LibJS: Make Error stack traces lazier

Instead of eagerly populating the stack trace with a textual
representation of every call frame, just store the raw source code range
(code, start offset, end offset). From that, we can generate the full
rich backtrace when requested, and save ourselves the trouble otherwise.

This makes test-wasm take ~7 seconds on my machine instead of ~60. :^)

1 files changed, 8 insertions, 1 deletions

diff --git a/Userland/Libraries/LibJS/Runtime/Error.h b/Userland/Libraries/LibJS/Runtime/Error.h
index 26be07495f..cf9315dfc8 100644
--- a/Userland/Libraries/LibJS/Runtime/Error.h
+++ b/Userland/Libraries/LibJS/Runtime/Error.h
@@ -17,7 +17,14 @@ namespace JS {
 
 struct TracebackFrame {
     DeprecatedFlyString function_name;
-    SourceRange source_range;
+    [[nodiscard]] SourceRange const& source_range() const;
+
+    struct UnrealizedSourceRange {
+        u32 start_offset { 0 };
+        u32 end_offset { 0 };
+        RefPtr<JS::SourceCode const> source_code;
+    };
+    mutable Variant<SourceRange, UnrealizedSourceRange> source_range_storage;
 };
 
 class Error : public Object {