rustc_passes: Enforce `rustc::potential_query_instability` lint

Stop allowing `rustc::potential_query_instability` in all of
`rustc_passes` and instead allow it on a case-by-case basis if it is
safe. In this case, all instances of the lint are safe to allow.

compiler/rustc_passes/src/diagnostic_items.rs

@@ -83,6 +83,9 @@ fn all_diagnostic_items(tcx: TyCtxt<'_>, (): ()) -> DiagnosticItems {
 
     // Collect diagnostic items in other crates.
     for &cnum in tcx.crates(()).iter().chain(std::iter::once(&LOCAL_CRATE)) {
+        // We are collecting many DiagnosticItems hash maps into one
+        // DiagnosticItems hash map. The iteration order does not matter.
+        #[allow(rustc::potential_query_instability)]
         for (&name, &def_id) in &tcx.diagnostic_items(cnum).name_to_id {
             collect_item(tcx, &mut items, name, def_id);
         }

compiler/rustc_passes/src/hir_stats.rs

@@ -121,6 +121,8 @@ impl<'k> StatCollector<'k> {
     }
 
     fn print(&self, title: &str, prefix: &str) {
+        // We will soon sort, so the initial order does not matter.
+        #[allow(rustc::potential_query_instability)]
         let mut nodes: Vec<_> = self.nodes.iter().collect();
         nodes.sort_by_key(|(_, node)| node.stats.count * node.stats.size);
 
@@ -147,6 +149,8 @@ impl<'k> StatCollector<'k> {
                 to_readable_str(node.stats.size)
             );
             if !node.subnodes.is_empty() {
+                // We will soon sort, so the initial order does not matter.
+                #[allow(rustc::potential_query_instability)]
                 let mut subnodes: Vec<_> = node.subnodes.iter().collect();
                 subnodes.sort_by_key(|(_, subnode)| subnode.count * subnode.size);
 

compiler/rustc_passes/src/lib.rs

@@ -4,7 +4,6 @@
 //!
 //! This API is completely unstable and subject to change.
 
-#![allow(rustc::potential_query_instability)]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(rustdoc_internals)]

compiler/rustc_passes/src/stability.rs

@@ -1048,6 +1048,9 @@ pub fn check_unused_or_stable_features(tcx: TyCtxt<'_>) {
         tcx.sess.emit_err(errors::UnknownFeature { span, feature: *feature });
     }
 
+    // We only use the hash map contents to emit errors, and the order of
+    // emitted errors do not affect query stability.
+    #[allow(rustc::potential_query_instability)]
     for (implied_by, feature) in remaining_implications {
         let local_defined_features = tcx.lib_features(LOCAL_CRATE);
         let span = local_defined_features