Rollup merge of #137399 - lukas-code:oopsie-woopsie, r=compiler-errors

fix ICE in layout computation with unnormalizable const

The first commit reverts half of 7a667d2, where I removed a case from `layout_of` for handling non-generic unevaluated consts in array length, that I incorrectly assumed to be unreachable. This can actually happen with the combination of `feature(generic_const_exprs)` and `feature(trivial_bounds)`, because GCE makes anon consts inherit their parent's predicates and with an impossible predicate like `u8: A` it's possible to have an array whose length is an associated const like `<u8 as A>::B` that is not generic, but also can't be normalized:

```rust
#![feature(generic_const_exprs)]
#![feature(trivial_bounds)]

trait A {
    const B: usize;
}

// With GCE + trivial bounds this definition is not a compile error.
// Computing the layout of this type shouldn't ICE.
struct S([u8; <u8 as A>::B])
where
    u8: A;
```

---

The first commit also incidentally fixes #137308, which also managed to get an unnormalizable assoc const into an array length:

```rust
trait A {
    const B: usize;
}

impl<C: ?Sized> A for u8 { //~ ERROR: the type parameter `C` is not constrained
    const B: usize = 42;
}

// Computing the layout of this type shouldn't ICE, even with the compile error above.
struct S([u8; <u8 as A>::B]);
```

This happens, because we bail out from `codegen_select_candidate` with an error if the selected impl has unconstrained params to avoid leaking infer vars out of a query. `Instance::try_resolve` will then return `Ok(None)`, which for assoc consts roughly means "this const can't be evaluated in a generic context" and is treated as such: https://github.com/rust-lang/rust/blob/71e06b9c59d6af50fdc55aed75620493d29baf98/compiler/rustc_middle/src/mir/interpret/queries.rs#L84 (and this can ICE if the const isn't generic: #135617).

However, here `<u8 as A>::B` is definitely not "too generic" and also not unresolvable due to an unsatisfiable `u8: A` bound, so I've included the second commit to change the result of `Instance::try_resolve` from `Ok(None)` to `Err(ErrorGuaranteed)` when resolving an assoc item to an impl with unconstrained generic params. This has the effect that `<u8 as A>::B` will now be normalized to `ConstKind::Error` in the example above.

This properly fixes #137308, by no longer treating `<u8 as A>::B` as unresolvable even though it clearly has a unique impl that it resolves to. It also has the effect of changing the layout error from `Unknown` ("the type may be valid but has no sensible layout") to `ReferencesError` ("a non-layout error is reported elsewhere") which seems more appropriate.

r? ```@compiler-errors```

Author: matthiaskrgr

compiler/rustc_middle/src/traits/mod.rs

@@ -12,7 +12,7 @@ use std::borrow::Cow;
 use std::hash::{Hash, Hasher};
 use std::sync::Arc;
 
-use rustc_errors::{Applicability, Diag, EmissionGuarantee};
+use rustc_errors::{Applicability, Diag, EmissionGuarantee, ErrorGuaranteed};
 use rustc_hir as hir;
 use rustc_hir::HirId;
 use rustc_hir::def_id::DefId;
@@ -996,4 +996,7 @@ pub enum CodegenObligationError {
     /// but was included during typeck due to the trivial_bounds feature.
     Unimplemented,
     FulfillmentError,
+    /// The selected impl has unconstrained generic parameters. This will emit an error
+    /// during impl WF checking.
+    UnconstrainedParam(ErrorGuaranteed),
 }

compiler/rustc_traits/src/codegen.rs

@@ -80,16 +80,14 @@ pub(crate) fn codegen_select_candidate<'tcx>(
         // but never resolved, causing the return value of a query to contain inference
         // vars. We do not have a concept for this and will in fact ICE in stable hashing
         // of the return value. So bail out instead.
-        match impl_source {
-            ImplSource::UserDefined(impl_) => {
-                tcx.dcx().span_delayed_bug(
-                    tcx.def_span(impl_.impl_def_id),
-                    "this impl has unconstrained generic parameters",
-                );
-            }
+        let guar = match impl_source {
+            ImplSource::UserDefined(impl_) => tcx.dcx().span_delayed_bug(
+                tcx.def_span(impl_.impl_def_id),
+                "this impl has unconstrained generic parameters",
+            ),
             _ => unreachable!(),
-        }
-        return Err(CodegenObligationError::FulfillmentError);
+        };
+        return Err(CodegenObligationError::UnconstrainedParam(guar));
     }
 
     Ok(&*tcx.arena.alloc(impl_source))

compiler/rustc_ty_utils/src/instance.rs

@@ -112,6 +112,7 @@ fn resolve_associated_item<'tcx>(
             | CodegenObligationError::Unimplemented
             | CodegenObligationError::FulfillmentError,
         ) => return Ok(None),
+        Err(CodegenObligationError::UnconstrainedParam(guar)) => return Err(guar),
     };
 
