Fix failures to resolve primitives

Previously, when looking for the associated items for primitives,
rustdoc would look for primitives in the current namespace.
But all primitives are in the type namespace. To fix this, rustdoc now
always looks for primitives in the namespace when considering them as a
stepping stone to the associated item.

However, fixing that bug caused several duplicate errors because rustdoc
now reports the same error in each namespace. To avoid this, rustdoc now
ignores all duplicate errors when issuing them.

Author: jyn514

src/librustdoc/passes/collect_intra_doc_links.rs

@@ -331,7 +331,7 @@ impl<'a, 'tcx> LinkCollector<'a, 'tcx> {
                     ErrorKind::Resolve(ResolutionFailure::NotInScope(item_name.to_string().into()))
                 })?;
 
-            if let Some((path, prim)) = is_primitive(&path_root, ns) {
+            if let Some((path, prim)) = is_primitive(&path_root, TypeNS) {
                 let impls = primitive_impl(cx, &path).ok_or_else(|| {
                     ErrorKind::Resolve(ResolutionFailure::NoPrimitiveImpl(prim, path_root.into()))
                 })?;
@@ -355,6 +355,12 @@ impl<'a, 'tcx> LinkCollector<'a, 'tcx> {
                         return Ok(link);
                     }
                 }
+                debug!(
+                    "returning primitive error for {}::{} in {} namespace",
+                    path,
+                    item_name,
+                    ns.descr()
+                );
                 return Err(ErrorKind::Resolve(ResolutionFailure::NoPrimitiveAssocItem {
                     res: prim,
                     prim_name: path,
@@ -1404,7 +1410,6 @@ fn resolution_failure(
         &link_range,
         |diag, sp| {
             let in_scope = kinds.iter().any(|kind| kind.res().is_some());
-            let mut reported_not_in_scope = false;
             let item = |res: Res| {
                 format!("the {} `{}`", res.descr(), cx.tcx.item_name(res.def_id()).to_string())
             };
@@ -1419,14 +1424,19 @@ fn resolution_failure(
                 );
                 diag.note(&note);
             };
+            // ignore duplicates
+            let mut variants_seen = SmallVec::<[_; 3]>::new();
             for failure in kinds {
+                let variant = std::mem::discriminant(&failure);
+                if variants_seen.contains(&variant) {
+                    continue;
+                }
+                variants_seen.push(variant);
                 match failure {
-                    // already handled above
                     ResolutionFailure::NotInScope(base) => {
-                        if in_scope || reported_not_in_scope {
+                        if in_scope {
                             continue;
                         }
-                        reported_not_in_scope = true;
                         diag.note(&format!("no item named `{}` is in scope", base));
                         diag.help(r#"to escape `[` and `]` characters, add '\' before them like `\[` or `\]`"#);
                     }