;
@@ -992,6 +992,11 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
issued_borrow.borrowed_place,
&issued_spans,
);
+ self.explain_iterator_advancement_in_for_loop_if_applicable(
+ &mut err,
+ span,
+ &issued_spans,
+ );
err
}
@@ -1278,6 +1283,73 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
}
}
+ /// Suggest using `while let` for call `next` on an iterator in a for loop.
+ ///
+ /// For example:
+ /// ```ignore (illustrative)
+ ///
+ /// for x in iter {
+ /// ...
+ /// iter.next()
+ /// }
+ /// ```
+ pub(crate) fn explain_iterator_advancement_in_for_loop_if_applicable(
+ &self,
+ err: &mut Diagnostic,
+ span: Span,
+ issued_spans: &UseSpans<'tcx>,
+ ) {
+ let issue_span = issued_spans.args_or_use();
+ let tcx = self.infcx.tcx;
+ let hir = tcx.hir();
+
+ let Some(body_id) = hir.get(self.mir_hir_id()).body_id() else { return };
+ let typeck_results = tcx.typeck(self.mir_def_id());
+
+ struct ExprFinder<'hir> {
+ issue_span: Span,
+ expr_span: Span,
+ body_expr: Option<&'hir hir::Expr<'hir>>,
+ loop_bind: Option<Symbol>,
+ }
+ impl<'hir> Visitor<'hir> for ExprFinder<'hir> {
+ fn visit_expr(&mut self, ex: &'hir hir::Expr<'hir>) {
+ if let hir::ExprKind::Loop(hir::Block{ stmts: [stmt, ..], ..}, _, hir::LoopSource::ForLoop, _) = ex.kind &&
+ let hir::StmtKind::Expr(hir::Expr{ kind: hir::ExprKind::Match(call, [_, bind, ..], _), ..}) = stmt.kind &&
+ let hir::ExprKind::Call(path, _args) = call.kind &&
+ let hir::ExprKind::Path(hir::QPath::LangItem(LangItem::IteratorNext, _, _, )) = path.kind &&
+ let hir::PatKind::Struct(path, [field, ..], _) = bind.pat.kind &&
+ let hir::QPath::LangItem(LangItem::OptionSome, _, _) = path &&
+ let PatField { pat: hir::Pat{ kind: hir::PatKind::Binding(_, _, ident, ..), .. }, ..} = field &&
+ self.issue_span.source_equal(call.span) {
+ self.loop_bind = Some(ident.name);
+ }
+
+ if let hir::ExprKind::MethodCall(body_call, _recv, ..) = ex.kind &&
+ body_call.ident.name == sym::next && ex.span.source_equal(self.expr_span) {
+ self.body_expr = Some(ex);
+ }
+
+ hir::intravisit::walk_expr(self, ex);
+ }
+ }
+ let mut finder =
+ ExprFinder { expr_span: span, issue_span, loop_bind: None, body_expr: None };
+ finder.visit_expr(hir.body(body_id).value);
+
+ if let Some(loop_bind) = finder.loop_bind &&
+ let Some(body_expr) = finder.body_expr &&
+ let Some(def_id) = typeck_results.type_dependent_def_id(body_expr.hir_id) &&
+ let Some(trait_did) = tcx.trait_of_item(def_id) &&
+ tcx.is_diagnostic_item(sym::Iterator, trait_did) {
+ err.note(format!(
+ "a for loop advances the iterator for you, the result is stored in `{}`.",
+ loop_bind
+ ));
+ err.help("if you want to call `next` on a iterator within the loop, consider using `while let`.");
+ }
+ }
+
/// Suggest using closure argument instead of capture.
///
/// For example:
diff --git a/compiler/rustc_borrowck/src/diagnostics/mod.rs b/compiler/rustc_borrowck/src/diagnostics/mod.rs
index 2f8c970f806d3..42e50fd0fad07 100644
--- a/compiler/rustc_borrowck/src/diagnostics/mod.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/mod.rs
@@ -1146,6 +1146,12 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
// Avoid pointing to the same function in multiple different
// error messages.
if span != DUMMY_SP && self.fn_self_span_reported.insert(self_arg.span) {
+ self.explain_iterator_advancement_in_for_loop_if_applicable(
+ err,
+ span,
+ &move_spans,
+ );
+
let func = tcx.def_path_str(method_did);
err.subdiagnostic(CaptureReasonNote::FuncTakeSelf {
func,
diff --git a/tests/ui/suggestions/issue-102972.rs b/tests/ui/suggestions/issue-102972.rs
new file mode 100644
index 0000000000000..106288b054d5d
--- /dev/null
+++ b/tests/ui/suggestions/issue-102972.rs
@@ -0,0 +1,16 @@
+fn test1() {
+ let mut chars = "Hello".chars();
+ for _c in chars.by_ref() {
+ chars.next(); //~ ERROR cannot borrow `chars` as mutable more than once at a time
+ }
+}
+
+fn test2() {
+ let v = vec![1, 2, 3];
+ let mut iter = v.iter();
+ for _i in iter {
+ iter.next(); //~ ERROR borrow of moved value: `iter`
+ }
+}
+
+fn main() { }
diff --git a/tests/ui/suggestions/issue-102972.stderr b/tests/ui/suggestions/issue-102972.stderr
new file mode 100644
index 0000000000000..03f9dbb6c895a
--- /dev/null
+++ b/tests/ui/suggestions/issue-102972.stderr
@@ -0,0 +1,33 @@
+error[E0499]: cannot borrow `chars` as mutable more than once at a time
+ --> $DIR/issue-102972.rs:4:9
+ |
+LL | for _c in chars.by_ref() {
+ | --------------
+ | |
+ | first mutable borrow occurs here
+ | first borrow later used here
+LL | chars.next();
+ | ^^^^^^^^^^^^ second mutable borrow occurs here
+ |
+ = note: a for loop advances the iterator for you, the result is stored in `_c`.
+ = help: if you want to call `next` on a iterator within the loop, consider using `while let`.
+
+error[E0382]: borrow of moved value: `iter`
+ --> $DIR/issue-102972.rs:12:9
+ |
+LL | let mut iter = v.iter();
+ | -------- move occurs because `iter` has type `std::slice::Iter<'_, i32>`, which does not implement the `Copy` trait
+LL | for _i in iter {
+ | ---- `iter` moved due to this implicit call to `.into_iter()`
+LL | iter.next();
+ | ^^^^^^^^^^^ value borrowed here after move
+ |
+ = note: a for loop advances the iterator for you, the result is stored in `_i`.
+ = help: if you want to call `next` on a iterator within the loop, consider using `while let`.
+note: `into_iter` takes ownership of the receiver `self`, which moves `iter`
+ --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0382, E0499.
+For more information about an error, try `rustc --explain E0382`.