From d562f487c92698cb901df89cab0682ec2cbef222 Mon Sep 17 00:00:00 2001
From: Takayuki Maeda <takoyaki0316@gmail.com>
Date: Fri, 12 Nov 2021 01:47:43 +0900
Subject: [PATCH] suggest `&str.chars()` on attempt to `&str.iter()`

check if `String` or `&String` or `&str`

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

Co-authored-by: Esteban Kuber <estebank@users.noreply.github.com>

remove some trailing whitespace
---
 .../rustc_typeck/src/check/method/suggest.rs  | 20 ++++++++
 src/test/ui/suggest-using-chars.rs            |  7 +++
 src/test/ui/suggest-using-chars.stderr        | 48 +++++++++++++++++++
 3 files changed, 75 insertions(+)
 create mode 100644 src/test/ui/suggest-using-chars.rs
 create mode 100644 src/test/ui/suggest-using-chars.stderr

diff --git a/compiler/rustc_typeck/src/check/method/suggest.rs b/compiler/rustc_typeck/src/check/method/suggest.rs
index 71cd8a43329c5..661ced952c736 100644
--- a/compiler/rustc_typeck/src/check/method/suggest.rs
+++ b/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
diff --git a/src/test/ui/suggest-using-chars.rs b/src/test/ui/suggest-using-chars.rs
new file mode 100644
index 0000000000000..95732881baf09
--- /dev/null
+++ b/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
+}
diff --git a/src/test/ui/suggest-using-chars.stderr b/src/test/ui/suggest-using-chars.stderr
new file mode 100644
index 0000000000000..99bcfb08a0892
--- /dev/null
+++ b/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`.