misc coercion cleanups and handle safety correctly

compiler/rustc_hir_typeck/src/coercion.rs

@@ -856,9 +856,6 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         debug_assert!(self.shallow_resolve(a) == a);
         debug_assert!(self.shallow_resolve(b) == b);
 
-        let InferOk { value: b, mut obligations } =
-            self.at(&self.cause, self.param_env).normalize(b);
-
         match b.kind() {
             ty::FnPtr(_, b_hdr) => {
                 let mut a_sig = a.fn_sig(self.tcx);
@@ -890,9 +887,10 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                     }
                 }
 
-                let InferOk { value: a_sig, obligations: o1 } =
+                // FIXME: we shouldn't be normalizing here as coercion is inside of
+                // a probe. This can probably cause ICEs.
+                let InferOk { value: a_sig, mut obligations } =
                     self.at(&self.cause, self.param_env).normalize(a_sig);
-                obligations.extend(o1);
 
                 let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
                     a_sig,
@@ -907,8 +905,8 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         }
     }
 
-    /// Attempts to coerce from the type of a non-capturing closure
-    /// into a function pointer.
+    /// Attempts to coerce from a closure to a function pointer. Fails
+    /// if the closure has any upvars.
     fn coerce_closure_to_fn(
         &self,
         a: Ty<'tcx>,

tests/ui/coercion/hr_alias_normalization_leaking_vars.rs

@@ -0,0 +1,36 @@
+// We have two function parameters with types:
+// - `&?0`
+// - `Box<for<'a> fn(<?0 as Trait<'a>>::Item)>`
+//
+// As the alias in the second parameter has a `?0` it is an ambig
+// alias, and as it references bound vars it can't be normalized to
+// an infer var.
+//
+// When checking function arguments we try to coerce both:
+// - `&()` to `&?0`
+// - `FnDef(f)` to `Box<for<'a> fn(<?0 as Trait<'a>>::Item)>`
+//
+// The first coercion infers `?0=()`. Previously when handling
+// the second coercion we wound *re-normalize* the alias, which
+// now that `?0` has been inferred allowed us to determine this
+// alias is not wellformed and normalize it to some infer var `?1`.
+//
+// We would then see that `FnDef(f)` can't be coerced to `Box<fn(?1)>`
+// and return a `TypeError` referencing this new variable `?1`. This
+// then caused ICEs as diagnostics would encounter inferences variables
+// from the result of normalization inside of the probe used be coercion.
+
+
+trait LendingIterator {
+    type Item<'q>;
+    fn for_each(&self, _f: Box<fn(Self::Item<'_>)>) {}
+}
+
+fn f(_: ()) {}
+
+fn main() {
+    LendingIterator::for_each(&(), f);
+    //~^ ERROR: the trait bound `(): LendingIterator` is not satisfied
+    //~| ERROR: the trait bound `(): LendingIterator` is not satisfied
+    //~| ERROR: mismatched types
+}

tests/ui/coercion/hr_alias_normalization_leaking_vars.stderr

@@ -0,0 +1,46 @@
+error[E0277]: the trait bound `(): LendingIterator` is not satisfied
+  --> $DIR/hr_alias_normalization_leaking_vars.rs:32:31
+   |
+LL |     LendingIterator::for_each(&(), f);
+   |     ------------------------- ^^^ the trait `LendingIterator` is not implemented for `()`
+   |     |
+   |     required by a bound introduced by this call
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/hr_alias_normalization_leaking_vars.rs:24:1
+   |
+LL | trait LendingIterator {
+   | ^^^^^^^^^^^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/hr_alias_normalization_leaking_vars.rs:32:36
+   |
+LL |     LendingIterator::for_each(&(), f);
+   |     -------------------------      ^ expected `Box<fn(...)>`, found fn item
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected struct `Box<for<'a> fn(<() as LendingIterator>::Item<'a>)>`
+             found fn item `fn(()) {f}`
+note: method defined here
+  --> $DIR/hr_alias_normalization_leaking_vars.rs:26:8
+   |
+LL |     fn for_each(&self, _f: Box<fn(Self::Item<'_>)>) {}
+   |        ^^^^^^^^        ---------------------------
+
+error[E0277]: the trait bound `(): LendingIterator` is not satisfied
+  --> $DIR/hr_alias_normalization_leaking_vars.rs:32:36
+   |
+LL |     LendingIterator::for_each(&(), f);
+   |                                    ^ the trait `LendingIterator` is not implemented for `()`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/hr_alias_normalization_leaking_vars.rs:24:1
+   |
+LL | trait LendingIterator {
+   | ^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0277, E0308.
+For more information about an error, try `rustc --explain E0277`.