Matcher messages must be a function.

[cpojer]

For performance reasons, we should make a breaking change that all matcher messages are functions typed () => string rather than string. We already do that within all of Jest's matchers, but we don't enforce it. We should do this for 20.1.

[thymikee]

Maybe worth scheduling for 21 if that's breaking?

[cpojer]

20.1 is fine.

[anilreddykatta]

@cpojer Can you provide an example of the possible change? I didn't quite get what is the intended behavior. I can take it up!

[cpojer]

Basically remove this if-check and always treat message as a function: https://github.com/facebook/jest/blob/master/packages/jest-matchers/src/index.js#L105

and then update everywhere that may need updating, including the types.

[jseminck]

If no one is taking a look at this, then I'd like to give it a shot!

[cpojer]

Go for it!

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