ImportC: Add __module declarations

changelog/dmd.import-c-modules.dd

@@ -0,0 +1,45 @@
+C files can now include a module statement
+
+Similar to the `__import` extension, the `__module` keyword brings D's module declaration to C.
+
+It's particularly useful when you want to import multiple C files with the same name
+(e.g. hello/utils.c and world/utils.c), since both have to be imported with `import utils` when
+they are listed on the command line, resulting in conflicts.
+
+Now you can do:
+
+hello/utils.c:
+
+```C
+
+#if __IMPORTC__
+__module hello.utils;
+#endif
+
+int sqr(int x) { return x * x; }
+```
+
+world/utils.c:
+```C
+
+#if __IMPORTC__
+__module world.utils;
+#endif
+
+int max(int a, int b) { return a > b ? a : b; }
+```
+
+app.d:
+```D
+import hello.utils;
+import world.utils;
+
+static assert(sqr(3) == 9);
+static assert(max(3, 5) == 5);
+```
+
+A __module declaration can appear anywhere in the top level scope.
+When there are multiple, the first one will be used.
+Therefore, every `#include` containing a __module declaration should come after the file's own module declaration,
+or it will be overwritten.
+When you always put the __module declaration at the very top like in D, there won't be such problems.

compiler/src/dmd/cparse.d

@@ -143,6 +143,20 @@ final class CParser(AST) : Parser!AST
                 return wrap;
             }
 
+            if (token.value == TOK._module)
+            {
+                token.value = TOK.module_;
+                auto oldMd = this.md;
+                parseModuleDeclaration();
+                if (oldMd)
+                {
+                    // We only use the first module declaration,
+                    // subsequent __module statements should only come from #included files
+                    this.md = oldMd;
+                }
+                continue;
+            }
+
             /* GNU Extensions
              * external-declaration:
              *    simple-asm-expr ;

compiler/src/dmd/dmodule.d

@@ -806,7 +806,13 @@ extern (C++) final class Module : Package
             p.nextToken();
             checkCompiledImport();
             members = p.parseModule();
-            assert(!p.md); // C doesn't have module declarations
+            md = p.md;
+            if (md)
+            {
+                this.ident = md.id;
+                dst = Package.resolve(md.packages, &this.parent, &ppack);
+            }
+
             numlines = p.scanloc.linnum;
         }
         else

compiler/src/dmd/frontend.h

@@ -3011,13 +3011,14 @@ enum class TOK : uint8_t
     _Thread_local_ = 216u,
     _assert_ = 217u,
     _import_ = 218u,
-    __cdecl_ = 219u,
-    __declspec_ = 220u,
-    __stdcall_ = 221u,
-    __thread_ = 222u,
-    __pragma_ = 223u,
-    __int128_ = 224u,
-    __attribute___ = 225u,
+    _module = 219u,
+    __cdecl_ = 220u,
+    __declspec_ = 221u,
+    __stdcall_ = 222u,
+    __thread_ = 223u,
+    __pragma_ = 224u,
+    __int128_ = 225u,
+    __attribute___ = 226u,
 };
 
 class FuncExp final : public Expression

compiler/src/dmd/tokens.d

@@ -276,6 +276,7 @@ enum TOK : ubyte
     // C only extended keywords
     _assert,
     _import,
+    _module,
     __cdecl,
     __declspec,
     __stdcall,
@@ -584,6 +585,7 @@ private immutable TOK[] keywords =
     // C only extended keywords
     TOK._assert,
     TOK._import,
+    TOK._module,
     TOK.__cdecl,
     TOK.__declspec,
     TOK.__stdcall,
@@ -619,7 +621,7 @@ static immutable TOK[TOK.max + 1] Ckeywords =
                        union_, unsigned, void_, volatile, while_, asm_, typeof_,
                        _Alignas, _Alignof, _Atomic, _Bool, _Complex, _Generic, _Imaginary, _Noreturn,
                        _Static_assert, _Thread_local,
-                       _import, __cdecl, __declspec, __stdcall, __thread, __pragma, __int128, __attribute__,
+                       _import, _module, __cdecl, __declspec, __stdcall, __thread, __pragma, __int128, __attribute__,
                        _assert ];
 
         foreach (kw; Ckwds)
@@ -898,6 +900,7 @@ extern (C++) struct Token
         // C only extended keywords
         TOK._assert       : "__check",
         TOK._import       : "__import",
+        TOK._module       : "__module",
         TOK.__cdecl        : "__cdecl",
         TOK.__declspec     : "__declspec",
         TOK.__stdcall      : "__stdcall",

compiler/src/dmd/tokens.h

@@ -282,6 +282,7 @@ enum class TOK : unsigned char
     // C only extended keywords
     _assert,
     _import,
+    _module,
     cdecl_,
     declspec,
     stdcall,

compiler/test/compilable/cmodules.d

@@ -0,0 +1,9 @@
+/**
+EXTRA_SOURCES: imports/cpkg/cmodule.c
+*/
+
+import imports.cpkg.cmodule;
+
+static assert(sqr(3) == 9);
+
+void main() {}

compiler/test/compilable/imports/cpkg/cmodule.c

@@ -0,0 +1,13 @@
+// D module declaration
+
+#if __IMPORTC__
+
+__module imports.cpkg.cmodule;
+
+// Only the first module statement is used,
+// subsequent __module declarations are assumed to come from #included other files
+__module some.header;
+
+#endif
+
+int sqr(int i) { return i * i; }

compiler/test/fail_compilation/cmodule_malformed.c

@@ -0,0 +1,31 @@
+/**
+TEST_OUTPUT:
+---
+fail_compilation/cmodule_malformed.c(15): Error: identifier expected following `module`
+fail_compilation/cmodule_malformed.c(15): Error: no type for declarator before `"a"`
+fail_compilation/cmodule_malformed.c(21): Error: no type-specifier for struct member
+fail_compilation/cmodule_malformed.c(21): Error: identifier or `(` expected
+fail_compilation/cmodule_malformed.c(21): Error: expected identifier for declarator
+fail_compilation/cmodule_malformed.c(26): Error: found `__module` instead of statement
+---
+*/
+
+#if __IMPORTC__
+
+__module "a";
+
+typedef struct S
+{
+    int x;
+
+    __module b;
+} S;
+
+void main(void)
+{
+    __module c.d;
+}
+
+__module e;
+
+#endif

compiler/test/unit/lexer/location_offset.d

@@ -542,6 +542,7 @@ enum ignoreTokens
 
     _assert,
     _import,
+    _module,
     __cdecl,
     __declspec,
     __stdcall,