Tuple field names in debuginfo are prefixed with __.

[jswrenn]

The DWARF debugging information entries for tuple field names (and for the field names of tuple-like structs and variants) are prefixed with __. For instance, Option's Some variant is recorded as having a field named __0, rather than 0.

I think this is a little unfortunate. Tuple indices behave quite like field names in Rust, insofar that they're accessed with . and that you can instantiate tuple-like structs and variants as if they are records; e.g.:

Some { 0: true };

It's odd, then, that these fields would not appear in a debugger as they would in Rust.

My own stake in this is deflect, allows you to get fields by name from structs and variants. Users reflecting on tuple-like structs and variants must know to prefix their queries with __, and the pretty-printer and serializers provided by deflect cannot generally strip away the __ from field names.

[tromey]

I'd imagine this approach was desirable before gdb had direct support for Rust. Back then you'd have to debug in C++ mode and that wouldn't understand x.0 as referring to a field.

Nowadays, gdb knows about the __ convention. It would be pretty simple to add support for numeric field names (for Rust) in gdb, but it would be nice to have some warning before a change is made; and on the other hand I probably wouldn't bother modifying gdb unless I knew Rust was planning to change.

[tromey]

Nowadays, gdb knows about the __ convention. It would be pretty simple to add support for numeric field names (for Rust) in gdb, but it would be nice to have some warning before a change is made; and on the other hand I probably wouldn't bother modifying gdb unless I knew Rust was planning to change.

FTR, gccrs emits tuple fields without the underscores, so I've submitted a gdb patch for that: https://sourceware.org/pipermail/gdb-patches/2025-March/216383.html. This should be in gdb 17.