{flat_,}map_identity: recognize |[x, y]| [x, y] as an identity function as well

[ada4a]

changelog: [map_identity,flat_map_identity]: also recognize |[x, y]| [x, y]

fixes #15198

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[ada4a]

should I add struct destructuring as well? It would be completely irrelevant flat_map_identity, but definitely so for map_identity, e.g. for |Foo {bar, baz}| Foo {bar, baz}

EDIT: on another thought... maybe not -- even the "simpler" thing, tuple structs, would require delving into the horrors of ExprKind::Call again, which I would rather avoid

[y21]

This looks good to me, thanks! Nice that you thought of match ergonomics, in the past they've caused some trouble in these identity lints :)

As for struct destructuring, I suppose conceptually it could make sense for this lint to catch them, but yeah, definitely not something you need to do here, this is a solid improvement on its own.

[y21]

Could you squash the commits?

[ada4a]

squashed (and rebased)