Debugger

Debug wado compiler with rust-gdb.

Build first

dev sets debug = "line-tables-only" and raises the workspace crates to opt-level = 1, so info locals / info args come back empty in every compiler frame. Build the debugger profile instead — full DWARF, no optimization on the crates being stepped through, its own target/debugger/ dir so the dev cache stays warm:

cargo build --profile debugger --bin wado

Usage

cat > scratchpad/gdb_commands.txt << 'EOF'
file ./target/debugger/wado
set pagination off
break emit_function
run compile -o scratchpad/out.wasm example/hello.wado
info locals
print func.name
bt 5
quit
EOF
rust-gdb --batch -x scratchpad/gdb_commands.txt

Break on a function name, not a file:line: a line number drifts with the next edit above it. gdb matches a bare name in every module, so qualify one that more than one module defines.

Ask one question per run, not one per build

A breakpoint that fires thousands of times and gets greped answers one question and costs a rebuild for the next. Make the breakpoint itself select:

break wado_compiler::wir_build::calls::local_get if $_streq(name->data_ptr, "…")
break substitute_types_in_expr
commands
silent
bt 6
continue
end

bt at a conditional hit gives the origin outright — the thing print debugging cannot produce without guessing where to put the next eprintln!.

Printing Rust values

printf "%s", s fails on a Rust String (it is a struct, not a char*) and aborts the whole command file with Value can't be converted to integer. Use print, which rust-gdb's pretty printers handle:

print fq              # "core:prelude/string.wado/String^Eq::eq"
print *expr
print info.struct_name

$_streq(s->data_ptr, "lit") is the way to compare one inside a breakpoint condition.

Batch runs

rust-gdb --batch exits on the first command error, so a typo in a commands block silently truncates the rest of the run — check the tail of the output for Error in sourced command file before trusting an empty result. Redirect to a file and grep it; the DWO-loading noise otherwise buries the hits:

rust-gdb --batch -x scratchpad/gdb_commands.txt > scratchpad/gdb.log 2>&1
grep -a '^\$[0-9]* = ' scratchpad/gdb.log | sort -u

When a guard beats a breakpoint

The debugger answers "what is this value here". When the question is "where else does this invariant break", an assertion at the point the invariant must hold enumerates every violation in one run and keeps doing so afterwards — the same reason a newtype that makes an illegal name unconstructible beats chasing one miscompile at a time. Reach for debug_assert! / assert! in the merge or registration step first, and for gdb once it fires and you need the values behind it.

Common commands

Notes

lldb does not work in Claude Code Web due to ptrace restrictions:

error: Cannot launch '...': personality get failed: Invalid argument

Use rust-gdb instead.