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rustlldb

Is there any way to view the Debug representation of a Rust variable in the LLDB debugger?

Rust code can be debugged using LLDB. The representation of variables used by tools like CodeLLDB, though, is simply a breakdown of the in-memory contents of the variable and does not show any information from the standard Debug trait.

CodeLLDB Variable View

Is there any way using LLDB I can invoke the actual Debug representation of a variable at runtime? This often has a significant amount of useful information which is not clear from a simple memory snapshot.

Solution

  • The "Debug" trait in Rust looks like a pretty close equivalent to the ObjC and Swift object description methods: it provides a to-string method that prints a developer-friendly view of the object. From what I can tell this trait cooperates with the standard formatted printing machinery in Rust.

    If that's right, the natural way to give access to the Rust Debug Trait in lldb would be to implement the "Object Description" part of lldb's "Rust LanguageRuntime" and call the print function under the covers. This would be accessed by the po or print-object command in lldb.

    Unfortunately, Rust doesn't have a "Rust LanguageRuntime" in lldb or really much of any support currently. The only mentions of Rust in the lldb sources are a recognizer for the Rust mangling scheme and a define that says "other than mangling, pretend Rust is C++". So that isn't a viable option at present.

    You could also try calling Rust's print directly in the expression evaluator, but YMMV as calling Rust code in lldb doesn't always work: as it turns out, Rust is not C++...