Fix parsing erb with bare yield

[Earlopain]

Motivation

Some things that are valid in erb would not be valid in a standalone ruby file. This particularly impacts yield.
Instead treat the provided code as being evaled

Closes #2442

Implementation

Switch prism into eval mode. Prior work for the prism parser translator at ruby/prism#2741

Automated Tests

Yup

Manual Tests

I'd guess that this is neovim/editor specific since I'm not seeing anything for syntax errors in vscode but the added test does fail previously.

[andyw8]

Are there any possible downsides to using eval mode here? (cc @kddnewton)

[st0012]

Thanks for the fix. Let's add a comment to explain what it does as well.

[kddnewton]

Are there any possible downsides to using eval mode here? (cc @kddnewton)

It will also turn off unused local variable warnings and void statement warnings, which is probably fine.

[natematykiewicz]

It will also turn off unused local variable warnings and void statement warnings, which is probably fine.

From what I've seen of using Rubocop on ERB, this is actually preferred, because if you assign a variable in one ERB block and use it in another, Rubocop thinks the variable is unused, because it checks each ERB block independently.

For example:

<% foos = [1,2,3] %>

<div>
  <% foos.each do |foo| %>
    <span><%= foo %></span>
  <% end %>
</div>

Rubocop via ERB Lint will complain on line 1 that foos is unused.

[vinistock]

That wouldn't be the case for Kevin's comment though. We strip out the Ruby code from the ERB template and parse it with Prism as if it were a regular Ruby file, so in that particular example it would be able to tell you if the local variable was indeed unused inside the entire template.

We don't yet run RuboCop or ERB Lint on ERB files, so all of the existing diagnostics were coming from Prism itself.