Suggest removing Box::new

Author: camsteffen

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -2955,6 +2955,28 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  )
  {
  let deref_kind = if checked_ty.is_box() {
+ // detect Box::new(..)
+ if let ExprKind::Call(box_new, [_]) = expr.kind
+ && let ExprKind::Path(qpath) = &box_new.kind
+ && let Res::Def(DefKind::AssocFn, fn_id) =
+ self.typeck_results.borrow().qpath_res(qpath, box_new.hir_id)
+ && let Some(impl_id) = self.tcx.inherent_impl_of_assoc(fn_id)
+ && self.tcx.type_of(impl_id).skip_binder().is_box()
+ && self.tcx.item_name(fn_id) == sym::new
+ {
+ let l_paren = self.tcx.sess.source_map().next_point(box_new.span);
+ let r_paren = self.tcx.sess.source_map().end_point(expr.span);
+ return Some((
+ vec![
+ (box_new.span.to(l_paren), String::new()),
+ (r_paren, String::new()),
+ ],
+ "consider removing the Box".to_string(),
+ Applicability::MachineApplicable,
+ false,
+ false,
+ ));
+ }
  "unboxing the value"
  } else if checked_ty.is_ref() {
  "dereferencing the borrow"

tests/ui/coercion/coerce-block-tail.stderr

@@ -6,10 +6,11 @@ LL | let _: &i32 = & { Box::new(1i32) };
  |
  = note: expected type `i32`
  found struct `Box<i32>`
-help: consider unboxing the value
+help: consider removing the Box
+ |
+LL - let _: &i32 = & { Box::new(1i32) };
+LL + let _: &i32 = & { 1i32 };
  |
-LL | let _: &i32 = & { *Box::new(1i32) };
- | +
 
 error: aborting due to 1 previous error
 

tests/ui/coercion/coerce-box-new-to-unboxed.rs

@@ -0,0 +1,4 @@
+fn main() {
+ let _: String = Box::new(String::new());
+ //~^ ERROR mismatched types
+}

tests/ui/coercion/coerce-box-new-to-unboxed.stderr

@@ -0,0 +1,19 @@
+error[E0308]: mismatched types
+ --> $DIR/coerce-box-new-to-unboxed.rs:2:21
+ |
+LL | let _: String = Box::new(String::new());
+ | ------ ^^^^^^^^^^^^^^^^^^^^^^^ expected `String`, found `Box<String>`
+ | |
+ | expected due to this
+ |
+ = note: expected struct `String`
+ found struct `Box<String>`
+help: consider removing the Box
+ |
+LL - let _: String = Box::new(String::new());
+LL + let _: String = String::new();
+ |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.