- ### Problem

  ActionPack requires "action_view/base" at boot time, this
  causes a variety of issue that I described in detail in #38024.

  There is no real reason to require av/base in the
  ActionDispatch::Debugexceptions class.

  ### Solution

  Like any other components (such as ActiveRecord, ActiveJob...),
  ActionView::Base shouldn't be loaded at boot time.

  Here are the two main changes needed for this:

  1) Actionview has a special initializer that needs to run
     before the app is fully booted (adding a executor needs to be done
     before application is done booting)
  https://github.com/rails/rails/blob/63ec70e700e321b22e9baf2ad2d45cd3f4febc79/actionview/lib/action_view/railtie.rb#L81-L84

     That initializer used a lazy load hooks but we can't do that anymore
     because Action::Base view won't be triggered during booting process.
     When it will get triggered, (presumably on the first request),
     it's too late to add an executor.

  ------------------------------------------------

  2) Compare to other components, ActionView doesn't use `Base` for
     configuration flag. A lot of flags ares instead set on modules
     (FormHelper, FormTagHelper).
     The problem is that those module depends on AV::Base to be
     loaded, as otherwise configuration set by the user aren't applied.
     (Since the lazy load hooks hasn't been triggered)
     https://github.com/rails/rails/blob/63ec70e700e321b22e9baf2ad2d45cd3f4febc79/actionview/lib/action_view/railtie.rb#L66-L69

     We shouldn't wait for AB::Base to be loaded in order to set these
     configuration. However, we need to do it inside an
     `after_initialize` block in order to let application
     set it to the value they want.

  Closes #28538

  Co-authored-by: betesh <iybetesh@gmail.com>"

When a SyntaxError is detected in a template we raise this exception. On
a first request to the server the exception we get a NameError since the
exception is not required from `active_view/template/error.rb` yet.
However later on it gets required and a second request will succeed.
On the first request we see the rails "Something Wen Wrong" page and not
the expected syntax error in template error page with the webconsole and
stacktrace. By autoloading the constant we fix this issue.
Co-authored-by: NGannon McGibbon <gannon.mcgibbon@gmail.com>

Previously it's possible to have multiple copies of the "same" Template.
For example, if index.html.erb is found both the :en and :fr locale, it
will return a different Template object for each. The same can happen
with formats, variants, and handlers.

This commit de-duplicates templates, there will now only be one template
per file/virtual_path/locals tuple.

We need to consider virtual_path because both `render "index"`, and
`render "index.html"` can both find the same file but will have
different virtual_paths. IMO this is rare and should be
deprecated/removed, but it exists now so we need to consider it in order
to cache correctly.

This commit introduces a new UnboundTemplate class, which represents a
template with unknown locals. Template objects can be built from it by
using `#with_locals`. Currently, this is just a convenience around
caching templates, but I hope it's a helpful concept that could have
more utility in the future.

This is unnecessary now that we can just provide a file source

Now we can throw away the subclass and the generated methods will get
GC'd too

Every template that specifies a "virtual path" loses the template source
when the template gets compiled:

  https://github.com/rails/rails/blob/eda0f574f129fcd5ad1fc58b55cb6d1db71ea95c/actionview/lib/action_view/template.rb#L275

The "refresh" method seems to think that the source code for a template
can be recovered if there is a virtual path:

  https://github.com/rails/rails/blob/eda0f574f129fcd5ad1fc58b55cb6d1db71ea95c/actionview/lib/action_view/template.rb#L171-L188

Every call site that allocates a template object *and* provides a
"virtual path" reads the template contents from the filesystem:

  https://github.com/rails/rails/blob/eda0f574f129fcd5ad1fc58b55cb6d1db71ea95c/actionview/lib/action_view/template/resolver.rb#L229-L231

Templates that are inline or literals don't provide a "virtual path":

  https://github.com/rails/rails/blob/eda0f574f129fcd5ad1fc58b55cb6d1db71ea95c/actionview/lib/action_view/renderer/template_renderer.rb#L34

This commit introduces a `FileTemplate` type that subclasses `Template`.
The `FileTemplate` keeps a reference to the filename, and reads the
source from the filesystem.  This effectively makes the template source
immutable.

Other classes depended on the source to be mutated while being compiled,
so this commit also introduces a temporary way to pass the mutated
source to the ERB (or whatever) compiler.  See `LegacyTemplate`.

I think we should consider it an error to provide a virtual path on a
non file type template an non-file templates can't recover their source.
Here is an example:

  https://github.com/rails/rails/blob/eda0f574f129fcd5ad1fc58b55cb6d1db71ea95c/actionview/lib/action_view/testing/resolvers.rb#L53

This provides a "virtual path" so the source code (a string literal) is
thrown away after compilation.  Clearly we can't recover that string, so
I think this should be an error.

`MissingRequestError` is no longer used since 1e2b0ce9.

This basically reverts c4d1a4ef

This reverts commit 3420a145, reversing
changes made to afb66a5a.

".. with __dir__ we can restore order in the Universe." - by @fxn

Related to 5b8738c2

Per https://www.timeanddate.com/counters/firstnewyear.html, it's already
2017 in a lot of places, so we should bump the Rails license years to
2017.

[ci skip]

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

LookupContext is eagerly loaded, and FallbackFileSystemResolver is
referenced at the class level.  Just require the resolver from the
eagerly loaded class rather than jumping through autoload hoops

just like all the other modules do require their version file

HTML Scanner is part of ActionView and it should be loaded along with it

Name was singular, should be plural

This commit creates structure for Action View gem and is first of a
series of commits extracting Action View from Action Pack.

Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>