Rule Request: no_constant_condition

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

New rule request

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

Conditional branches where the condition constantly evaluates to true or false are useless because only one branch is executed always or never.

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

Violating Examples

if true {
    print("foo")
}

if false {
    print("foo")
}

if x && false {
    print("foo")
}

if x || true {
    print("foo")
}

switch "b" {
case "a":
    print("foo")
case "c":
    print("bar")
default:
    break
}

Non-Violating Examples

if x {
    print("foo")
}

switch x {
case "a":
    print("foo")
case "c":
    print("bar")
default:
    break
}

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

This rule doesn’t need to be configurable.

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.

I think this rule would be a good candidate for the default rules since it points out a possible programming error.

Read README.md’s opt-in-rules guidance first, then use the Swift examples in the issue as the behavior specification. Done means the rule detects each listed constant-condition violation, leaves the non-violating examples unchanged, and requires no configuration.