rustdoc: fix min_const_generics with ty::Param

Author: lcnr

src/librustdoc/clean/mod.rs

@@ -1364,16 +1364,16 @@ impl Clean<Type> for hir::Ty<'_> {
             TyKind::Slice(ref ty) => Slice(box ty.clean(cx)),
             TyKind::Array(ref ty, ref length) => {
                 let def_id = cx.tcx.hir().local_def_id(length.hir_id);
-                let length = match cx.tcx.const_eval_poly(def_id.to_def_id()) {
-                    Ok(length) => {
-                        print_const(cx, ty::Const::from_value(cx.tcx, length, cx.tcx.types.usize))
-                    }
-                    Err(_) => cx
-                        .sess()
-                        .source_map()
-                        .span_to_snippet(cx.tcx.def_span(def_id))
-                        .unwrap_or_else(|_| "_".to_string()),
-                };
+                // NOTE(min_const_generics): We can't use `const_eval_poly` for constants
+                // as we currently do not supply the parent generics to anonymous constants
+                // but do allow `ConstKind::Param`.
+                //
+                // `const_eval_poly` tries to to first substitute generic parameters which
+                // results in an ICE while manually constructing the constant and using `eval`
+                // does nothing for `ConstKind::Param`.
+                let ct = ty::Const::from_anon_const(cx.tcx, def_id);
+                let param_env = cx.tcx.param_env(def_id);
+                let length = print_const(cx, ct.eval(cx.tcx, param_env));
                 Array(box ty.clean(cx), length)
             }
             TyKind::Tup(ref tys) => Tuple(tys.clean(cx)),

src/test/rustdoc/const-generics/type-alias.rs

@@ -0,0 +1,6 @@
+// ignore-tidy-linelength
+#![feature(min_const_generics)]
+#![crate_name = "foo"]
+
+// @has foo/type.CellIndex.html '//pre[@class="rust typedef"]' 'type CellIndex<const D: usize> = [i64; D];'
+pub type CellIndex<const D: usize> = [i64; D];