#[exception] should check that the exception exists on the target core

[jonas-schievink]

In reference to this comment:

cortex-m-rt/macros/src/lib.rs

Lines 289 to 290 in 7050c73

	// NOTE that at this point we don't check if the exception is available on the target (e.g.
	// MemoryManagement is not available on Cortex-M0)

I've lost a lot of time due to not realizing that MemoryManagement does not exist on M0(+) chips (instead, HardFault is invoked). At least in this particular case it should be easy to forbid it since it never exists on thumbv6-based targets.

Exceptions that don't exist on ARMv6:

• MemoryManagement
• BusFault
• UsageFault
• DebugMonitor

Exceptions that only exist on ARMv8:

• SecureFault

[jonas-schievink]

Looks like this is currently unimplementable: Proc macros have no access to the build target, for some reason (TARGET is unset), and there are no #[cfg] flags we can use to detect a specific ARM version.

[adamgreig]

I guess the proc macro is being built for the host so it makes some sense there's no target. Could we use cfg flags set from build.rs or are they only visible to the parent crate?

[jonas-schievink]

They're only visible to the crate that owns the build script, I think

[adamgreig]

Could we do a similar trick to interrupts and generate a dummy reference to the named exception from c-m-rt which will error if the variant doesn't exist there?

[jonas-schievink]

Yes, I think that should work!