rustdoc: error when rustc_doc_primitive has an unknown type

Currently rustdoc silently does nothing if it isn't aware of the
specified primitive type. Change this to raise an error instead.

Author: tgross35

src/librustdoc/clean/types.rs

@@ -23,6 +23,7 @@ use rustc_hir::{BodyId, Mutability};
 use rustc_hir_analysis::check::intrinsic::intrinsic_operation_unsafety;
 use rustc_index::IndexVec;
 use rustc_metadata::rendered_const;
+use rustc_middle::span_bug;
 use rustc_middle::ty::fast_reject::SimplifiedType;
 use rustc_middle::ty::{self, TyCtxt, Visibility};
 use rustc_resolve::rustdoc::{
@@ -266,8 +267,15 @@ impl ExternalCrate {
         let as_primitive = |res: Res<!>| {
             let Res::Def(DefKind::Mod, def_id) = res else { return None };
             tcx.get_attrs(def_id, sym::rustc_doc_primitive).find_map(|attr| {
-                // FIXME: should warn on unknown primitives?
-                Some((def_id, PrimitiveType::from_symbol(attr.value_str()?)?))
+                let attr_value = attr.value_str().expect("syntax should already be validated");
+                let Some(prim) = PrimitiveType::from_symbol(attr_value) else {
+                    span_bug!(
+                        attr.span,
+                        "primitive `{attr_value}` is not a member of `PrimitiveType`"
+                    );
+                };
+
+                Some((def_id, prim))
             })
         };