Introduce a module alias option to the frontend

include/swift/AST/DiagnosticsFrontend.def

@@ -150,9 +150,14 @@ ERROR(error_stdlib_module_name,none,
       "module name \"%0\" is reserved for the standard library"
       "%select{|; use -module-name flag to specify an alternate name}1",
       (StringRef, bool))
-
 ERROR(error_stdlib_not_found,Fatal,
       "unable to load standard library for target '%0'", (StringRef))
+ERROR(error_module_alias_invalid_format,none,
+      "invalid module alias format \"%0\"; make sure to use the format '-module-alias alias_name=underlying_name'", (StringRef))
+ERROR(error_module_alias_forbidden_name,none,
+      "invalid module alias \"%0\"; make sure the alias differs from the module name, module ABI name, module link name, and a standard library name", (StringRef))
+ERROR(error_module_alias_duplicate,none,
+      "duplicate module alias; the name \"%0\" is already used for a module alias or an underlying name", (StringRef))
 
 ERROR(error_unable_to_load_supplementary_output_file_map, none,
       "unable to load supplementary output file map '%0': %1",

include/swift/Frontend/FrontendOptions.h

@@ -18,6 +18,7 @@
 #include "swift/Frontend/FrontendInputsAndOutputs.h"
 #include "swift/Frontend/InputFile.h"
 #include "llvm/ADT/Hashing.h"
+#include "llvm/ADT/StringMap.h"
 
 #include <string>
 #include <vector>
@@ -48,6 +49,9 @@ class FrontendOptions {
   /// An Objective-C header to import and make implicitly visible.
   std::string ImplicitObjCHeaderPath;
 
+  /// The map of aliases and underlying names of imported or referenced modules.
+  llvm::StringMap<StringRef> ModuleAliasMap;
+
   /// The name of the module that the frontend is building.
   std::string ModuleName;
 

include/swift/Option/Options.td

@@ -430,6 +430,11 @@ def module_name : Separate<["-"], "module-name">,
 def module_name_EQ : Joined<["-"], "module-name=">, Flags<[FrontendOption]>,
   Alias<module_name>;
 
+def module_alias : Separate<["-"], "module-alias">,
+  Flags<[FrontendOption]>,
+  MetaVarName<"<alias_name=underlying_name>">,
+  HelpText<"If a source file imports or references module <alias_name>, the underlying name is used for the contents of the file">;
+
 def module_link_name : Separate<["-"], "module-link-name">,
   Flags<[FrontendOption, ModuleInterfaceOption]>,
   HelpText<"Library to link against when using this module">;

lib/Frontend/ArgsToFrontendOptionsConverter.cpp

@@ -244,6 +244,11 @@ bool ArgsToFrontendOptionsConverter::convert(
   if (const Arg *A = Args.getLastArg(OPT_module_link_name))
     Opts.ModuleLinkName = A->getValue();
 
+  // This must be called after computing module name, module abi name,
+  // and module link name.
+  if (computeModuleAliases())
+    return true;
+
   if (const Arg *A = Args.getLastArg(OPT_access_notes_path))
     Opts.AccessNotesPath = A->getValue();
 
@@ -498,7 +503,65 @@ bool ArgsToFrontendOptionsConverter::setUpImmediateArgs() {
   return false;
 }
 
+bool ArgsToFrontendOptionsConverter::computeModuleAliases() {
+  auto list = Args.getAllArgValues(options::OPT_module_alias);
+  if (!list.empty()) {
+    auto validate = [this](StringRef value, bool allowModuleName) -> bool
+    {
+      if (!allowModuleName) {
+        if (value == Opts.ModuleName ||
+            value == Opts.ModuleABIName ||
+            value == Opts.ModuleLinkName) {
+          Diags.diagnose(SourceLoc(), diag::error_module_alias_forbidden_name, value);
+          return false;
+        }
+      }
+      if (value == STDLIB_NAME) {
+        Diags.diagnose(SourceLoc(), diag::error_module_alias_forbidden_name, value);
+        return false;
+      }
+      if (!Lexer::isIdentifier(value)) {
+        Diags.diagnose(SourceLoc(), diag::error_bad_module_name, value, false);
+        return false;
+      }
+      return true;
+    };
+
+    for (auto item: list) {
+      auto str = StringRef(item);
+      // splits to an alias and the underlying name
+      auto pair = str.split('=');
+      auto lhs = pair.first;
+      auto rhs = pair.second;
+
+      if (rhs.empty()) { // '=' is missing
+          Diags.diagnose(SourceLoc(), diag::error_module_alias_invalid_format, str);
+          return true;
+      }
+      if (!validate(lhs, false) || !validate(rhs, true)) {
+          return true;
+      }
+
+      // First, add the underlying name as a key to prevent it from being
+      // used as an alias
+      if (!Opts.ModuleAliasMap.insert({rhs, StringRef()}).second) {
+        Diags.diagnose(SourceLoc(), diag::error_module_alias_duplicate, rhs);
+        return true;
+      }
+      // Next, add the alias as a key and the underlying name as a value to the map
+      auto underlyingName = Opts.ModuleAliasMap.find(rhs)->first();
+      if (!Opts.ModuleAliasMap.insert({lhs, underlyingName}).second) {
+          Diags.diagnose(SourceLoc(), diag::error_module_alias_duplicate, lhs);
+          return true;
+      }
+    }
+  }
+  return false;
+}
+
 bool ArgsToFrontendOptionsConverter::computeModuleName() {
+  assert(Opts.ModuleAliasMap.empty() && "Module name must be computed before computing module aliases");
+
   const Arg *A = Args.getLastArg(options::OPT_module_name);
   if (A) {
     Opts.ModuleName = A->getValue();

