Lower fn items as ZST valtrees and delay a bug

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -2154,11 +2154,15 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 span_bug!(span, "use of bare `static` ConstArgKind::Path's not yet supported")
             }
             // FIXME(const_generics): create real const to allow fn items as const paths
-            Res::Def(DefKind::Fn | DefKind::AssocFn, _) => ty::Const::new_error_with_message(
-                tcx,
-                span,
-                "fn items cannot be used as const args",
-            ),
+            Res::Def(DefKind::Fn | DefKind::AssocFn, did) => {
+                self.dcx().span_delayed_bug(span, "function items cannot be used as const args");
+                let args = self.lower_generic_args_of_path_segment(
+                    span,
+                    did,
+                    path.segments.last().unwrap(),
+                );
+                ty::Const::new_value(tcx, ty::ValTree::zst(), Ty::new_fn_def(tcx, did, args))
+            }
 
             // Exhaustive match to be clear about what exactly we're considering to be
             // an invalid Res for a const path.

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -1800,7 +1800,7 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
         }
 
         let u8_type = self.tcx().types.u8;
-        match (cv.valtree, cv.ty.kind()) {
+        match (cv.valtree, *cv.ty.kind()) {
             (ty::ValTree::Branch(_), ty::Ref(_, inner_ty, _)) => match inner_ty.kind() {
                 ty::Slice(t) if *t == u8_type => {
                     let bytes = cv.try_to_raw_bytes(self.tcx()).unwrap_or_else(|| {
@@ -1820,13 +1820,13 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
                     return Ok(());
                 }
                 _ => {
-                    let cv = ty::Value { valtree: cv.valtree, ty: *inner_ty };
+                    let cv = ty::Value { valtree: cv.valtree, ty: inner_ty };
                     p!("&");
                     p!(pretty_print_const_valtree(cv, print_ty));
                     return Ok(());
                 }
             },
-            (ty::ValTree::Branch(_), ty::Array(t, _)) if *t == u8_type => {
+            (ty::ValTree::Branch(_), ty::Array(t, _)) if t == u8_type => {
                 let bytes = cv.try_to_raw_bytes(self.tcx()).unwrap_or_else(|| {
                     bug!("expected to convert valtree to raw bytes for type {:?}", t)
                 });
@@ -1893,11 +1893,16 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
             }
             (ty::ValTree::Leaf(leaf), ty::Ref(_, inner_ty, _)) => {
                 p!(write("&"));
-                return self.pretty_print_const_scalar_int(leaf, *inner_ty, print_ty);
+                return self.pretty_print_const_scalar_int(leaf, inner_ty, print_ty);
             }
             (ty::ValTree::Leaf(leaf), _) => {
                 return self.pretty_print_const_scalar_int(leaf, cv.ty, print_ty);
             }
+            (_, ty::FnDef(def_id, args)) => {
+                // Never allowed today, but we still encounter them in invalid const args.
+                p!(print_value_path(def_id, args));
+                return Ok(());
+            }
             // FIXME(oli-obk): also pretty print arrays and other aggregate constants by reading
             // their fields instead of just dumping the memory.
             _ => {}

tests/ui/const-generics/mgca/unexpected-fn-item-in-array.rs

@@ -0,0 +1,13 @@
+// Make sure we don't ICE when encountering an fn item during lowering in mGCA.
+
+#![feature(min_generic_const_args)]
+//~^ WARN the feature `min_generic_const_args` is incomplete
+
+trait A<T> {}
+
+impl A<[usize; fn_item]> for () {}
+//~^ ERROR the constant `fn_item` is not of type `usize`
+
+fn fn_item() {}
+
+fn main() {}

tests/ui/const-generics/mgca/unexpected-fn-item-in-array.stderr

@@ -0,0 +1,19 @@
+warning: the feature `min_generic_const_args` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/unexpected-fn-item-in-array.rs:3:12
+   |
+LL | #![feature(min_generic_const_args)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #132980 <https://github.com/rust-lang/rust/issues/132980> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: the constant `fn_item` is not of type `usize`
+  --> $DIR/unexpected-fn-item-in-array.rs:8:6
+   |
+LL | impl A<[usize; fn_item]> for () {}
+   |      ^^^^^^^^^^^^^^^^^^^ expected `usize`, found fn item
+   |
+   = note: the length of array `[usize; fn_item]` must be type `usize`
+
+error: aborting due to 1 previous error; 1 warning emitted
+