Suggest the right help message for as_ref

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2078,7 +2078,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                             diag.span_suggestion(
                                 span,
                                 *msg,
-                                format!("{}.as_ref()", snippet),
+                                // HACK: fix issue# 100605, suggesting convert from &Option<T> to Option<&T>, remove the extra `&`
+                                format!("{}.as_ref()", snippet.trim_start_matches('&')),
                                 Applicability::MachineApplicable,
                             );
                         }

src/test/ui/issues/issue-100605.rs

@@ -0,0 +1,9 @@
+fn takes_option(_arg: Option<&String>) {}
+
+fn main() {
+    takes_option(&None); //~ ERROR 4:18: 4:23: mismatched types [E0308]
+
+    let x = String::from("x");
+    let res = Some(x);
+    takes_option(&res); //~ ERROR 8:18: 8:22: mismatched types [E0308]
+}

src/test/ui/issues/issue-100605.stderr

@@ -0,0 +1,46 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-100605.rs:4:18
+   |
+LL |     takes_option(&None);
+   |     ------------ ^^^^^ expected enum `Option`, found `&Option<_>`
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note:   expected enum `Option<&String>`
+           found reference `&Option<_>`
+note: function defined here
+  --> $DIR/issue-100605.rs:1:4
+   |
+LL | fn takes_option(_arg: Option<&String>) {}
+   |    ^^^^^^^^^^^^ ---------------------
+help: you can convert from `&Option<T>` to `Option<&T>` using `.as_ref()`
+   |
+LL |     takes_option(None.as_ref());
+   |                  ~~~~~~~~~~~~~
+help: consider removing the borrow
+   |
+LL -     takes_option(&None);
+LL +     takes_option(None);
+   |
+
+error[E0308]: mismatched types
+  --> $DIR/issue-100605.rs:8:18
+   |
+LL |     takes_option(&res);
+   |     ------------ ^^^^
+   |     |            |
+   |     |            expected enum `Option`, found `&Option<String>`
+   |     |            help: you can convert from `&Option<T>` to `Option<&T>` using `.as_ref()`: `res.as_ref()`
+   |     arguments to this function are incorrect
+   |
+   = note:   expected enum `Option<&String>`
+           found reference `&Option<String>`
+note: function defined here
+  --> $DIR/issue-100605.rs:1:4
+   |
+LL | fn takes_option(_arg: Option<&String>) {}
+   |    ^^^^^^^^^^^^ ---------------------
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.