Suggest changing iter/into_iter when the other was meant

compiler/rustc_span/src/symbol.rs

@@ -270,6 +270,7 @@ symbols! {
         Into,
         IntoFuture,
         IntoIterator,
+        IntoIteratorItem,
         IoBufRead,
         IoLines,
         IoRead,

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -4390,6 +4390,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         param_env: ty::ParamEnv<'tcx>,
         path_segment: &hir::PathSegment<'_>,
         args: &[hir::Expr<'_>],
+        prev_ty: Ty<'_>,
         err: &mut Diag<'_, G>,
     ) {
         let tcx = self.tcx;
@@ -4403,6 +4404,47 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 let TypeError::Sorts(expected_found) = diff else {
                     continue;
                 };
+                if tcx.is_diagnostic_item(sym::IntoIteratorItem, *def_id)
+                    && path_segment.ident.name == sym::iter
+                    && self.can_eq(
+                        param_env,
+                        Ty::new_ref(
+                            tcx,
+                            tcx.lifetimes.re_erased,
+                            expected_found.found,
+                            ty::Mutability::Not,
+                        ),
+                        *ty,
+                    )
+                    && let [] = args
+                {
+                    // Used `.iter()` when `.into_iter()` was likely meant.
+                    err.span_suggestion_verbose(
+                        path_segment.ident.span,
+                        format!("consider consuming the `{prev_ty}` to construct the `Iterator`"),
+                        "into_iter".to_string(),
+                        Applicability::MachineApplicable,
+                    );
+                }
+                if tcx.is_diagnostic_item(sym::IntoIteratorItem, *def_id)
+                    && path_segment.ident.name == sym::into_iter
+                    && self.can_eq(
+                        param_env,
+                        expected_found.found,
+                        Ty::new_ref(tcx, tcx.lifetimes.re_erased, *ty, ty::Mutability::Not),
+                    )
+                    && let [] = args
+                {
+                    // Used `.into_iter()` when `.iter()` was likely meant.
+                    err.span_suggestion_verbose(
+                        path_segment.ident.span,
+                        format!(
+                            "consider not consuming the `{prev_ty}` to construct the `Iterator`"
+                        ),
+                        "iter".to_string(),
+                        Applicability::MachineApplicable,
+                    );
+                }
                 if tcx.is_diagnostic_item(sym::IteratorItem, *def_id)
                     && path_segment.ident.name == sym::map
                     && self.can_eq(param_env, expected_found.found, *ty)
@@ -4515,19 +4557,20 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             expr = rcvr_expr;
             let assocs_in_this_method =
                 self.probe_assoc_types_at_expr(&type_diffs, span, prev_ty, expr.hir_id, param_env);
+            prev_ty = self.resolve_vars_if_possible(
+                typeck_results.expr_ty_adjusted_opt(expr).unwrap_or(Ty::new_misc_error(tcx)),
+            );
             self.look_for_iterator_item_mistakes(
                 &assocs_in_this_method,
                 typeck_results,
                 &type_diffs,
                 param_env,
                 path_segment,
                 args,
+                prev_ty,
                 err,
             );
             assocs.push(assocs_in_this_method);
-            prev_ty = self.resolve_vars_if_possible(
-                typeck_results.expr_ty_adjusted_opt(expr).unwrap_or(Ty::new_misc_error(tcx)),
-            );
 
             if let hir::ExprKind::Path(hir::QPath::Resolved(None, path)) = expr.kind
                 && let hir::Path { res: Res::Local(hir_id), .. } = path

library/core/src/iter/traits/collect.rs

@@ -281,6 +281,7 @@ pub trait FromIterator<A>: Sized {
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait IntoIterator {
     /// The type of the elements being iterated over.
+    #[rustc_diagnostic_item = "IntoIteratorItem"]
     #[stable(feature = "rust1", since = "1.0.0")]
     type Item;
 

