fix some rough edges around stack traces

hide irrelevant CallFrames
fix span info for the implicit stack frame

crates/haku2/src/haku2.zig

@@ -196,27 +196,32 @@ export fn haku2_vm_stackframe_count(vm: *const Vm) usize {
     return vm.stackFrameCount();
 }
 
+export fn haku2_vm_stackframe_is_return_marker(vm: *const Vm, index: usize) bool {
+    const stack_frame = vm.stackFrame(index);
+    return stack_frame.closure == null;
+}
+
 export fn haku2_vm_stackframe_is_system(vm: *const Vm, index: usize) bool {
     const stack_frame = vm.stackFrame(index);
-    return stack_frame.closure.impl == .system;
+    return stack_frame.closure.?.impl == .system;
 }
 
 export fn haku2_vm_stackframe_pc(vm: *const Vm, index: usize) i32 {
     const stack_frame = vm.stackFrame(index);
-    if (stack_frame.closure.impl == .bytecode)
+    if (stack_frame.closure.?.impl == .bytecode)
         if (stack_frame.ip) |ip|
-            return stack_frame.closure.programCounter(ip);
+            return stack_frame.closure.?.programCounter(ip);
     return -1; // no return address
 }
 
 export fn haku2_vm_stackframe_function_name_len(vm: *const Vm, index: usize) usize {
     const stack_frame = vm.stackFrame(index);
-    return stack_frame.closure.name.len;
+    return stack_frame.closure.?.name.len;
 }
 
 export fn haku2_vm_stackframe_function_name(vm: *const Vm, index: usize) [*]const u8 {
     const stack_frame = vm.stackFrame(index);
-    return stack_frame.closure.name.ptr;
+    return stack_frame.closure.?.name.ptr;
 }
 
 // Renderer

crates/haku2/src/vm.zig

@@ -30,7 +30,7 @@ pub const Limits = struct {
 };
 
 pub const CallFrame = struct {
-    closure: *const value.Closure,
+    closure: ?*const value.Closure,
     return_addr: ?[*]const u8,
     bottom: u32,
 };
@@ -100,7 +100,7 @@ pub fn outOfMemory(vm: *Vm) Error {
 
 // NOTE: Different from CallFrame, this is a helper type for obtaining stack traces.
 pub const StackFrame = struct {
-    closure: *const value.Closure,
+    closure: ?*const value.Closure,
     ip: ?[*]const u8,
 };
 
@@ -117,7 +117,17 @@ pub fn stackFrame(vm: *const Vm, index: usize) StackFrame {
         if (index == 0) {
             return .{
                 .closure = closure,
-                .ip = vm.ip,
+                // Span info hairiness:
+                // Because call frames place the instruction pointer past the call instruction,
+                // the frontend subtracts 1 from it to get a position within the chunk that
+                // has the correct line info.
+                // This subtracting by one doesn't work for the implicit stack frame though, because
+                // its ip gets updated at the _start_ of the instruction---meaning that if you
+                // subtract one from it, it'll point to the _previous_ instruction, while the actual
+                // error occurred in the current one.
+                // To alleviate this, add one here to point to either the middle or the end of the
+                // current instruction.
+                .ip = if (vm.ip) |ip| ip + 1 else null,
             };
         } else {
             // NOTE: Minus two, because remember index == 0 is occupied by the current stack frame.
@@ -269,7 +279,7 @@ pub fn run(
     }
 
     try vm.pushCall(.{
-        .closure = closure,
+        .closure = null,
         .return_addr = null, // nowhere to return to
         .bottom = bottom,
     });
@@ -591,7 +601,7 @@ pub fn run(
             try vm.push(result);
 
             if (call_frame.return_addr) |addr| {
-                closure = call_frame.closure;
+                closure = call_frame.closure.?;
                 ip = addr;
                 bottom = call_frame.bottom;