Peel borrows before suggesting as_ref/as_deref

compiler/rustc_hir_typeck/src/errors.rs

@@ -329,15 +329,16 @@ pub struct CtorIsPrivate {
 }
 
 #[derive(Subdiagnostic)]
-#[suggestion(
+#[multipart_suggestion(
     hir_typeck_convert_using_method,
-    code = "{sugg}",
     applicability = "machine-applicable",
     style = "verbose"
 )]
 pub struct SuggestConvertViaMethod<'tcx> {
-    #[primary_span]
+    #[suggestion_part(code = "{sugg}")]
     pub span: Span,
+    #[suggestion_part(code = "")]
+    pub borrow_removal_span: Option<Span>,
     pub sugg: &'static str,
     pub expected: Ty<'tcx>,
     pub found: Ty<'tcx>,

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -413,7 +413,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             self.deconstruct_option_or_result(found, expected)
             && let ty::Ref(_, peeled, hir::Mutability::Not) = *expected_ty_inner.kind()
         {
-            // Check that given `Result<_, E>`, our expected ty is `Result<_, &E>`
+            // Suggest removing any stray borrows (unless there's macro shenanigans involved).
+            let inner_expr = expr.peel_borrows();
+            if !inner_expr.span.eq_ctxt(expr.span) {
+                return false;
+            }
+            let borrow_removal_span = if inner_expr.hir_id == expr.hir_id {
+                None
+            } else {
+                Some(expr.span.shrink_to_lo().until(inner_expr.span))
+            };
+            // Given `Result<_, E>`, check our expected ty is `Result<_, &E>` for
+            // `as_ref` and `as_deref` compatibility.
             let error_tys_equate_as_ref = error_tys.map_or(true, |(found, expected)| {
                 self.can_eq(self.param_env, self.tcx.mk_imm_ref(self.tcx.lifetimes.re_erased, found), expected)
             });
@@ -425,6 +436,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     sugg: ".as_ref()",
                     expected,
                     found,
+                    borrow_removal_span,
                 });
                 return true;
             } else if let Some((deref_ty, _)) =
@@ -437,11 +449,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     sugg: ".as_deref()",
                     expected,
                     found,
+                    borrow_removal_span,
                 });
                 return true;
             } else if let ty::Adt(adt, _) = found_ty_inner.peel_refs().kind()
                 && Some(adt.did()) == self.tcx.lang_items().string()
                 && peeled.is_str()
+                // `Result::map`, conversely, does not take ref of the error type.
                 && error_tys.map_or(true, |(found, expected)| {
                     self.can_eq(self.param_env, found, expected)
                 })

tests/ui/issues/issue-100605.stderr

@@ -36,8 +36,9 @@ LL | fn takes_option(_arg: Option<&String>) {}
    |    ^^^^^^^^^^^^ ---------------------
 help: try using `.as_ref()` to convert `&Option<String>` to `Option<&String>`
    |
-LL |     takes_option(&res.as_ref());
+LL -     takes_option(&res);
-   |                      +++++++++
+LL +     takes_option(res.as_ref());
+   |
 
 error: aborting due to 2 previous errors
 

tests/ui/let-else/let-else-ref-bindings.stderr

@@ -21,8 +21,9 @@ LL |     let Some(ref a): Option<&[u8]> = &some else { return };
            found reference `&Option<Vec<u8>>`
 help: try using `.as_deref()` to convert `&Option<Vec<u8>>` to `Option<&[u8]>`
    |
-LL |     let Some(ref a): Option<&[u8]> = &some.as_deref() else { return };
+LL -     let Some(ref a): Option<&[u8]> = &some else { return };
-   |                                           +++++++++++
+LL +     let Some(ref a): Option<&[u8]> = some.as_deref() else { return };
+   |
 
 error[E0308]: mismatched types
   --> $DIR/let-else-ref-bindings.rs:24:34
@@ -51,8 +52,9 @@ LL |     let Some(a): Option<&[u8]> = &some else { return };
            found reference `&Option<Vec<u8>>`
 help: try using `.as_deref()` to convert `&Option<Vec<u8>>` to `Option<&[u8]>`
    |
-LL |     let Some(a): Option<&[u8]> = &some.as_deref() else { return };
+LL -     let Some(a): Option<&[u8]> = &some else { return };
-   |                                       +++++++++++
+LL +     let Some(a): Option<&[u8]> = some.as_deref() else { return };
+   |
 
 error[E0308]: mismatched types
   --> $DIR/let-else-ref-bindings.rs:44:46