IC: a backend stage stops building the interface half it cannot read

`populateInterfaceTablesFromIndex` stubbed a `PSym` for every global symbol a
module declares and inserted it into `interf` and/or `interfHidden`. On a
68-module target that is 2.37M stubs across the build — and 2.04M of them go
into `interfHidden` alone, which a `cg`/`emit` stage has no way to reach.

`interfHidden` is selected exclusively by `modulegraphs.interfSelect`, and only
when `optImportHidden` is in the module's options. That flag is set in exactly
one place — `importer.importModuleAs`, i.e. during sem. A backend stage builds
its module symbols in `moduleFromNifFile` and never imports anything, so the
flag cannot be set and the table cannot be selected. Skipping the hidden-only
branch there is unobservable, not merely unlikely to be observed.

Exported symbols still go into both tables. That is 0.33M inserts against the
2.0M saved, and it leaves `interfHidden` a coherent view — a module with no
hidden symbols — rather than an empty one, should anything ever consult it.

    interface tables   1401ms -> 863ms
    loadDepClosure     2771ms -> 2309ms
    cold --ic:on build  9.38s -> 8.91s

Skipping the tables WHOLESALE in a backend stage does not work, and the failure
is worth recording since it is the obvious next thought: the exported half is
what `magicsys.getSysSym` resolves through, so the stage dies with "system
module needs: pointer". Skipping them in the FRONTEND is worse still — `nim m`
loses `defined`.

Verified: both configurations build; `tests/ic` 40/40; 67/67 generated `.c`
byte-identical, cursor still identical to `PNode`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FMyRHByv7hhaQJ4Pa1bHbE

compiler/ast2nif.nim

@@ -3688,13 +3688,32 @@ proc populateInterfaceTablesFromIndex(c: var DecodeContext; module: FileIndex;
   var indexTab = move c.mods[module].index
 
   # Add all symbols to interf (exported interface) and interfHidden
+  # A BACKEND stage cannot read the hidden-only half, so it does not build it.
+  # `interfHidden` is reached exclusively through `modulegraphs.interfSelect`,
+  # which picks it only when `optImportHidden` is in the module's options — and
+  # that flag is set in exactly one place, `importer.importModuleAs`, i.e.
+  # during sem. `cg`/`emit` build their module symbols in `moduleFromNifFile`
+  # and never import anything, so the flag cannot be set and the table cannot be
+  # selected. The saving is the bulk of this proc: on a 68-module target the
+  # hidden branch stubs 2.0M symbols against the exported branch's 0.33M.
+  #
+  # Exported symbols are still added to BOTH tables. That costs little beside
+  # the above and leaves `interfHidden` a coherent view (a module with no hidden
+  # symbols) rather than an empty one, should anything ever consult it.
+  let conf = c.infos.config
+  let backendStage = conf.cmd == cmdNifC and
+                     (conf.icBackendStage == "cg" or conf.icBackendStage == "emit")
+
+  prof pIfaceModules
   for nifName, entry in indexTab:
     if entry.vis == Exported:
+      prof pIfaceExported
       let sym = loadSymFromIndexEntry(c, module, nifName, entry, thisModule)
       if sym != nil:
         strTableAdd(interf, sym)
         strTableAdd(interfHidden, sym)
-    elif not nifName.startsWith("`t"):
+    elif not backendStage and not nifName.startsWith("`t"):
+      prof pIfaceHidden
       # do not load types, they are not part of an interface but an implementation detail!
       #echo "LOADING SYM ", nifName, " ", entry.offset
       let sym = loadSymFromIndexEntry(c, module, nifName, entry, thisModule)

compiler/icprof.nim

@@ -31,7 +31,7 @@ when defined(icBNodeProf):
     ProfSlot* = enum
       pKind, pTagKindHit, pTagKindMiss, pAstChildren, pSkip, pSon, pLen,
       pLastSon, pIterYield, pSym, pTyp, pTypTagLit, pOrigin, pNilType,
-      pGenBodyCalls, pInfo
+      pGenBodyCalls, pInfo, pIfaceExported, pIfaceHidden, pIfaceModules
     TimeSlot* = enum
       tLoadClosure, tModuleId, tBifLoad, tPosIndex, tTopLevel, tInterfTables,
       tTransform, tHandOff, tGenBody, tAnalyses,