Unrolled build for rust-lang#127596

Rollup merge of rust-lang#127596 - tesuji:help-unwrap-or, r=compiler-errors

More suggestion for converting `Option<&Vec<T>>` to `Option<&[T]>`

Please review commit-by-commit.

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -466,21 +466,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  borrow_removal_span,
  });
  return true;
- } else if let Some((deref_ty, _)) =
- self.autoderef(expr.span, found_ty_inner).silence_errors().nth(1)
- && self.can_eq(self.param_env, deref_ty, peeled)
- && error_tys_equate_as_ref
- {
- let sugg = prefix_wrap(".as_deref()");
- err.subdiagnostic(errors::SuggestConvertViaMethod {
- span: expr.span.shrink_to_hi(),
- sugg,
- expected,
- found,
- borrow_removal_span,
- });
- return true;
- } else if let ty::Adt(adt, _) = found_ty_inner.peel_refs().kind()
+ } else if let ty::Ref(_, peeled_found_ty, _) = found_ty_inner.kind()
+ && let ty::Adt(adt, _) = peeled_found_ty.peel_refs().kind()
  && self.tcx.is_lang_item(adt.did(), LangItem::String)
  && peeled.is_str()
  // `Result::map`, conversely, does not take ref of the error type.
@@ -496,12 +483,47 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  Applicability::MachineApplicable,
  );
  return true;
+ } else {
+ if !error_tys_equate_as_ref {
+ return false;
+ }
+ let mut steps = self.autoderef(expr.span, found_ty_inner).silence_errors();
+ if let Some((deref_ty, _)) = steps.nth(1)
+ && self.can_eq(self.param_env, deref_ty, peeled)
+ {
+ let sugg = prefix_wrap(".as_deref()");
+ err.subdiagnostic(errors::SuggestConvertViaMethod {
+ span: expr.span.shrink_to_hi(),
+ sugg,
+ expected,
+ found,
+ borrow_removal_span,
+ });
+ return true;
+ }
+ for (deref_ty, n_step) in steps {
+ if self.can_eq(self.param_env, deref_ty, peeled) {
+ let explicit_deref = "*".repeat(n_step);
+ let sugg = prefix_wrap(&format!(".map(|v| &{explicit_deref}v)"));
+ err.subdiagnostic(errors::SuggestConvertViaMethod {
+ span: expr.span.shrink_to_hi(),
+ sugg,
+ expected,
+ found,
+ borrow_removal_span,
+ });
+ return true;
+ }
+ }
  }
  }
 
  false
  }
 
+ /// If `ty` is `Option<T>`, returns `T, T, None`.
+ /// If `ty` is `Result<T, E>`, returns `T, T, Some(E, E)`.
+ /// Otherwise, returns `None`.
  fn deconstruct_option_or_result(
  &self,
  found_ty: Ty<'tcx>,

tests/ui/mismatched_types/transforming-option-ref-issue-127545.rs

@@ -0,0 +1,6 @@
+// Regression test for <https://github.com/rust-lang/rust/issues/127545>.
+#![crate_type = "lib"]
+
+pub fn foo(arg: Option<&Vec<i32>>) -> Option<&[i32]> {
+ arg //~ ERROR 5:5: 5:8: mismatched types [E0308]
+}

tests/ui/mismatched_types/transforming-option-ref-issue-127545.stderr

@@ -0,0 +1,18 @@
+error[E0308]: mismatched types
+ --> $DIR/transforming-option-ref-issue-127545.rs:5:5
+ |
+LL | pub fn foo(arg: Option<&Vec<i32>>) -> Option<&[i32]> {
+ | -------------- expected `Option<&[i32]>` because of return type
+LL | arg
+ | ^^^ expected `Option<&[i32]>`, found `Option<&Vec<i32>>`
+ |
+ = note: expected enum `Option<&[i32]>`
+ found enum `Option<&Vec<i32>>`
+help: try using `.map(|v| &**v)` to convert `Option<&Vec<i32>>` to `Option<&[i32]>`
+ |
+LL | arg.map(|v| &**v)
+ | ++++++++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.