lib/Frontend/ArgsToFrontendOptionsConverter.h

@@ -37,6 +37,7 @@ class ArgsToFrontendOptionsConverter {
   void computeDebugTimeOptions();
   bool computeFallbackModuleName();
   bool computeModuleName();
+  bool computeModuleAliases();
   bool computeMainAndSupplementaryOutputFilenames();
   void computeDumpScopeMapLocations();
   void computeHelpOptions();

test/Frontend/Inputs/invalid-module-alias.swift

@@ -0,0 +1,3 @@
+func quip() {
+}
+

test/Frontend/invalid-module-alias.swift

@@ -0,0 +1,25 @@
+// Tests for invalid module alias format and values.
+
+// RUN: not %target-swift-frontend -emit-silgen -parse-as-library %S/Inputs/invalid-module-alias.swift -module-name foo -module-alias foo=bar 2>&1 | %FileCheck -check-prefix=INVALID_MODULE_ALIAS %s
+// INVALID_MODULE_ALIAS: error: invalid module alias "foo"; make sure the alias differs from the module name, module ABI name, module link name, and a standard library name
+
+// RUN: not %target-swift-frontend -emit-silgen -parse-as-library %S/Inputs/invalid-module-alias.swift -module-name foo -module-alias Swift=Bar 2>&1 | %FileCheck -check-prefix=INVALID_MODULE_ALIAS1 %s
+// INVALID_MODULE_ALIAS1: error: invalid module alias "Swift"; make sure the alias differs from the module name, module ABI name, module link name, and a standard library name
+
+// RUN: not %target-swift-frontend -emit-silgen -parse-as-library %S/Inputs/invalid-module-alias.swift -module-name foo -module-alias bar=bar 2>&1 | %FileCheck -check-prefix=INVALID_MODULE_ALIAS2 %s
+// INVALID_MODULE_ALIAS2: error: duplicate module alias; the name "bar" is already used for a module alias or an underlying name
+
+// RUN: not %target-swift-frontend -emit-silgen -parse-as-library %S/Inputs/invalid-module-alias.swift -module-name foo -module-alias bar=baz -module-alias baz=cat 2>&1 | %FileCheck -check-prefix=INVALID_MODULE_ALIAS3 %s
+// INVALID_MODULE_ALIAS3: error: duplicate module alias; the name "baz" is already used for a module alias or an underlying name
+
+// RUN: not %target-swift-frontend -emit-silgen -parse-as-library %S/Inputs/invalid-module-alias.swift -module-name foo -module-alias bar 2>&1 | %FileCheck -check-prefix=INVALID_MODULE_ALIAS4 %s
+// INVALID_MODULE_ALIAS4: error: invalid module alias format "bar"; make sure to use the format '-module-alias alias_name=underlying_name'
+
+// RUN: not %target-swift-frontend -emit-silgen -parse-as-library %S/Inputs/invalid-module-alias.swift -module-name foo -module-alias bar=c-a.t 2>&1 | %FileCheck -check-prefix=INVALID_MODULE_NAME %s
+// INVALID_MODULE_NAME: error: module name "c-a.t" is not a valid identifier
+
+// These should succeed.
+// RUN: %target-swift-frontend -emit-silgen %S/Inputs/invalid-module-alias.swift > /dev/null
+// RUN: %target-swift-frontend -emit-silgen -parse-as-library -module-name foo %S/Inputs/invalid-module-alias.swift -module-alias bar=cat > /dev/null
+// RUN: %target-swift-frontend -typecheck -parse-as-library -module-name foo %S/Inputs/invalid-module-alias.swift -module-alias bar=cat
+