Eagerly resolve vars in predicate during coercion loop

compiler/rustc_hir_typeck/src/coercion.rs

@@ -636,6 +636,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 Some(ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_pred)))
                     if traits.contains(&trait_pred.def_id()) =>
                 {
+                    let trait_pred = self.resolve_vars_if_possible(trait_pred);
                     if unsize_did == trait_pred.def_id() {
                         let self_ty = trait_pred.self_ty();
                         let unsize_ty = trait_pred.trait_ref.substs[1].expect_ty();
@@ -662,7 +663,6 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 // Uncertain or unimplemented.
                 Ok(None) => {
                     if trait_pred.def_id() == unsize_did {
-                        let trait_pred = self.resolve_vars_if_possible(trait_pred);
                         let self_ty = trait_pred.self_ty();
                         let unsize_ty = trait_pred.trait_ref.substs[1].expect_ty();
                         debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_pred);

src/tools/tidy/src/ui_tests.rs

@@ -10,7 +10,7 @@ use std::path::{Path, PathBuf};
 
 const ENTRY_LIMIT: usize = 900;
 // FIXME: The following limits should be reduced eventually.
-const ISSUES_ENTRY_LIMIT: usize = 1896;
+const ISSUES_ENTRY_LIMIT: usize = 1894;
 const ROOT_ENTRY_LIMIT: usize = 870;
 
 const EXPECTED_TEST_FILE_EXTENSIONS: &[&str] = &[

tests/ui/traits/trait-upcasting/issue-11515.current.stderr

@@ -1,5 +1,5 @@
 error[E0658]: cannot cast `dyn Fn()` to `dyn FnMut()`, trait upcasting coercion is experimental
-  --> $DIR/issue-11515.rs:9:38
+  --> $DIR/issue-11515.rs:10:38
    |
 LL |     let test = Box::new(Test { func: closure });
    |                                      ^^^^^^^

tests/ui/traits/trait-upcasting/issue-11515.next.stderr

@@ -0,0 +1,13 @@
+error[E0658]: cannot cast `dyn Fn()` to `dyn FnMut()`, trait upcasting coercion is experimental
+  --> $DIR/issue-11515.rs:10:38
+   |
+LL |     let test = Box::new(Test { func: closure });
+   |                                      ^^^^^^^
+   |
+   = note: see issue #65991 <https://github.com/rust-lang/rust/issues/65991> for more information
+   = help: add `#![feature(trait_upcasting)]` to the crate attributes to enable
+   = note: required when coercing `Box<(dyn Fn() + 'static)>` into `Box<(dyn FnMut() + 'static)>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/traits/trait-upcasting/issue-11515.rs

@@ -1,9 +1,10 @@
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
+
 struct Test {
     func: Box<dyn FnMut() + 'static>,
 }
 
-
-
 fn main() {
     let closure: Box<dyn Fn() + 'static> = Box::new(|| ());
     let test = Box::new(Test { func: closure }); //~ ERROR trait upcasting coercion is experimental [E0658]