tests/ui/iterators/into_iter-when-iter-was-intended.fixed

@@ -0,0 +1,10 @@
+//@ run-rustfix
+//@ edition:2021
+// Suggest using the right `IntoIterator` method. #68095
+fn main() {
+    let _a = [0, 1, 2].iter().chain([3, 4, 5].iter()); //~ ERROR E0271
+    let _b = [0, 1, 2].into_iter().chain([3, 4, 5].into_iter()); //~ ERROR E0271
+    // These don't have appropriate suggestions yet.
+    // let c = [0, 1, 2].iter().chain([3, 4, 5]);
+    // let d = [0, 1, 2].iter().chain(vec![3, 4, 5]);
+}

tests/ui/iterators/into_iter-when-iter-was-intended.rs

@@ -0,0 +1,10 @@
+//@ run-rustfix
+//@ edition:2021
+// Suggest using the right `IntoIterator` method. #68095
+fn main() {
+    let _a = [0, 1, 2].iter().chain([3, 4, 5].into_iter()); //~ ERROR E0271
+    let _b = [0, 1, 2].into_iter().chain([3, 4, 5].iter()); //~ ERROR E0271
+    // These don't have appropriate suggestions yet.
+    // let c = [0, 1, 2].iter().chain([3, 4, 5]);
+    // let d = [0, 1, 2].iter().chain(vec![3, 4, 5]);
+}

tests/ui/iterators/into_iter-when-iter-was-intended.stderr

@@ -0,0 +1,48 @@
+error[E0271]: type mismatch resolving `<IntoIter<{integer}, 3> as IntoIterator>::Item == &{integer}`
+  --> $DIR/into_iter-when-iter-was-intended.rs:5:37
+   |
+LL |     let _a = [0, 1, 2].iter().chain([3, 4, 5].into_iter());
+   |                               ----- ^^^^^^^^^^^^^^^^^^^^^ expected `&{integer}`, found integer
+   |                               |
+   |                               required by a bound introduced by this call
+   |
+note: the method call chain might not have had the expected associated types
+  --> $DIR/into_iter-when-iter-was-intended.rs:5:47
+   |
+LL |     let _a = [0, 1, 2].iter().chain([3, 4, 5].into_iter());
+   |                                     --------- ^^^^^^^^^^^ `IntoIterator::Item` is `{integer}` here
+   |                                     |
+   |                                     this expression has type `[{integer}; 3]`
+note: required by a bound in `std::iter::Iterator::chain`
+  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
+help: consider not consuming the `[{integer}; 3]` to construct the `Iterator`
+   |
+LL -     let _a = [0, 1, 2].iter().chain([3, 4, 5].into_iter());
+LL +     let _a = [0, 1, 2].iter().chain([3, 4, 5].iter());
+   |
+
+error[E0271]: type mismatch resolving `<Iter<'_, {integer}> as IntoIterator>::Item == {integer}`
+  --> $DIR/into_iter-when-iter-was-intended.rs:6:42
+   |
+LL |     let _b = [0, 1, 2].into_iter().chain([3, 4, 5].iter());
+   |                                    ----- ^^^^^^^^^^^^^^^^ expected integer, found `&{integer}`
+   |                                    |
+   |                                    required by a bound introduced by this call
+   |
+note: the method call chain might not have had the expected associated types
+  --> $DIR/into_iter-when-iter-was-intended.rs:6:52
+   |
+LL |     let _b = [0, 1, 2].into_iter().chain([3, 4, 5].iter());
+   |                                          --------- ^^^^^^ `IntoIterator::Item` is `&{integer}` here
+   |                                          |
+   |                                          this expression has type `[{integer}; 3]`
+note: required by a bound in `std::iter::Iterator::chain`
+  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
+help: consider consuming the `&[{integer}]` to construct the `Iterator`
+   |
+LL |     let _b = [0, 1, 2].into_iter().chain([3, 4, 5].into_iter());
+   |                                                    +++++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0271`.