Rule request: fallthrough should be avoided

[marcelofabri]

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Rule Request

Avoid fallthrough inside switch statements.

1. Why should this rule be added? Share links to existing discussion about what
   the community thinks about this.

Using fallthrough makes code harder to read and, in most cases, can be replaced by combining cases. LinkedIn's Swift Style Guide also thinks that (item 3.4.5).

2. Provide several examples of what would and wouldn't trigger violations.

// should trigger
switch foo {
case .bar:
    fallthrough
case .bar2:
    fallthrough
case .bar3
    something()
}


// shouldn't trigger
switch foo {
case .bar, .bar2, .bar3:
    something()
}

3. Should the rule be configurable, if so what parameters should be configurable?

Only severity.

4. Should the rule be opt-in or enabled by default? Why?
   See README.md for guidelines on when to mark a rule as opt-in.

Opt-in. In the end, this is still a matter of style and there are situations where you can't just combine cases.