Implement diagnostic for String conversion

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

compiler/rustc_typeck/src/check/fn_ctxt/suggestions.rs

@@ -14,6 +14,7 @@ use rustc_middle::lint::in_external_macro;
 use rustc_middle::ty::{self, Binder, Ty};
 use rustc_span::symbol::{kw, sym};
 
+use rustc_middle::ty::subst::GenericArgKind;
 use std::iter;
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
@@ -232,6 +233,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let is_struct_pat_shorthand_field =
                 self.is_hir_id_from_struct_pattern_shorthand_field(expr.hir_id, expr.span);
             let methods = self.get_conversion_methods(expr.span, expected, found, expr.hir_id);
+            if !methods.is_empty() {
                 if let Ok(expr_text) = self.sess().source_map().span_to_snippet(expr.span) {
                     let mut suggestions = iter::zip(iter::repeat(&expr_text), &methods)
                         .filter_map(|(receiver, method)| {
@@ -249,7 +251,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                         format!("{}{}", &receiver[..max_len], method_call),
                                     )]
                                 } else {
-                                if expr.precedence().order() < ExprPrecedence::MethodCall.order() {
+                                    if expr.precedence().order()
+                                        < ExprPrecedence::MethodCall.order()
+                                    {
                                         vec![
                                             (expr.span.shrink_to_lo(), "(".to_string()),
                                             (expr.span.shrink_to_hi(), format!("){}", method_call)),
@@ -276,6 +280,27 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         );
                     }
                 }
+            } else if found.to_string().starts_with("Option<")
+                && expected.to_string() == "Option<&str>"
+            {
+                if let ty::Adt(_def, subst) = found.kind() {
+                    if subst.len() != 0 {
+                        if let GenericArgKind::Type(ty) = subst[0].unpack() {
+                            let peeled = ty.peel_refs().to_string();
+                            if peeled == "String" {
+                                let ref_cnt = ty.to_string().len() - peeled.len();
+                                let result = format!(".map(|x| &*{}x)", "*".repeat(ref_cnt));
+                                err.span_suggestion_verbose(
+                                    expr.span.shrink_to_hi(),
+                                    "try converting the passed type into a `&str`",
+                                    result,
+                                    Applicability::MaybeIncorrect,
+                                );
+                            }
+                        }
+                    }
+                }
+            }
         }
     }
 

src/test/ui/typeck/issue-89856.rs

@@ -0,0 +1,8 @@
+fn take_str_maybe(x: Option<&str>) -> Option<&str> { None }
+
+fn main() {
+    let string = String::from("Hello, world");
+    let option = Some(&string);
+    take_str_maybe(option);
+    //~^ ERROR: mismatched types [E0308]
+}

src/test/ui/typeck/issue-89856.stderr

@@ -0,0 +1,16 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-89856.rs:6:20
+   |
+LL |     take_str_maybe(option);
+   |                    ^^^^^^ expected `str`, found struct `String`
+   |
+   = note: expected enum `Option<&str>`
+              found enum `Option<&String>`
+help: try converting the passed type into a `&str`
+   |
+LL |     take_str_maybe(option.map(|x| &**x));
+   |                          ++++++++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.