Silence TooGeneric error

This error may be produced during intermediate failed attempts at evaluation of a generic const, which may nevertheless succeed later.

src/librustc/traits/error_reporting.rs

@@ -12,6 +12,7 @@ use crate::hir::Node;
 use crate::infer::error_reporting::TypeAnnotationNeeded as ErrorCode;
 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use crate::infer::{self, InferCtxt};
+use crate::mir::interpret::ErrorHandled;
 use crate::session::DiagnosticMessageId;
 use crate::ty::error::ExpectedFound;
 use crate::ty::fast_reject;
@@ -1086,6 +1087,11 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
             // already reported in the query
             ConstEvalFailure(err) => {
+                if let ErrorHandled::TooGeneric = err {
+                    // Silence this error, as it can be produced during intermediate steps
+                    // when a constant is not yet able to be evaluated (but will be later).
+                    return;
+                }
                 self.tcx.sess.delay_span_bug(
                     span,
                     &format!("constant in type had an ignored error: {:?}", err),

src/librustc_typeck/impl_wf_check.rs

@@ -98,7 +98,10 @@ fn enforce_impl_params_are_constrained(
         // (#36836)
         tcx.sess.delay_span_bug(
             tcx.def_span(impl_def_id),
-            "potentially unconstrained type parameters weren't evaluated",
+            &format!(
+                "potentially unconstrained type parameters weren't evaluated: {:?}",
+                impl_self_ty,
+            ),
         );
         return;
     }

src/test/ui/const-generics/array-size-in-generic-struct-param.rs

@@ -0,0 +1,23 @@
+// run-pass
+
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+#[allow(dead_code)]
+struct ArithArrayLen<const N: usize>([u32; 0 + N]); // ok
+
+#[derive(PartialEq, Eq)]
+struct Config {
+    arr_size: usize,
+}
+
+struct B<const CFG: Config> {
+    arr: [u8; CFG.arr_size], // ok
+}
+
+const C: Config = Config { arr_size: 5 };
+
+fn main() {
+    let b = B::<C> { arr: [1, 2, 3, 4, 5] };
+    assert_eq!(b.arr.len(), 5);
+}

src/test/ui/const-generics/array-size-in-generic-struct-param.stderr

@@ -0,0 +1,8 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/array-size-in-generic-struct-param.rs:3:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+