Replace a single range in an `if let` by a boolean `if`

What it does

Matching against a single range pattern is equivalent to either an inequality or a range.contains() (ie two inequalities) or trivial. This lint would replace let range = ... in if let statements and let chains.

Advantage

• Easier to read
• Range pattern matches only work with integers and chars, while inequalities work more generally, so changing the type of the variable being matched would require a smaller change once this lint is applied

Drawbacks

In const contexts and when the range is either Range or RangeInclusive, the lint may not be an improvement:

• Range::contains() uses PartialOrd so cannot be used in const contexts
• if 3 <= x && x < 5 may be harder to read compared to if let 3..5 = x and if x is a complex expression then it requires a new binding to maintain the same semantics.

Example

if let 2.. = x && let ..10 = y && let 'a'..='z' = c {
    ...
}

Could be written as:

if x >= 2 && y < 10 && ('a'..='z').contains(&c) {
    ...
}

Comparison with existing lints

Instead of being a new lint, this could, or perhaps should, be part of redundant_pattern_matching. The lint equatable_if_let is also similar.

Additional Context

No response

Start by reading the existing redundant_pattern_matching and equatable_if_let lints, since the issue proposes either extending one of them or adding a new lint. Compare the listed range-pattern cases and drawbacks, including const contexts and complex expressions; done means the chosen lint handles the intended cases without changing semantics.