When a GODEBUG setting is not listed in the environment variable,
its value is derived from three sources:
the defaults for the Go toolchain used to build the program,
amended to match the Go version listed in `go.mod`,
and then overridden by explicit `//go:debug` lines in the program.

The [GODEBUG History](#history) gives the exact defaults for each Go toolchain version.
For example, Go 1.21 introduces the `panicnil` setting,