Nim Debugging

Nim Debugging

Debug by reducing uncertainty: preserve the failure, state one hypothesis, and collect the smallest piece of evidence that can reject it. Find the first violated invariant rather than stopping at the final crash.

Rules

Preserve the failure

• Record the exact input, command, working directory, backend, memory manager, thread setting, build mode, and relevant environment.
• Make the failure deterministic before changing code. If it is intermittent, preserve a seed, event order, or failing fixture.
• Change one diagnostic dimension at a time. A debug build that no longer fails is a comparison point, not a reproduction.
• Turn each confirmed defect into a regression test before removing instrumentation.

Choose evidence by failure class

Stack traces and build modes

Default build behavior:

• Add --lineTrace:on to the failing release or danger command to restore the call chain and line numbers. It implies --stackTrace:on.
• Use --excessiveStackTrace:off when filenames are sufficient; the default on form includes full paths.
• Call writeStackTrace() when the current path matters before an exception exists.
• Read the trace from the failure backward, then inspect values along that path until the first invalid state is found.

Runtime inspection

• Use echo for ordinary values and repr when a type lacks useful $ formatting.
• Use debugEcho inside {.noSideEffect.} code; ordinary echo is rejected there.
• echo flushes automatically. Flush stdout.write explicitly when a crash could occur before the buffer drains.
• Label diagnostic values with the phase and invariant being checked. Avoid dumping large structures when one length, index, tag, or identity answers the hypothesis.
• compiles(expr) is a compile-time boolean probe. If it returns false, compile the candidate normally to obtain the actual diagnostic.
• fields and fieldPairs use the expression's static type, not its runtime subtype. Use when, not if, for code that depends on a field's name or type.

Compiler-generated code

• Run nim c --expandMacro:<name> file.nim to inspect a macro's expanded AST. The compiler emits an [ExpandMacro] hint.
• Run nim c --expandArc:<proc> file.nim to inspect injected copy, sink, move, and destroy operations.
• Keep an --expandArc target reachable from the entry point and use the target project's --mm: mode.
• Measure copy behavior inside a named proc; last-use analysis does not fire at module top level.
• Treat expansion output as evidence about generated operations, not as a reason to optimize by itself. move is destructive and requires deliberate last use plus behavior tests.
• Nim defaults to ORC. Read the compiler build hint because project configuration may select ARC or atomic ARC.

AddressSanitizer

Use ASan for ptr, addr, cstring, cstringArray, manual allocation, and C-FFI memory faults. The required Linux/GCC recipe is:

bashCopy code
nim c \
  --passC:"-fsanitize=address -fno-omit-frame-pointer" \
  --passL:"-fsanitize=address -fno-omit-frame-pointer" \
  -g \
  -d:useMalloc \
  -d:noSignalHandler \
  file.nim

• Pass sanitizer flags to both compilation and linking.
• -d:useMalloc is required so ASan observes memory obtained through Nim's allocator.
• -d:noSignalHandler is required so ASan, rather than Nim's signal handler, reports signal faults.
• -g gives symbolized Nim locations with GCC. Add --cc:clang for Clang and ensure llvm-symbolizer is in PATH.
• Preserve the project's memory manager and failing input.
• Read both the invalid-access stack and the allocation/free stack before editing code.

See references/memory_sanitizers.md for a complete runnable example. Platform recipes not verified in this repository are intentionally omitted.

Valgrind

Use Valgrind as an independent native-memory check:

bashCopy code
nim c -g -o:program file.nim
valgrind --leak-check=full --error-exitcode=1 ./program

It requires no sanitizer instrumentation and is slower than ASan. A non-zero exit distinguishes detected errors in automated checks.

Debug symbols and native debuggers

• -g and --debugger:native enable native debug information and line directives.
• This project does not use GDB: Nim name mangling and generated C make variable inspection slower and less reliable than stack traces, focused inspection, ASan, Valgrind, and compiler expansion.

Workflow

1. Reproduce the failure with an exact command and fixed input.
2. Minimize unrelated setup while keeping the same failure and configuration.
3. State one falsifiable hypothesis and the invariant it predicts.
4. Select the narrowest evidence source from the failure-class table.
5. Follow the failing path backward and locate the first invalid state.
6. Fix the invariant at its owner instead of suppressing the final symptom.
7. Re-run the original reproduction and relevant sanitizer or expansion check.
8. Add a regression test, then remove temporary prints and scratch artifacts.

Common Mistakes

References

• references/stack_trace_diagnosis.md — Boundary instrumentation for finding the first bad value behind a crash.
• references/memory_sanitizers.md — ASan and Valgrind commands for unsafe access and leaks.
• references/arc_optimization.md — expandArc comparison for unexpected ownership copies.