[ruby-core:122178] [Ruby Feature#21346] Introduce `String#ensure_suffix`

Issue #21346 has been updated by nobu (Nobuyoshi Nakada).


matheusrich (Matheus Richard) wrote in #note-3:
> > You say "queries that might find this pattern". That seems to say that you haven't found it yet.
> 
> I'm not sure what you mean. I just meant that the regex can find this pattern, but I can't guarantee it will _only_ find that.

Maybe like this?

```ruby
"Hell".sub(/(?<!o!)\z/, "o!")   #=> "Hello!"
"Hello!".sub(/(?<!o!)\z/, "o!") #=> "Hello!"
"o!Hell".sub(/(?<!o!)\z/, "o!") #=> "o!Hello!"
```

> > What's the result for
> >
> > "Hello".ensure_suffix("o!")
> >
> > Is it "Helloo!", or is it "Hello!"?
> 
> As currently implemented, it returns `"Helloo!"`

You should describe the corner case in the doc, and the test preferably.

----------------------------------------
Feature #21346: Introduce `String#ensure_suffix`
https://bugs.ruby-lang.org/issues/21346#change-113325

* Author: matheusrich (Matheus Richard)
* Status: Open
----------------------------------------

## Problem

Ensuring a string has a specific suffix or prefix is a common operation in many applications.
Bundler itself uses it:

https://github.com/rubygems/rubygems/blob/d409ec8b5fc647fabe30e37e17cd1ea857634f6b/bundler/lib/bundler/uri_normalizer.rb#L17

Here are [GitHub search](https://github.com/search) queries that might find this pattern in other places:
1. for Ruby: `/end(?:s)?_with\?\(['"].*['"]\) \?/ lang:ruby -is:fork`
1. for Crystal (a language very similar to Ruby): `/ends_with\?\(['"].*['"]\) \?/ lang:crystal -is:fork`

## Suggested solution

I believe Ruby would benefit from having a first-class method for this purpose.
I suggest the `String#ensure_suffix` and `String#ensure_prefix` methods.

I think these names are intuitive enough (here are 2 examples of people using `ensure` for this purpose ([1](https://github.com/boltops-tools/ufo/blob/796104fdb89163d09a58fad42add697923c18294/lib/ufo/cfn/stack/builder/resources/dns.rb#L76), [2](https://github.com/mumuki/mumuki-domain/blob/6194089d82b1a0c8805ecba98e006deb1694dc8e/lib/mumuki/domain/extensions/string.rb#L6))).

I've gone ahead and implemented `String#ensure_suffix` in a [pull request](https://github.com/ruby/ruby/pull/13366) but the suggested behavior is this:

```rb
"Hell".ensure_suffix("o!")   # => "Hello!"
"Hello!".ensure_suffix("o!") # => "Hello!"

s = "Hello!"
s.ensure_suffix("!").equal?(s) # => true # returns same object if already suffixed
```





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