Detect cases when user written object assoc bound differs from elaborated non-self-referential bound

Author: compiler-errors

compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs

@@ -1,4 +1,4 @@
-use rustc_data_structures::fx::{FxHashSet, FxIndexMap, FxIndexSet};
+use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap, FxIndexSet};
 use rustc_errors::codes::*;
 use rustc_errors::struct_span_code_err;
 use rustc_hir as hir;
@@ -62,6 +62,8 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         let mut trait_bounds = vec![];
         let mut projection_bounds = vec![];
+        let mut uwu = FxHashMap::default();
+
         for (pred, span) in bounds.clauses() {
             let bound_pred = pred.kind();
             match bound_pred.skip_binder() {
@@ -71,6 +73,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 }
                 ty::ClauseKind::Projection(proj) => {
                     projection_bounds.push((bound_pred.rebind(proj), span));
+                    uwu.insert(proj.def_id(), (bound_pred.rebind(proj.term), span));
                 }
                 ty::ClauseKind::TypeOutlives(_) => {
                     // Do nothing, we deal with regions separately
@@ -176,6 +179,20 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                             projection_bounds.push((pred, original_span));
                         }
 
+                        if !references_self
+                            && let Some(&(user_written, span)) = uwu.get(&pred.projection_def_id())
+                        {
+                            let user_written = tcx.anonymize_bound_vars(user_written);
+                            let elaborated =
+                                tcx.anonymize_bound_vars(pred.map_bound(|pred| pred.term));
+                            if user_written != elaborated {
+                                self.dcx().span_err(
+                                    span,
+                                    format!("expected {user_written}, found {elaborated}"),
+                                );
+                            }
+                        }
+
                         self.check_elaborated_projection_mentions_input_lifetimes(
                             pred,
                             original_span,

tests/ui/associated-types/associated-types-overridden-binding-2.stderr

@@ -1,3 +1,9 @@
+error: expected u32, found i32
+  --> $DIR/associated-types-overridden-binding-2.rs:6:29
+   |
+LL |     let _: &dyn I32Iterator<Item = u32> = &vec![42].into_iter();
+   |                             ^^^^^^^^^^
+
 error[E0271]: expected `IntoIter<u32>` to be an iterator that yields `i32`, but it yields `u32`
   --> $DIR/associated-types-overridden-binding-2.rs:6:43
    |
@@ -6,6 +12,6 @@ LL |     let _: &dyn I32Iterator<Item = u32> = &vec![42].into_iter();
    |
    = note: required for the cast from `&std::vec::IntoIter<u32>` to `&dyn Iterator<Item = u32, Item = i32>`
 
-error: aborting due to 1 previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0271`.

tests/ui/associated-types/associated-types-overridden-binding.stderr

@@ -22,6 +22,12 @@ note: required by a bound in `I32Iterator`
 LL | trait I32Iterator = Iterator<Item = i32>;
    |                              ^^^^^^^^^^ required by this bound in `I32Iterator`
 
-error: aborting due to 2 previous errors
+error: expected u32, found i32
+  --> $DIR/associated-types-overridden-binding.rs:10:29
+   |
+LL |     let _: &dyn I32Iterator<Item = u32>;
+   |                             ^^^^^^^^^^
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0284`.

tests/ui/traits/object/pretty.stderr

@@ -1,3 +1,15 @@
+error: expected u16, found u8
+  --> $DIR/pretty.rs:28:34
+   |
+LL | fn dyn_fixed_multi(x: &dyn Fixed<Assoc = u16>) { x }
+   |                                  ^^^^^^^^^^^
+
+error: expected &'a u8, found for<'a> &'a u8
+  --> $DIR/pretty.rs:36:62
+   |
+LL | fn dyn_fixed_generic_multi(x: &dyn for<'a> FixedGeneric1<'a, Assoc2 = &u8>) { x }
+   |                                                              ^^^^^^^^^^^^
+
 warning: unnecessary associated type bound for dyn-incompatible associated type
   --> $DIR/pretty.rs:41:35
    |
@@ -161,6 +173,6 @@ LL | fn dyn_has_gat(x: &dyn HasGat<u8, Assoc<bool> = ()>) { x }
    = note: expected unit type `()`
               found reference `&dyn HasGat<u8, Assoc<bool> = ()>`
 
-error: aborting due to 14 previous errors; 1 warning emitted
+error: aborting due to 16 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0308`.