[clang][Modules] Reporting Errors for Duplicating Link Declarations in modulemaps (#148959)

This PR teaches the modulemap parsing logic to report warnings that
default to errors if the parsing logic sees duplicating link
declarations in the same module. Specifically, duplicating link
declarations means multiple link declarations with the same
string-literal in the same module. No errors are reported if a same link
declaration exist in a submodule and its enclosing module.

The warning can be disabled with `-Wno-module-link-redeclaration`. 

rdar://155880064

clang/include/clang/Basic/DiagnosticLexKinds.td

@@ -912,6 +912,11 @@ def err_mmap_nested_submodule_id : Error<
   "qualified module name can only be used to define modules at the top level">;
 def err_mmap_expected_feature : Error<"expected a feature name">;
 def err_mmap_expected_attribute : Error<"expected an attribute name">;
+def warn_mmap_link_redeclaration : Warning<"redeclaration of link library '%0'">,
+  InGroup<DiagGroup<"module-link-redeclaration">>, DefaultError;
+def note_mmap_prev_link_declaration : Note<"previously declared here">;
+def err_mmap_submodule_link_decl
+    : Error<"link declaration is not allowed in submodules">;
 def warn_mmap_unknown_attribute : Warning<"unknown attribute '%0'">,
   InGroup<IgnoredAttributes>;
 def warn_mmap_mismatched_private_submodule : Warning<

clang/lib/Lex/ModuleMapFile.cpp

@@ -118,7 +118,8 @@ struct ModuleMapFileParser {
   std::optional<ExcludeDecl> parseExcludeDecl(clang::SourceLocation LeadingLoc);
   std::optional<UmbrellaDirDecl>
   parseUmbrellaDirDecl(SourceLocation UmbrellaLoc);
-  std::optional<LinkDecl> parseLinkDecl();
+  std::optional<LinkDecl>
+  parseLinkDecl(llvm::StringMap<SourceLocation> &SeenLinkDecl, bool Allowed);
 
   SourceLocation consumeToken();
   void skipUntil(MMToken::TokenKind K);
@@ -325,6 +326,7 @@ std::optional<ModuleDecl> ModuleMapFileParser::parseModuleDecl(bool TopLevel) {
   SourceLocation LBraceLoc = consumeToken();
 
   bool Done = false;
+  llvm::StringMap<SourceLocation> SeenLinkDecl;
   do {
     std::optional<Decl> SubDecl;
     switch (Tok.Kind) {
@@ -405,7 +407,9 @@ std::optional<ModuleDecl> ModuleMapFileParser::parseModuleDecl(bool TopLevel) {
       break;
 
     case MMToken::LinkKeyword:
-      SubDecl = parseLinkDecl();
+      // Link decls are only allowed in top level modules or explicit
+      // submodules.
+      SubDecl = parseLinkDecl(SeenLinkDecl, TopLevel || MDecl.Explicit);
       break;
 
     default:
@@ -822,7 +826,8 @@ ModuleMapFileParser::parseUmbrellaDirDecl(clang::SourceLocation UmbrellaLoc) {
 ///
 ///   module-declaration:
 ///     'link' 'framework'[opt] string-literal
-std::optional<LinkDecl> ModuleMapFileParser::parseLinkDecl() {
+std::optional<LinkDecl> ModuleMapFileParser::parseLinkDecl(
+    llvm::StringMap<SourceLocation> &SeenLinkDecl, bool Allowed) {
   assert(Tok.is(MMToken::LinkKeyword));
   LinkDecl LD;
   LD.Location = consumeToken();
@@ -838,12 +843,33 @@ std::optional<LinkDecl> ModuleMapFileParser::parseLinkDecl() {
   if (!Tok.is(MMToken::StringLiteral)) {
     Diags.Report(Tok.getLocation(), diag::err_mmap_expected_library_name)
         << LD.Framework << SourceRange(LD.Location);
+    consumeToken();
     HadError = true;
     return std::nullopt;
   }
 
-  LD.Library = Tok.getString();
+  StringRef Library = Tok.getString();
+
+  LD.Library = Library;
   consumeToken();
+
+  // Make sure we eat all the tokens when we report the errors so parsing
+  // can continue.
+  if (!Allowed) {
+    Diags.Report(LD.Location, diag::err_mmap_submodule_link_decl);
+    HadError = true;
+    return std::nullopt;
+  }
+
+  auto [It, Inserted] =
+      SeenLinkDecl.insert(std::make_pair(Library, LD.Location));
+  if (!Inserted) {
+    Diags.Report(LD.Location, diag::warn_mmap_link_redeclaration) << Library;
+    Diags.Report(It->second, diag::note_mmap_prev_link_declaration);
+    HadError = true;
+    return std::nullopt;
+  }
+
   return std::move(LD);
 }
 

clang/test/ClangScanDeps/link-libraries-diag-dup.c

@@ -0,0 +1,57 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file %s %t
+
+//--- module.modulemap
+module A {
+  umbrella header "A.h"
+
+  module B {
+    header "B.h"
+    link "libraryB"
+  }
+
+  explicit module D {
+    header "D.h"
+    link "libraryD"
+  }
+
+  link "libraryA"
+  link "libraryA"
+}
+
+module C {
+  header "C.h"
+  link "libraryA"
+}
+
+//--- A.h
+#include "B.h"
+//--- B.h
+// empty
+//--- C.h
+// empty
+//--- D.h
+// empty
+//--- TU.c
+#include "A.h"
+#include "C.h"
+#include "D.h"
+
+//--- cdb.json.template
+[{
+  "file": "DIR/TU.c",
+  "directory": "DIR",
+  "command": "clang -fmodules -fmodules-cache-path=DIR/cache -I DIR -c DIR/TU.c"
+}]
+
+// RUN: sed "s|DIR|%/t|g" %t/cdb.json.template > %t/cdb.json
+// RUN: not clang-scan-deps -compilation-database %t/cdb.json -format \
+// RUN:   experimental-full 2>&1 | FileCheck %s
+
+// Note that module D does not report an error because it is explicit.
+// Therefore we can use CHECK-NEXT for the redeclaration error on line 15.
+// CHECK:      module.modulemap:6:5:  error: link declaration is not allowed in submodules
+// CHECK-NEXT: module.modulemap:15:3: error: redeclaration of link library 'libraryA' [-Wmodule-link-redeclaration]
+// CHECK-NEXT: module.modulemap:14:3: note: previously declared here
+// CHECK-NOT:  module.modulemap:20:3: error: redeclaration of link library 'libraryA'