     // Now that we know which impl is being used, we can dispatch to

compiler/rustc_ty_utils/src/layout.rs

@@ -147,12 +147,25 @@ fn extract_const_value<'tcx>(
 ) -> Result<ty::Value<'tcx>, &'tcx LayoutError<'tcx>> {
     match ct.kind() {
         ty::ConstKind::Value(cv) => Ok(cv),
-        ty::ConstKind::Param(_) | ty::ConstKind::Expr(_) | ty::ConstKind::Unevaluated(_) => {
+        ty::ConstKind::Param(_) | ty::ConstKind::Expr(_) => {
             if !ct.has_param() {
                 bug!("failed to normalize const, but it is not generic: {ct:?}");
             }
             Err(error(cx, LayoutError::TooGeneric(ty)))
         }
+        ty::ConstKind::Unevaluated(_) => {
+            let err = if ct.has_param() {
+                LayoutError::TooGeneric(ty)
+            } else {
+                // This case is reachable with unsatisfiable predicates and GCE (which will
+                // cause anon consts to inherit the unsatisfiable predicates). For example
+                // if we have an unsatisfiable `u8: Trait` bound, then it's not a compile
+                // error to mention `[u8; <u8 as Trait>::CONST]`, but we can't compute its
+                // layout.
+                LayoutError::Unknown(ty)
+            };
+            Err(error(cx, err))
+        }
         ty::ConstKind::Infer(_)
         | ty::ConstKind::Bound(..)
         | ty::ConstKind::Placeholder(_)

tests/ui/layout/gce-rigid-const-in-array-len.rs

@@ -0,0 +1,27 @@
+//! With `feature(generic_const_exprs)`, anon consts (e.g. length in array types) will
+//! inherit their parent's predicates. When combined with `feature(trivial_bounds)`, it
+//! is possible to have an unevaluated constant that is rigid, but not generic.
+//!
+//! This is what happens below: `u8: A` does not hold in the global environment, but
+//! with trivial bounds + GCE it it possible that `<u8 as A>::B` can appear in an array
+//! length without causing a compile error. This constant is *rigid* (i.e. it cannot be
+//! normalized further), but it is *not generic* (i.e. it does not depend on any generic
+//! parameters).
+//!
+//! This test ensures that we do not ICE in layout computation when encountering such a
+//! constant.
+
+#![feature(rustc_attrs)]
+#![feature(generic_const_exprs)] //~ WARNING: the feature `generic_const_exprs` is incomplete
+#![feature(trivial_bounds)]
+
+#![crate_type = "lib"]
+
+trait A {
+    const B: usize;
+}
+
+#[rustc_layout(debug)]
+struct S([u8; <u8 as A>::B]) //~ ERROR: the type `[u8; <u8 as A>::B]` has an unknown layout
+where
+    u8: A;

tests/ui/layout/gce-rigid-const-in-array-len.stderr

@@ -0,0 +1,17 @@
+warning: the feature `generic_const_exprs` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/gce-rigid-const-in-array-len.rs:15:12
+   |
+LL | #![feature(generic_const_exprs)]
+   |            ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #76560 <https://github.com/rust-lang/rust/issues/76560> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: the type `[u8; <u8 as A>::B]` has an unknown layout
+  --> $DIR/gce-rigid-const-in-array-len.rs:25:1
+   |
+LL | struct S([u8; <u8 as A>::B])
+   | ^^^^^^^^
+
+error: aborting due to 1 previous error; 1 warning emitted
+

tests/ui/layout/unconstrained-param-ice-137308.rs

@@ -0,0 +1,18 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/137308>.
+//!
+//! This used to ICE in layout computation, because `<u8 as A>::B` fails to normalize
+//! due to the unconstrained param on the impl.
+
+#![feature(rustc_attrs)]
+#![crate_type = "lib"]
+
+trait A {
+    const B: usize;
+}
+
+impl<C: ?Sized> A for u8 { //~ ERROR: the type parameter `C` is not constrained
+    const B: usize = 42;
+}
+
+#[rustc_layout(debug)]
+struct S([u8; <u8 as A>::B]); //~ ERROR: the type has an unknown layout

tests/ui/layout/unconstrained-param-ice-137308.stderr

@@ -0,0 +1,15 @@
+error[E0207]: the type parameter `C` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/unconstrained-param-ice-137308.rs:13:6
+   |
+LL | impl<C: ?Sized> A for u8 {
+   |      ^ unconstrained type parameter
+
+error: the type has an unknown layout
+  --> $DIR/unconstrained-param-ice-137308.rs:18:1
+   |
+LL | struct S([u8; <u8 as A>::B]);
+   | ^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0207`.