[simonhorlick]

This PR resurrects the hls-retrie-plugin.

The previous retrie plugin had a few issues that I wanted to ensure were solved by the new plugin:

• imports were often missing or incorrect
• grafting often introduced syntax errors
• precedence was not handled correctly
• test coverage was quite sparse
• several correctness issues have also been fixed upstream

This is the result of many attempts and failures, each leading to a hopefully better implementation. I have tried to document below the known shortcomings, many of these I intend to address with follow up PRs.

AI disclaimer: Some of the test cases were suggested by Claude. The plugin code itself is almost completely hand written.

What's supported

• Inlining of top-level definitions, infix operators and sections
• Inlining of let and where bindings
• Patterns in the definitions arguments
• Bindings in the expression are renamed to avoid capture
• Where bindings in the definition are rewritten as let bindings in the rewrite
• Use sites that are not fully applied get lambdas inserted
• Cross-module support (including adding of imports)

What's missing?

• Definitions with several clauses, guards or a record wildcard construction. Previously these were handled incorrectly, now they are refused
• We do not emit changes for non-active CPP branches
• Visible type application
• Recursive functions
• Functions that reference a type variable in the expression
• Cross-module inlining where the target module is missing a language pragma
• Type constraints solved by the expressions signature are not propagated to the inlining site
• Duplication of arguments (inlining could cause an argument to be evaluated multiple times)
• Capture avoidance outside of the substitution (if a binder somewhere on the path to the splice point captures a free variable in the template)
• Rewrites into CPP modules have incorrect indentation (see Layout issues when using Replacements xich/retrie#24)
• Capture avoidence from binders introduced by RecordWildCards dots
• Removal of the definition after inlining

Testing

This PR has undergone the following tests:

• As many distinct golden tests as I could think of. Including previous inlining tests from the hls-retrie-plugin.
• Running the inline action at a load of locations in the haskell-language-server repo

Fixes #279
Fixes #307
Fixes #1758
Partially fixes #4374 (by rejecting defs with guards, for now)
Fixes #4408 (except for the ghc10 support as noted in the comments)

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