[rustdoc] Don't try to print the value of a non-primitive const

Author: yotamofek

src/librustdoc/clean/utils.rs

@@ -371,6 +371,9 @@ pub(crate) fn print_evaluated_const(
 ) -> Option<String> {
     tcx.const_eval_poly(def_id).ok().and_then(|val| {
         let ty = tcx.type_of(def_id).instantiate_identity();
+        if !ty.is_primitive() {
+            return None;
+        }
         match (val, ty.kind()) {
             (_, &ty::Ref(..)) => None,
             (mir::ConstValue::Scalar(_), &ty::Adt(_, _)) => None,

tests/rustdoc/non-primitive-assoc-const-150312.rs

@@ -0,0 +1,18 @@
+#![crate_name = "foo"]
+
+pub trait Foo {
+    type AssocType;
+
+    const THE_CONST: Self::AssocType;
+}
+
+pub struct Bar;
+
+impl Foo for Bar {
+    type AssocType = (u32,);
+
+    //@ has foo/struct.Bar.html
+    //@ matches - '//section[@id="associatedconstant.THE_CONST"]/h4[@class="code-header"]' '^const THE_CONST: Self::AssocType'
+    //@ !matches - '//section[@id="associatedconstant.THE_CONST"]//*[last()]' '{transmute'
+    const THE_CONST: Self::AssocType = (1u32,);
+}