`HardFault` trampoline makes it difficult for `probe-rs` debugger to unwind past the exception handler.

The HardFault trampoline overwrites the EXC_RETURN magic values when it writes to the lr register.

Currently, the probe-rs debugger relies on those magic values during stack unwinding, to identify exception handlers, and unwind the stack that caused the exception. For instance, it used to be possible to get a stack like this:

__cortex_m_rt_HardFault (/Users/jn/dev/debug/probe-rs-debugger-test/src/thumbv7em-none-eabihf/nRF52833_xxAA.rs:43)
"HardFault handler. Cause: Escalated BusFault (Precise data access error) at location: 0x3ffffffc." (Unknown Source:0)
div_half_u128 (/rustc/0ecbd0605770f45c9151715e66ba2b3cae367fcb/library/core/src/ptr/mod.rs:1636)
__cortex_m_rt_SysTick (/Users/jn/dev/debug/probe-rs-debugger-test/src/thumbv7em-none-eabihf/nRF52833_xxAA.rs:32)
__cortex_m_rt_SysTick_trampoline (/Users/jn/dev/debug/probe-rs-debugger-test/src/thumbv7em-none-eabihf/nRF52833_xxAA.rs:17)
"Systick handler." (Unknown Source:0)
__delay #[inline] (/Users/jn/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cortex-m-0.7.7/asm/inline.rs:62)
delay (/Users/jn/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cortex-m-0.7.7/src/asm.rs:29)
__cortex_m_rt_main (/Users/jn/dev/debug/probe-rs-debugger-test/src/thumbv7em-none-eabihf/nRF52833_xxAA.rs:85)
__cortex_m_rt_main (/Users/jn/dev/debug/probe-rs-debugger-test/src/thumbv7em-none-eabihf/nRF52833_xxAA.rs:62)

With the trampoline, the debugger has no way of knowing that it is an exception handler that needs register substitutions before the calling frames can be identified, and it stops unwinding at the first frame.

I've notice in the unreleased code, that users can bypass the trampoline, with#[exception(trampoline = false)], and this restores the previous behaviour of the stack unwind.

Unfortunately, this means that the users have to modify their code before they can use the debugger in these situations.

Is there any way we can make the default to be trampoline = false?

Start in cortex-m-rt/src/lib.rs at the HardFault trampoline around lines 446-469, then trace the existing #[exception(trampoline = false)] handling. Compare the generated exception-entry behavior and determine how the default could preserve probe-rs stack unwinding without breaking existing handlers. Done means the default behavior supports unwinding while retaining documented compatibility.