This generates additional macros to help keeping implementation simple,
guarded with WIDL_USING ifdefs, like this:

    #ifdef WIDL_USING_WINDOWS_FOO_IFOO

I would find it more readable if we didn't follow all-uppercase for macro rule here, something like:

#define WIDL_using_Windows_Foo_IFoo

But it's a matter of taste, so I'm mentioning it just for consideration.

    #define IFooVtbl __x_ABI_CWindows_CFoo_CIFooVtbl
    #define IFoo __x_ABI_CWindows_CFoo_CIFoo

Typedefs for those would express it a bit more precisely in C (although we will need macros for other things anyway, so...).

    #define IFoo_DoFoo __x_ABI_CWindows_CFoo_CIFoo_DoFoo
    #endif /* WIDL_USING_WINDOWS_FOO_IFOO */

Implementation files can define the desired WIDL_USING preprocessor
macros before including the header, and then implement or use the
interface methods with the simple non-prefixed names instead.

How about extending it a bit more to allow specifying entire namespaces in using statements, so something like:

#define WIDL_using_Windows_Foo

would have an effect on all interfaces in this exact namespace.

Signed-off-by: Rémi Bernon <rbernon@codeweavers.com>
---
 tools/widl/header.c    | 68 ++++++++++++++++++++++++++++++++++++++++++
 tools/widl/typetree.c  | 19 ++++++++++++
 tools/widl/widltypes.h |  1 +
 3 files changed, 88 insertions(+)

diff --git a/tools/widl/header.c b/tools/widl/header.c
index 82aad0f11ca..8ca74d5a6f4 100644
--- a/tools/widl/header.c
+++ b/tools/widl/header.c
@@ -48,6 +48,8 @@ static void write_type_v(FILE *f, const decl_spec_t *t, int is_field, int declon
 static void write_apicontract_guard_start(FILE *header, const expr_t *expr);
 static void write_apicontract_guard_end(FILE *header, const expr_t *expr);
 
+static void write_widl_using_macros(FILE *header, type_t *iface);
+
 static void indent(FILE *h, int delta)
 {
   int c;
@@ -604,6 +606,7 @@ static void write_type_definition(FILE *f, type_t *t, int declonly)
         fprintf(f, "#else\n");
         write_type_left(f, &ds, NAME_C, declonly, TRUE);
         fprintf(f, ";\n");
+        if (winrt_mode) write_widl_using_macros(f, t);
         fprintf(f, "#endif\n\n");
     }
     if (contract) write_apicontract_guard_end(f, contract);
@@ -1536,6 +1539,70 @@ static void write_com_interface_start(FILE *header, const type_t *iface)
   fprintf(header,"#define __%s_%sINTERFACE_DEFINED__\n\n", iface->c_name, dispinterface ? "DISP" : "");
 }
 
+static char *get_winrt_guard_macro(type_t *iface)
+{
+    unsigned int len;
+    char *macro, *tmp = (char *)iface->c_name;
+    int i;
+
+    if (!strncmp(tmp, "__x", 3)) tmp += 3;
+    if (!strncmp(tmp, "_ABI", 4)) tmp += 4;
+    macro = xstrdup(tmp);
+
+    len = strlen(macro) + 1;
+    for (tmp = strstr(macro, "__F"); tmp; tmp = strstr(tmp, "__F"))
+        memmove(tmp + 1, tmp + 3, len - (tmp - macro) - 3);
+    for (tmp = strstr(macro, "__C"); tmp; tmp = strstr(tmp, "__C"))
+        memmove(tmp + 1, tmp + 3, len - (tmp - macro) - 3);
+    for (tmp = strstr(macro, "_C"); tmp; tmp = strstr(tmp, "_C"))
+        memmove(tmp + 1, tmp + 2, len - (tmp - macro) - 2);
+
+    for (i = strlen(macro); i > 0; --i) macro[i - 1] = toupper(macro[i - 1]);
+
+    return macro;
+}

Could you just compute it from namespace and name stored in type_t instead of parsing a previously computed string? It looks like a job for format_namespace().

Thanks,

Jacek