Add MBED_FALLTHROUGH attribute

[kjbracey]

Summary of changes

C++17 standardised [[fallthrough]] for switch statements to suppress compiler warnings. Provide access to it, or compiler-specific alternatives.

Impact of changes

MBED_FALLTHROUGH attribute added to mbed_toolchain.h - a portable equivalent for C++17's [[fallthrough]].

Migration actions required

None

Documentation

Toolchain attributes not covered by docs

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[X] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

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Reviewers

@JanneKiiskila

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[JanneKiiskila]

Looks good to me.

[kjbracey]

Just to note that the GCC form here requires GCC 7. I believe it will generate an "empty declaration" warning on GCC 6.

I could adjust if pre-GCC 7 compatibility is an issue.

[JanneKiiskila]

Well, Mbed OS uses GCC9 now in master and 5.15. onwards, so I doubt that is an issue.
We just switch the warning (that exists) into a) most cases no warning and b) using an old compiler into a slightly different warning. In both cases - the outcome is just fine, IMHO.

[kjbracey]

I agree here, but thought it was worth flagging for anyone considering stealing borrowing adapting the code for another environment, like ns_types.h.

[JanneKiiskila]

Hey, it's Apache 2.0, it's fair game!-)

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts

[0xc0170]

@ARMmbed/mbed-os-core Once you approve, this is ready to go in

[bulislaw]

That doesn't look like it fixes anything and may be problematic for some users so lets push it for 6.

[0xc0170]

Moved to 6.0.0 (@JanneKiiskila)

[JanneKiiskila]

@bulislaw - what is problematic about this, can you please explain?

[bulislaw]

We are past code freeze going through RCs, as a rule only bug fixes land in the RCs.

[0xc0170]

@ARMmbed/mbed-os-core Once you approve, this is ready to go in

Please review

[adbridge]

@bulislaw could you review this for Core ?

[evedon]

It would be good to turn on -Wimplicit-fallthrough at some point.