Fix backward compatibility for Bundler versions < 2.2.17

[Artes02]

Motivation

Hi!

One of the projects that I'm working with still uses Bundler version 2.1.4 (sad, I know!) and Ruby LSP errors out upon initialization with this version. In fact, it won't be able to initialize with any Bundler prior to version 2.2.17 (more details in Implementation section)!

I believe that Ruby LSP is now an essential part of any VS Code's Ruby project and I want to make it backward compatible with previous versions of Bundler so that people don't have to switch to other editors/IDEs when they can't simply upgrade to newer version of Bundler due to certain project limitations.

Luckily, the fix is not that complicated!

Implementation

Let's take a look at the PR that introduced the usage of Bundler::Settings.key_for(e) in Ruby LSP: #2535.

It might look okay at first until you realize that def self.key_for(key) for Bundler::Settings was only introduced in bundler-v2.2.17, meaning that Bundler::Settings.key_for(e) won't work for any Bundler version < 2.2.17.

Considering that instance method def key_for(key) is still in place in latest Bundler version and don't seem to be going away - I suggest we use it instead! Plus we already have an instance of Bundler::Settings available in our local variable settings.

I was also thinking about an explicit Bundler version check approach but didn't find any benefits. There's even a drawback - we'd be missing out the caching mechanism (introduced by this PR) that instance method provides in newer versions of Bundler if we keep using the class method instead.

Automated Tests

I updated corresponding test/setup_bundler_test.rb.

Manual Tests

Actual result:

GIVEN ruby project with Bundler version < 2.2.17
WHEN Ruby LSP is being initialized
THEN "undefined method `key_for' for Bundler::Settings:Class (NoMethodError)" error occurs
AND Ruby LSP fails to initialize

GIVEN ruby project with Bundler version >= 2.2.17
WHEN Ruby LSP is being initialized
THEN no errors occur
AND Ruby LSP initializes successfully

Attaching an error example that is currently happening: undefined_method_key_for_for_Bundler_Settings_Class_NoMethodError.txt.

Expected Result:

GIVEN ruby project with Bundler version < 2.2.17
WHEN Ruby LSP is being initialized
THEN no errors occur
AND Ruby LSP initializes successfully

GIVEN ruby project with Bundler version >= 2.2.17
WHEN Ruby LSP is being initialized
THEN no errors occur
AND Ruby LSP initializes successfully

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