Extensions versioning

[voodoos]

Presentation

This PR implements a new versioning system for syntax extensions.
When reading the dune-project file an error will be raised if an extension requires a minimal version of the dune language which is superior to the declared version of the dune language.

Fixes #2957

Changes

• Extensions should now define explicitly the minimal dune lang version for each one of their minor versions. For example:

  Dune_lang.Syntax.create ~name:"fmt"
    ~desc:"integration with automatic formatters"
    [ ((1, 0), `Since (1, 4))
    ; ((1, 1), `Since (1, 7))
    ; ((1, 2), `Since (1, 11)) ]

• When an extension is used without being explicitly declared the latest version supported under the actual dune lang version will be added automatically to the project file.

• If a mismatch is found the minimal dune lang version needed is shown along with the versions of the extensions available under the current dune lang version.

• To make it less a breaking change, only a warning is emitted in some cases:

Extension version	Dune lang version	Result
Exists in a greater dune lang	>= 2.5	Error
Exists in a greater dune lang	< 2.5	Warning
Does not exists at all	>= 2.5	Error
Does not exists at all	< 2.5	Error

Notes

• Some fatal uses of the Syntax.since function are now useless.
• The version of the dune lang itself is handled by the same mechanism as the extensions versions. It may be beneficial to better separate the two as their semantics have now diverged a little. For example the greatest supported version of the dune lang is always the greatest one while the greatest supported version of an extension depends on the active dune lang version.
• It is a breaking change: misconfigured projects won't build anymore.

Todo

• Make it a warning for older versions of dune lang and an error for >=2.5

[voodoos]

I just realized with the CI crashing that this may be too much of a breaking change ?

For example the CI (which apparently use the PR's dune version to build the opam switch) fails because ppxlib uses Cinaps 1.0 with a dune-lang < 1.11.

I see three options:

1. Make it a warning with a flag to turn it into an error
2. Make it a warning for older versions of dune lang and an error for >=2.5
3. ⚔️ Start a crusade to correct all the wrong dune projects of opam

And my choice would be option 2.

[ghost]

Options 2 seems best to me as well.

One thing we could do to help though is scan the dune-universe for invalid dune-project files and ping the corresponding authors/projects.

[ghost]

Looks good!

[voodoos]

I implemented Option 2 for backward compatibility. The current policy is the following:

Extension version	Dune lang version	Result
Exists in a greater dune lang	>= 2.5	Error
Exists in a greater dune lang	< 2.5	Warning
Does not exists at all	>= 2.5	Error
Does not exists at all	< 2.5	Error

• It is probably better to wait for the CI to finish at least once.
• I will also add some tests for the different warning/error cases (but we won't be able to test the first case for now...).

[ghost]

You can bump the version of the dune language to 2.5 in this PR. This way we can test it now. We usually bump it as soon as required after a release.

[voodoos]

You can bump the version of the dune language to 2.5 in this PR. This way we can test it now. We usually bump it as soon as required after a release.

However it is not sufficient to enable us to test the first case: for that we need an extension with a newer version introduced in 2.5 or later. Is there one already ?

[ghost]

There isn't indeed. Let's just rely on code review for the first case then.

BTW, reading this part of the code again, I just remembered that you can use the ~is_error argument of User_warning.emit for things that are sometimes warnings and sometimes errors.

[ghost]

@voodoos this PR looks ready to me, you should feel free to merge it.