Difference between debug=1 and debug=2 - rust

Question

As I was testing this, I ran a test program with the Cargo setting:
[profile.release]
debug = 1
I got exactly the same stacktrace as if with debug = 2:
$ RUST_BACKTRACE=1 cargo run --release
Finished release [optimized + debuginfo] target(s) in 0.02s
Running `target/release/an`
Error: param not big enough!
Stack backtrace:
0: an::bb
at ./src/main.rs:18:5
an::aa
at ./src/main.rs:13:2
an::main
at ./src/main.rs:6:2
1: core::ops::function::FnOnce::call_once
at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/core/src/ops/function.rs:227:5
std::sys_common::backtrace::__rust_begin_short_backtrace
at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/std/src/sys_common/backtrace.rs:123:18
2: std::rt::lang_start::{{closure}}
at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/std/src/rt.rs:145:18
3: core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &F>::call_once
at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/core/src/ops/function.rs:259:13
std::panicking::try::do_call
at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/std/src/panicking.rs:406:40
std::panicking::try
at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/std/src/panicking.rs:370:19
std::panic::catch_unwind
at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/std/src/panic.rs:133:14
std::rt::lang_start_internal::{{closure}}
at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/std/src/rt.rs:128:48
std::panicking::try::do_call
at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/std/src/panicking.rs:406:40
std::panicking::try
at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/std/src/panicking.rs:370:19
std::panic::catch_unwind
at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/std/src/panic.rs:133:14
std::rt::lang_start_internal
at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/std/src/rt.rs:128:20
4: main
5: __libc_start_main
6: _start
Why?
The doc says:
The valid options are:
0 or false: no debug info at all
1: line tables only
2 or true: full debug info
So I expected only line numbers would be present in the stack trace with debug = 1.
(BTW the binary file size with debug = 2 was considerably bigger than with debug = 1, so obviously there is some difference somewhere...)