Fix false positive in string_add.

[daxpedda]

clippy::string_add was popping up in macros.
I'm not sure what clippy's general direction is in these matters, but I can change it to be external macros only too.

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changelog: Fix false positives for string_add in macro expansions.

[phansch]

I'm not sure what clippy's general direction is in these matters

In general we want to always avoid linting in external macros. For user-defined macros it depends on the lint. Since macros can become very complex there can be weird issues sometimes.

What does this lint report if it's triggering in a user-defined macro?

[daxpedda]

It reports if it does string additions with an arithmetic operator instead of push_str.
Basically telling u to do s.push_str("asd") instead of s + "asd".

The lint works perfectly fine in user-defined macros, so there's no issue there. I suppose I should change it to checking in external macros only.

Is there a way to test external macros?

EDIT: Found out how to test for external macros.

[daxpedda]

Changed it to check only in external macros, as the lint was behaving as intended in internal macros.

[phansch]

@bors r+ thanks!

[bors]

📌 Commit 946961d has been approved by phansch

[bors]

⌛ Testing commit 946961d with merge ff1607e...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: phansch
Pushing ff1607e to master...