Don't treat unnormalized function arguments as well-formed

compiler/rustc_borrowck/src/type_check/free_region_relations.rs

@@ -258,7 +258,6 @@ impl UniversalRegionRelationsBuilder<'cx, 'tcx> {
                 debug!("build: input_or_output={:?}", ty);
                 // We add implied bounds from both the unnormalized and normalized ty
                 // See issue #87748
-                let constraints_implied_1 = self.add_implied_bounds(ty);
                 let TypeOpOutput { output: norm_ty, constraints: constraints1, .. } = self
                     .param_env
                     .and(type_op::normalize::Normalize::new(ty))
@@ -286,10 +285,9 @@ impl UniversalRegionRelationsBuilder<'cx, 'tcx> {
                 // }
                 // ```
                 // Both &Self::Bar and &() are WF
-                let constraints_implied_2 =
-                    if ty != norm_ty { self.add_implied_bounds(norm_ty) } else { None };
+                let constraints_implied = self.add_implied_bounds(norm_ty);
                 normalized_inputs_and_output.push(norm_ty);
-                constraints1.into_iter().chain(constraints_implied_1).chain(constraints_implied_2)
+                constraints1.into_iter().chain(constraints_implied)
             })
             .collect();
 

compiler/rustc_typeck/src/check/compare_method.rs

@@ -266,14 +266,9 @@ fn compare_predicate_entailment<'tcx>(
         // First liberate late bound regions and subst placeholders
         let trait_sig = tcx.liberate_late_bound_regions(impl_m.def_id, tcx.fn_sig(trait_m.def_id));
         let trait_sig = trait_sig.subst(tcx, trait_to_placeholder_substs);
-        // Next, add all inputs and output as well-formed tys. Importantly,
-        // we have to do this before normalization, since the normalized ty may
-        // not contain the input parameters. See issue #87748.
-        wf_tys.extend(trait_sig.inputs_and_output.iter());
         let trait_sig =
             inh.normalize_associated_types_in(impl_m_span, impl_m_hir_id, param_env, trait_sig);
-        // Also add the resulting inputs and output as well-formed.
-        // This probably isn't strictly necessary.
+        // Add the resulting inputs and output as well-formed.
         wf_tys.extend(trait_sig.inputs_and_output.iter());
         let trait_fty = tcx.mk_fn_ptr(ty::Binder::dummy(trait_sig));
 

compiler/rustc_typeck/src/check/mod.rs

@@ -391,7 +391,6 @@ fn typeck_with_fallback<'tcx>(
             let mut wf_tys = FxHashSet::default();
             // Compute the fty from point of view of inside the fn.
             let fn_sig = tcx.liberate_late_bound_regions(def_id.to_def_id(), fn_sig);
-            wf_tys.extend(fn_sig.inputs_and_output.iter());
             let fn_sig = inh.normalize_associated_types_in(
                 body.value.span,
                 body_id.hir_id,

compiler/rustc_typeck/src/check/wfcheck.rs

@@ -960,11 +960,6 @@ fn check_fn_or_method<'fcx, 'tcx>(
 ) {
     let sig = fcx.tcx.liberate_late_bound_regions(def_id, sig);
 
-    // Unnormalized types in signature are WF too
-    implied_bounds.extend(sig.inputs());
-    // FIXME(#27579) return types should not be implied bounds
-    implied_bounds.insert(sig.output());
-
     // Normalize the input and output types one at a time, using a different
     // `WellFormedLoc` for each. We cannot call `normalize_associated_types`
     // on the entire `FnSig`, since this would use the same `WellFormedLoc`

src/test/ui/fn/implied-bounds-unnorm-associated-type.rs

@@ -0,0 +1,22 @@
+// check-fail
+// See issue #91068. Types in the substs of an associated type can't be implied
+// to be WF, since they don't actually have to be constructed.
+
+trait Trait {
+    type Type;
+}
+
+impl<T> Trait for T {
+    type Type = ();
+}
+
+fn f<'a, 'b>(s: &'b str, _: <&'a &'b () as Trait>::Type) -> &'a str {
+    s //~ ERROR lifetime mismatch [E0623]
+}
+
+fn main() {
+    let x = String::from("Hello World!");
+    let y = f(&x, ());
+    drop(x);
+    println!("{}", y);
+}

src/test/ui/fn/implied-bounds-unnorm-associated-type.stderr

@@ -0,0 +1,13 @@
+error[E0623]: lifetime mismatch
+  --> $DIR/implied-bounds-unnorm-associated-type.rs:14:5
+   |
+LL | fn f<'a, 'b>(s: &'b str, _: <&'a &'b () as Trait>::Type) -> &'a str {
+   |                 -------      ----------
+   |                 |
+   |                 these two types are declared with different lifetimes...
+LL |     s
+   |     ^ ...but data from `s` flows here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0623`.