[flang] Safer hermetic module file reading

When a hermetic module file is read, use a new scope to hold its
dependent modules so that they don't conflict with any modules in
the global scope.

Author: klausler

flang/docs/ModFiles.md

@@ -164,3 +164,13 @@ a diagnostic but we still wouldn't have line numbers.
 To provide line numbers and character positions or source lines as the user
 wrote them we would have to save some amount of provenance information in the
 module file as well.
+
+## Hermetic modules files
+
+Top-level module files for libraries can be build with `-fhermetic-module-files`.
+This option causes these module files to contain copies of all of the non-intrinsic
+modules on which they depend, so that non-top-level local modules and the
+modules of dependent libraries need not also be packaged with the library.
+When the compiler reads a hermetic module file, the copies of the dependent
+modules are read into their own scope, and will not conflict with other modules
+of the same name that client code might `USE`.

flang/include/flang/Semantics/semantics.h

@@ -110,6 +110,12 @@ class SemanticsContext {
   }
   Scope &globalScope() { return globalScope_; }
   Scope &intrinsicModulesScope() { return intrinsicModulesScope_; }
+  Scope *currentHermeticModuleFileScope() {
+    return currentHermeticModuleFileScope_;
+  }
+  void set_currentHermeticModuleFileScope(Scope *scope) {
+    currentHermeticModuleFileScope_ = scope;
+  }
   parser::Messages &messages() { return messages_; }
   evaluate::FoldingContext &foldingContext() { return foldingContext_; }
   parser::AllCookedSources &allCookedSources() { return allCookedSources_; }
@@ -313,6 +319,7 @@ class SemanticsContext {
   evaluate::TargetCharacteristics targetCharacteristics_;
   Scope globalScope_;
   Scope &intrinsicModulesScope_;
+  Scope *currentHermeticModuleFileScope_{nullptr};
   ScopeIndex scopeIndex_;
   parser::Messages messages_;
   evaluate::FoldingContext foldingContext_;

flang/lib/Semantics/mod-file.cpp

@@ -1366,6 +1366,12 @@ Scope *ModFileReader::Read(SourceName name, std::optional<bool> isIntrinsic,
       name.ToString(), isIntrinsic.value_or(false))};
   if (!isIntrinsic.value_or(false) && !ancestor) {
     // Already present in the symbol table as a usable non-intrinsic module?
+    if (Scope * hermeticScope{context_.currentHermeticModuleFileScope()}) {
+      auto it{hermeticScope->find(name)};
+      if (it != hermeticScope->end()) {
+        return it->second->scope();
+      }
+    }
     auto it{context_.globalScope().find(name)};
     if (it != context_.globalScope().end()) {
       Scope *scope{it->second->scope()};
@@ -1543,9 +1549,22 @@ Scope *ModFileReader::Read(SourceName name, std::optional<bool> isIntrinsic,
   // Process declarations from the module file
   auto wasModuleFileName{context_.foldingContext().moduleFileName()};
   context_.foldingContext().set_moduleFileName(name);
+  // Are there multiple modules in the module file due to it having been
+  // created under -fhermetic-module-files?  If so, process them first in
+  // their own nested scope that will be visible only to USE statements
+  // within the module file.
+  if (parseTree.v.size() > 1) {
+    parser::Program hermeticModules{std::move(parseTree.v)};
+    parseTree.v.emplace_back(std::move(hermeticModules.v.front()));
+    hermeticModules.v.pop_front();
+    Scope &hermeticScope{topScope.MakeScope(Scope::Kind::Global)};
+    context_.set_currentHermeticModuleFileScope(&hermeticScope);
+    ResolveNames(context_, hermeticModules, hermeticScope);
+  }
   GetModuleDependences(context_.moduleDependences(), sourceFile->content());
   ResolveNames(context_, parseTree, topScope);
   context_.foldingContext().set_moduleFileName(wasModuleFileName);
+  context_.set_currentHermeticModuleFileScope(nullptr);
   if (!moduleSymbol) {
     // Submodule symbols' storage are owned by their parents' scopes,
     // but their names are not in their parents' dictionaries -- we

flang/lib/Semantics/resolve-names.cpp

@@ -2548,9 +2548,11 @@ void ScopeHandler::PopScope() {
     ConvertToObjectEntity(*pair.second);
   }
   funcResultStack_.Pop();
-  // If popping back into a global scope, pop back to the main global scope.
-  SetScope(currScope_->parent().IsGlobal() ? context().globalScope()
-                                           : currScope_->parent());
+  // If popping back into a global scope, pop back to the top scope.
+  Scope *hermetic{context().currentHermeticModuleFileScope()};
+  SetScope(currScope_->parent().IsGlobal()
+          ? (hermetic ? *hermetic : context().globalScope())
+          : currScope_->parent());
 }
 void ScopeHandler::SetScope(Scope &scope) {
   currScope_ = &scope;
@@ -9375,6 +9377,12 @@ template <typename A> std::set<SourceName> GetUses(const A &x) {
 }
 
 bool ResolveNamesVisitor::Pre(const parser::Program &x) {
+  if (Scope * hermetic{context().currentHermeticModuleFileScope()}) {
+    // Processing either the dependent modules or first module of a
+    // hermetic module file; ensure that the hermetic module scope has
+    // its implicit rules map entry.
+    ImplicitRulesVisitor::BeginScope(*hermetic);
+  }
   std::map<SourceName, const parser::ProgramUnit *> modules;
   std::set<SourceName> uses;
   bool disordered{false};