OpenGL and Vulkan examples; ptr(int)/int(ptr); trailing commas in calls; drop .fn hint

- examples/opengl.fch: OpenGL 3.3 core, shaders, VAO/VBO, a spinning triangle (checked offscreen)
- examples/vulkan.fch: instance, GPUs with properties/memory/queues, logical device and queue
- ptr(n) and int(p) conversions for C APIs that pass offsets as pointers
- .ptr works on string literals; a comma after the last call argument is allowed
- a missing header included by an imported header is now named in the error
- the old .fn extension hint is gone (Finch is new, there are no old files)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/cimport.cpp

@@ -341,8 +341,14 @@ CImports importHeaders(const std::vector<Import> &imports, const std::vector<std
             if (clang_getDiagnosticSeverity(d) >= CXDiagnostic_Error) {
                 std::string msg = str(clang_getDiagnosticSpelling(d));
                 clang_disposeDiagnostic(d);
-                if (msg.find("file not found") != std::string::npos)
-                    failAt(im.pos.file, im.pos.line, im.pos.col, "can't find the C header '" + im.path + "'");
+                if (msg.find("file not found") != std::string::npos) {
+                    // "'GL/gl.h' file not found": it may be a header that this one includes
+                    std::string missing = msg.substr(1, msg.find('\'', 1) - 1);
+                    if (missing == im.path)
+                        failAt(im.pos.file, im.pos.line, im.pos.col, "can't find the C header '" + im.path + "'");
+                    failAt(im.pos.file, im.pos.line, im.pos.col, "can't find the C header '" + missing + "', which '" + im.path +
+                                                                     "' includes (is its library's development package installed?)");
+                }
                 failAt(im.pos.file, im.pos.line, im.pos.col, "C header '" + im.path + "' has an error: " + msg);
             }
             clang_disposeDiagnostic(d);

src/codegen.cpp

@@ -758,7 +758,9 @@ LRef Codegen::member(const MemberExpr &m, bool forWrite) {
             if (!obj.isPlace) release(obj.val);
             return {false, {}, {len, FType::I64}};
         }
-        if (!obj.isPlace && obj.val.fresh) failAt(p.file, p.line, p.col, ".ptr of a temporary value would point to freed memory");
+        // a literal lives as long as the program; other temporaries are freed right after this expression
+        if (!obj.isPlace && obj.val.fresh && !isa<Constant>(obj.val.v))
+            failAt(p.file, p.line, p.col, ".ptr of a temporary value would point to freed memory");
         FType pt = t.kind == FType::Str ? FType::ptrTo(FType::Char) : FType::ptrTo(*t.elem);
         return {false, {}, {t.kind == FType::Str ? cstr(v) : b.CreateExtractValue(v, 0), pt}};
     }

src/codegen_builtins.cpp

@@ -441,6 +441,8 @@ Value_ Codegen::convert(const FType &to, const std::vector<ExprPtr> &args, Pos,
     }
     if (to.kind == FType::Bool && from.isInt()) return {b.CreateICmpNE(v.v, ConstantInt::get(ty(from), 0)), to};
     if (to.isPtr() && (from.isPtr() || from.kind == FType::Null)) return {v.v, to};
+    if (to.isPtr() && from.isInt()) return {b.CreateIntToPtr(intCast(v.v, from, FType::U64), b.getPtrTy()), to};  // an address or offset for C
+    if (to.isInt() && from.isPtr()) return {b.CreatePtrToInt(v.v, ty(to)), to};
     if (to.isPtr() && from.kind == FType::Str) failAt(p.file, p.line, p.col, "use .ptr to get a str's address (it is only valid while the str lives)");
     failAt(p.file, p.line, p.col, "can't turn " + from.show() + " into " + name);
 }

src/main.cpp

@@ -68,12 +68,7 @@ struct Loader {
     void load(const std::string &path, const std::string &module, Pos from) {
         std::string text;
         if (!readFile(path, text)) {
-            if (from.line) {
-                std::string old = path.substr(0, path.size() - 4) + ".fn";
-                if (sys::fs::exists(old))
-                    failAt(from.file, from.line, from.col, "found " + old + ", but Finch files now end in .fch: rename it to " + path);
-                failAt(from.file, from.line, from.col, "can't find the module '" + module + "' (looked for " + path + ")");
-            }
+            if (from.line) failAt(from.file, from.line, from.col, "can't find the module '" + module + "' (looked for " + path + ")");
             std::fprintf(stderr, "error: can't open '%s'\n", path.c_str());
             std::exit(1);
         }

src/parser.cpp

@@ -456,6 +456,7 @@ private:
         parenDepth++;
         if (!at(Tok::RParen)) {
             do {
+                if (at(Tok::RParen)) break;  // a trailing comma is fine
                 std::string name;
                 if (at(Tok::Ident) && peekTok().kind == Tok::Colon) {
                     name = next().text;