Rollup merge of rust-lang#153574 - TaKO8Ki:self-referential-param-env-normalization-ice, r=lcnr

Avoid ICE when param-env normalization leaves unresolved inference variables

Fixes rust-lang#153354

Because the impl is already ill-formed, those variables are not fully constrained, so zfully_resolve` fails. We previously treated that as an immediate compiler bug with `span_bug!`, which caused an ICE on invalid input.

Author: JonathanBrouwer

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -300,19 +300,14 @@ fn do_normalize_predicates<'tcx>(
         Ok(predicates) => Ok(predicates),
         Err(fixup_err) => {
             // If we encounter a fixup error, it means that some type
-            // variable wound up unconstrained. I actually don't know
-            // if this can happen, and I certainly don't expect it to
-            // happen often, but if it did happen it probably
-            // represents a legitimate failure due to some kind of
-            // unconstrained variable.
-            //
-            // @lcnr: Let's still ICE here for now. I want a test case
-            // for that.
-            span_bug!(
+            // variable wound up unconstrained. That can happen for
+            // ill-formed impls, so we delay a bug here instead of
+            // immediately ICEing and let type checking report the
+            // actual user-facing errors.
+            Err(tcx.dcx().span_delayed_bug(
                 span,
-                "inference variables in normalized parameter environment: {}",
-                fixup_err
-            );
+                format!("inference variables in normalized parameter environment: {fixup_err}"),
+            ))
         }
     }
 }

tests/crashes/120033.rs

@@ -0,0 +1,22 @@
+//@ compile-flags: --crate-type=lib
+
+// Regression test for <https://github.com/rust-lang/rust/issues/120033>
+
+#![feature(rustc_attrs)]
+#![feature(non_lifetime_binders)]
+#![allow(incomplete_features)]
+#![rustc_no_implicit_bounds]
+
+pub trait Foo<T> {
+    type Bar<K>;
+}
+
+pub struct Bar<T: ?AutoTrait> {} //~ ERROR cannot find trait `AutoTrait`
+
+pub fn f<T1, T2>()
+where
+    T1: for<T> Foo<usize, Bar = Bar<T>>, //~ ERROR missing generics for associated type `Foo::Bar`
+    //~| ERROR missing generics for associated type `Foo::Bar`
+    T2: for<L, T> Foo<usize, Bar<T> = T1::Bar<T>>,
+{
+}

tests/ui/traits/non_lifetime_binders/normalized-param-env-unconstrained-type-120033.rs

@@ -0,0 +1,43 @@
+error[E0405]: cannot find trait `AutoTrait` in this scope
+  --> $DIR/normalized-param-env-unconstrained-type-120033.rs:14:20
+   |
+LL | pub struct Bar<T: ?AutoTrait> {}
+   |                    ^^^^^^^^^ not found in this scope
+
+error[E0107]: missing generics for associated type `Foo::Bar`
+  --> $DIR/normalized-param-env-unconstrained-type-120033.rs:18:27
+   |
+LL |     T1: for<T> Foo<usize, Bar = Bar<T>>,
+   |                           ^^^ expected 1 generic argument
+   |
+note: associated type defined here, with 1 generic parameter: `K`
+  --> $DIR/normalized-param-env-unconstrained-type-120033.rs:11:10
+   |
+LL |     type Bar<K>;
+   |          ^^^ -
+help: add missing generic argument
+   |
+LL |     T1: for<T> Foo<usize, Bar<K> = Bar<T>>,
+   |                              +++
+
+error[E0107]: missing generics for associated type `Foo::Bar`
+  --> $DIR/normalized-param-env-unconstrained-type-120033.rs:18:27
+   |
+LL |     T1: for<T> Foo<usize, Bar = Bar<T>>,
+   |                           ^^^ expected 1 generic argument
+   |
+note: associated type defined here, with 1 generic parameter: `K`
+  --> $DIR/normalized-param-env-unconstrained-type-120033.rs:11:10
+   |
+LL |     type Bar<K>;
+   |          ^^^ -
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+help: add missing generic argument
+   |
+LL |     T1: for<T> Foo<usize, Bar<K> = Bar<T>>,
+   |                              +++
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0107, E0405.
+For more information about an error, try `rustc --explain E0107`.

tests/ui/traits/non_lifetime_binders/normalized-param-env-unconstrained-type-120033.stderr

@@ -0,0 +1,18 @@
+//~ ERROR overflow evaluating the requirement `Self: StreamingIterator<'_>` [E0275]
+// Regression test for <https://github.com/rust-lang/rust/issues/153354>.
+
+trait StreamingIterator<'a> {
+    type Item: 'a;
+}
+
+impl<'b, I, T> StreamingIterator<'b> for I
+//~^ ERROR the type parameter `T` is not constrained by the impl trait, self type, or predicates [E0207]
+where
+    I: IntoIterator,
+    T: FnMut(Self::Item, I::Item),
+{
+    type Item = T;
+    //~^ ERROR overflow evaluating the requirement `I: IntoIterator` [E0275]
+}
+
+fn main() {}

tests/ui/traits/self-referential-param-env-normalization.rs

@@ -0,0 +1,42 @@
+error[E0275]: overflow evaluating the requirement `Self: StreamingIterator<'_>`
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`self_referential_param_env_normalization`)
+note: required for `Self` to implement `StreamingIterator<'_>`
+  --> $DIR/self-referential-param-env-normalization.rs:8:16
+   |
+LL | impl<'b, I, T> StreamingIterator<'b> for I
+   |                ^^^^^^^^^^^^^^^^^^^^^     ^
+...
+LL |     T: FnMut(Self::Item, I::Item),
+   |        -------------------------- unsatisfied trait bound introduced here
+   = note: 127 redundant requirements hidden
+   = note: required for `Self` to implement `StreamingIterator<'a>`
+
+error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/self-referential-param-env-normalization.rs:8:13
+   |
+LL | impl<'b, I, T> StreamingIterator<'b> for I
+   |             ^ unconstrained type parameter
+
+error[E0275]: overflow evaluating the requirement `I: IntoIterator`
+  --> $DIR/self-referential-param-env-normalization.rs:14:17
+   |
+LL |     type Item = T;
+   |                 ^
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`self_referential_param_env_normalization`)
+note: required for `I` to implement `StreamingIterator<'_>`
+  --> $DIR/self-referential-param-env-normalization.rs:8:16
+   |
+LL | impl<'b, I, T> StreamingIterator<'b> for I
+   |                ^^^^^^^^^^^^^^^^^^^^^     ^
+...
+LL |     T: FnMut(Self::Item, I::Item),
+   |        -------------------------- unsatisfied trait bound introduced here
+   = note: 127 redundant requirements hidden
+   = note: required for `I` to implement `StreamingIterator<'b>`
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0207, E0275.
+For more information about an error, try `rustc --explain E0207`.