Suggest &str.chars() on attempt to &str.iter()

compiler/rustc_typeck/src/check/method/suggest.rs

@@ -478,6 +478,26 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 let mut label_span_not_found = || {
                     if unsatisfied_predicates.is_empty() {
                         err.span_label(span, format!("{item_kind} not found in `{ty_str}`"));
+                        let is_string_or_ref_str = match actual.kind() {
+                            ty::Ref(_, ty, _) => {
+                                ty.is_str()
+                                    || matches!(
+                                        ty.kind(),
+                                        ty::Adt(adt, _) if self.tcx.is_diagnostic_item(sym::String, adt.did)
+                                    )
+                            }
+                            ty::Adt(adt, _) => self.tcx.is_diagnostic_item(sym::String, adt.did),
+                            _ => false,
+                        };
+                        if is_string_or_ref_str && item_name.name == sym::iter {
+                            err.span_suggestion_verbose(
+                                item_name.span,
+                                "because of the in-memory representation of `&str`, to obtain \
+                                 an `Iterator` over each of its codepoint use method `chars`",
+                                String::from("chars"),
+                                Applicability::MachineApplicable,
+                            );
+                        }
                         if let ty::Adt(adt, _) = rcvr_ty.kind() {
                             let mut inherent_impls_candidate = self
                                 .tcx

src/test/ui/suggest-using-chars.rs

@@ -0,0 +1,7 @@
+pub fn main() {
+    let _ = "foo".iter(); //~ ERROR no method named `iter` found for reference `&'static str` in the current scope
+    let _ = "foo".foo(); //~ ERROR no method named `foo` found for reference `&'static str` in the current scope
+    let _ = String::from("bar").iter(); //~ ERROR no method named `iter` found for struct `String` in the current scope
+    let _ = (&String::from("bar")).iter(); //~ ERROR no method named `iter` found for reference `&String` in the current scope
+    let _ = 0.iter(); //~ ERROR no method named `iter` found for type `{integer}` in the current scope
+}

src/test/ui/suggest-using-chars.stderr

@@ -0,0 +1,48 @@
+error[E0599]: no method named `iter` found for reference `&'static str` in the current scope
+  --> $DIR/suggest-using-chars.rs:2:19
+   |
+LL |     let _ = "foo".iter();
+   |                   ^^^^ method not found in `&'static str`
+   |
+help: because of the in-memory representation of `&str`, to obtain an `Iterator` over each of its codepoint use method `chars`
+   |
+LL |     let _ = "foo".chars();
+   |                   ~~~~~
+
+error[E0599]: no method named `foo` found for reference `&'static str` in the current scope
+  --> $DIR/suggest-using-chars.rs:3:19
+   |
+LL |     let _ = "foo".foo();
+   |                   ^^^ method not found in `&'static str`
+
+error[E0599]: no method named `iter` found for struct `String` in the current scope
+  --> $DIR/suggest-using-chars.rs:4:33
+   |
+LL |     let _ = String::from("bar").iter();
+   |                                 ^^^^ method not found in `String`
+   |
+help: because of the in-memory representation of `&str`, to obtain an `Iterator` over each of its codepoint use method `chars`
+   |
+LL |     let _ = String::from("bar").chars();
+   |                                 ~~~~~
+
+error[E0599]: no method named `iter` found for reference `&String` in the current scope
+  --> $DIR/suggest-using-chars.rs:5:36
+   |
+LL |     let _ = (&String::from("bar")).iter();
+   |                                    ^^^^ method not found in `&String`
+   |
+help: because of the in-memory representation of `&str`, to obtain an `Iterator` over each of its codepoint use method `chars`
+   |
+LL |     let _ = (&String::from("bar")).chars();
+   |                                    ~~~~~
+
+error[E0599]: no method named `iter` found for type `{integer}` in the current scope
+  --> $DIR/suggest-using-chars.rs:6:15
+   |
+LL |     let _ = 0.iter();
+   |               ^^^^ method not found in `{integer}`
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0599`.