[SourceKit] Add CollectVariableType request

[fwcd]

To provide accurate support for inlay type hints in SourceKit-LSP (see swiftlang/sourcekit-lsp#406 for details), this PR introduces a new request called CollectVariableType. Although similar to CollectExpressionType, the new request provides types for variable declarations and closure parameters specifically, together with the identifiers' ranges and a flag stating whether there already exists an explicit type annotation.

Example

let x: Int = 3
var y = "test"

yields the following response:

<VariableTypes>
(4, 5): Int (explicit type: 1)
(19, 20): String (explicit type: 0)
</VariableTypes>

Additions to the SourceKit protocol

The request looks like this:

{
    <key.request>:            (UID)     <source.request.variable.type>,
    <key.sourcefile>:         (string)  // Absolute path to the file.
    <key.compilerargs>:       [string*] // Array of zero or more strings for the compiler arguments,
                                        // e.g ["-sdk", "/path/to/sdk"]. If key.sourcefile is provided,
                                        // these must include the path to that file.
    [opt] <key.offset>:       (int64)   // Offset of the requested range. Defaults to zero.
    [opt] <key.length>:       (int64)   // Length of the requested range. Defaults to the entire file.
}

The response looks like this:

{
    <key.variable_type_list>: (array) [var-type-info*]   // A list of variable declarations and types
}

var-type-info ::=
{
    <key.variable_offset>:       (int64)    // Offset of a variable identifier in the source file
    <key.variable_length>:       (int64)    // Length of a variable identifier an expression in the source file
    <key.variable_type>:         (string)   // Printed type of the variable declaration
    <key.variable_type_explicit> (bool)     // Whether the declaration has an explicit type annotation
}

Implementation Progress

• Add VariableTypeCollector that traverses AST and collects variable type information
• Implement the new request handler
• Add tests verifying basic use-cases (local variables, fields, parameters, declarations with and without type annotations)
• Add support for ranged requests (key.offset, key.length)

[ahoppen]

Mostly looking good to me. I’ve added some questions and comments inline.

[fwcd]

Thanks for the feedback, I've followed the implementation of ExpressionTypeInfo pretty closely for now, but I will make sure to address these issues.

[ahoppen]

Sorry for the delay, I only now managed to look through the code. Unfortunately, after looking at the implementation of the new TypeVariableArray, I remembered why we needed the approach with the output stream + offsets in ExpressionTypeArrayBuilder and I think it’s also necessary for VariableTypeArray 😬. Sorry for the back and forth about this.

[ahoppen]

And I’ve got some more comments inline.

[fwcd]

I have updated the implementation to use SourceRanges as suggested and will now rebase the branch on main to resolve the merge conflict and squash related commits.

[ahoppen]

I’ve got some minor comments regarding variable names and doc comments again. Other than that, it’s looking great. Can’t wait to try this out with the editor.

[ahoppen]

Nice! Let’s get this 🚢ed.

[ahoppen]

@swift-ci Please smoke test