Expandable hover

[gabritto]

Fixes #59029.

The basic idea of how this feature works is that the current quickinfo response corresponds to verbosityLevel 0, and each increase after that means expanding all* of the aliases we currently display.

For instance:

interface BaseType<T> {
    baseProp: () => T;
}

interface SomeType<T> extends BaseType<T>{
    prop1: T;
    prop2: T extends symbol ? string : number;
}

declare const a: SomeType<string>;
declare const b: SomeType<symbol>;

Hovering over a shows you this:
At level 0 (initial hover):

const a: SomeType<string>

At level 1:

const a: {
    prop1: string;
    prop2: number;
    baseProp: () => string;
}

Note how we show you the fully resolved type, i.e. fully instantiated, with conditional types resolved, inherited members, etc.
That's what I think is one of the main advantages of this feature, compared to just using "go to definition".

For top-level declarations, we also now expand to show the full declaration. Considering the same code as above, if we hover over SomeType, this is what we see:
Level 0:

interface SomeType<T>

Level 1:

interface SomeType<T> extends BaseType<T> {
    prop1: T;
    prop2: T extends symbol ? string : number;
}

So this also makes it convenient to see the declaration without having to navigate to a different place, e.g. a different file.

Implementation

The first kind of hover, hover on a type, works via typeToString, i.e. type printing, and the second kind of hover, on declarations, works via the serializeX functions, i.e. the functions responsible for d.ts emit.
For the latter, some noteworthy changes are that I added a new symbolToDeclarations function to nodeBuilder, and I added truncation to those functions to match the truncation that we have in typeToString (see f9d2309). I also had to change how some constructs are printed, e.g. instead of showing #private in a class with private identifiers, we now show all private identifiers.
All of those changes to the d.ts emit functions should only happen when we are calling those functions for quickinfo purposes.

The expansion is controlled by new properties on the NodeBuilderContext that is shared for all of the functions mentioned above. We have a maximum depth of expansion that corresponds to a verbosity level, and we also keep track of how many levels of aliases/definitions we have expanded so far, as we visit the tree to produce a hover. Whenever we decide to expand something, we increase that by one level.

We also need to produce output when we call those functions. We do this by having an out property on the context.
The output is basically two booleans:

• couldUnfoldMore, which will correspond to canIncreaseVerbosityLevel, and is true if increasing the verbosity level by one would produce a new expansion.
• truncated, which is true if we did truncation. If we did truncation, we will always return false for canIncreaseVerbosityLevel, because increasing verbosity would only produce a larger hover, and the current one is already too large.

Testing

To test this feature in vscode, you need to downgrade your vscode version to the January version (1.97): https://code.visualstudio.com/updates/v1_97
For reasons, that's the version that has the experimental expandable hover setting:

I will make a vscode PR to re-enable this on the editor side, but that would require testing this feature with a local build of vscode, which is far less convenient.

[typescript-bot]

Thanks for the PR! It looks like you've changed the TSServer protocol in some way. Please ensure that any changes here don't break consumers of the current TSServer API. For some extra review, we'll ping @sheetalkamat, @mjbvz, @zkat, and @joj for you. Feel free to loop in other consumers/maintainers if necessary.

[typescript-bot]

Looks like you're introducing a change to the public API surface area. If this includes breaking changes, please document them on our wiki's API Breaking Changes page.

Also, please make sure @DanielRosenwasser and @RyanCavanaugh are aware of the changes, just as a heads up.

[Copilot]

Pull Request Overview

This PR implements an "Expandable hover" feature by adding a verbosity level to quick info responses so that increasingly detailed type information can be displayed on hover. Key changes include updates to type‐display and signature functions to accept verbosity level and output context, overloads and new properties in quick info interfaces, and corresponding adjustments in the language service, protocol, and fourslash harness test infrastructure.

Reviewed Changes

Copilot reviewed 65 out of 68 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/services/utilities.ts	Updated typeToDisplayParts and signatureToDisplayParts to pass verbosity level and writer context.
src/services/types.ts	Added an overload and a canIncreaseVerbosityLevel property to QuickInfo.
src/services/symbolDisplay.ts	Propagated verbosity level through symbol display computation and introduced unfolding helpers.
src/services/services.ts	Modified getQuickInfoAtPosition to forward verbosity level to type printing functions.
src/server/session.ts & src/server/protocol.ts	Updated quick info request/response signatures to include verbosity level and expanded type printing.
src/harness/*	Adjusted fourslash baseline and client code to allow controlling verbosity levels in tests.
src/compiler/types.ts	Extended internal type writer and symbol declaration functions with verbosity level support.

Files not reviewed (3)

• tests/baselines/reference/quickinfoVerbosity1.baseline: Language not supported
• tests/baselines/reference/quickinfoVerbosityConditionalType.baseline: Language not supported
• tests/baselines/reference/quickinfoVerbosityIntersection1.baseline: Language not supported

Comments suppressed due to low confidence (1)

src/services/symbolDisplay.ts:282

• [nitpick] Consider renaming 'typeWriterOut' to something more descriptive of its role (e.g. 'writerContextOut') to improve clarity about its purpose in managing verbosity state.

const typeWriterOut: WriterContextOut = { couldUnfoldMore: false, truncated: false };