[flang][Semantics] Ensure deterministic mod file output (#128655)

This PR adds a test to ensure deterministic ordering in `.mod` files. It
also includes related changes to prevent non-deterministic symbol
ordering caused by pointers outside the cooked source. This issue is
particularly noticeable when using Flang as a library and compiling the
same files multiple times.

Author: inaki-amatria

flang/lib/Semantics/mod-file.cpp

@@ -836,18 +836,6 @@ void ModFileWriter::PutUseExtraAttr(
   }
 }
 
-static inline SourceName NameInModuleFile(const Symbol &symbol) {
-  if (const auto *use{symbol.detailsIf<UseDetails>()}) {
-    if (use->symbol().attrs().test(Attr::PRIVATE)) {
-      // Avoid the use in sorting of names created to access private
-      // specific procedures as a result of generic resolution;
-      // they're not in the cooked source.
-      return use->symbol().name();
-    }
-  }
-  return symbol.name();
-}
-
 // Collect the symbols of this scope sorted by their original order, not name.
 // Generics and namelists are exceptions: they are sorted after other symbols.
 void CollectSymbols(const Scope &scope, SymbolVector &sorted,
@@ -882,13 +870,8 @@ void CollectSymbols(const Scope &scope, SymbolVector &sorted,
       sorted.push_back(symbol);
     }
   }
-  // Sort most symbols by name: use of Symbol::ReplaceName ensures the source
-  // location of a symbol's name is the first "real" use.
-  auto sorter{[](SymbolRef x, SymbolRef y) {
-    return NameInModuleFile(*x).begin() < NameInModuleFile(*y).begin();
-  }};
-  std::sort(sorted.begin(), sorted.end(), sorter);
-  std::sort(generics.begin(), generics.end(), sorter);
+  std::sort(sorted.begin(), sorted.end(), SymbolSourcePositionCompare{});
+  std::sort(generics.begin(), generics.end(), SymbolSourcePositionCompare{});
   sorted.insert(sorted.end(), generics.begin(), generics.end());
   sorted.insert(sorted.end(), namelist.begin(), namelist.end());
   for (const auto &pair : scope.commonBlocks()) {

flang/test/Semantics/Inputs/modfile72.f90

@@ -0,0 +1,13 @@
+module foo
+  interface do_foo
+    procedure do_foo_impl
+  end interface
+  interface do_bar
+    procedure do_bar_impl
+  end interface
+contains
+  subroutine do_foo_impl()
+  end
+  subroutine do_bar_impl()
+  end
+end

flang/test/Semantics/modfile72.f90

@@ -0,0 +1,28 @@
+! This test verifies that both invocations produce a consistent order in the
+! generated `.mod` file. Previous versions of Flang exhibited non-deterministic
+! behavior due to pointers outside the cooked source being used to order symbols
+! in the `.mod` file.
+
+! RUN: rm -rf %t && mkdir -p %t
+! RUN: %flang_fc1 -fsyntax-only -J%t %S/Inputs/modfile72.f90
+! RUN: %flang_fc1 -fsyntax-only -J%t %s
+! RUN: cat %t/bar.mod | FileCheck %s
+
+! RUN: rm -rf %t && mkdir -p %t
+! RUN: %flang_fc1 -fsyntax-only -J%t %S/Inputs/modfile72.f90 %s
+! RUN: cat %t/bar.mod | FileCheck %s
+
+module bar
+  use foo, only : do_foo
+  use foo, only : do_bar
+contains
+  subroutine do_baz()
+    call do_foo()
+    call do_bar()
+  end
+end
+
+!      CHECK: use foo,only:do_foo
+! CHECK-NEXT: use foo,only:do_bar
+! CHECK-NEXT: use foo,only:foo$foo$do_bar_impl=>do_bar_impl
+! CHECK-NEXT: use foo,only:foo$foo$do_foo_impl=>do_foo_impl