Add support for inlay hints from TypeScript language server

[shivjm]

This is an inexpert adaptation (mainly duplication) of the recently-added support for rust-analyzer’s inlay hints to ts-ls. Here’s how it looks in JavaScript and TypeScript, with bright backgrounds for the hints so they stand out in the image:

The language server suggests a fair bit of flexibility, almost all of which is reproduced in this PR.¹ For example, the return new Entity line that the cursor is on in the top window would normally have hints for the parameters too, but I’ve set lsp-javascript-display-parameter-name-hints-when-argument-matches-name to nil, so, for example, passing a status variable as the status parameter suppresses the hint.

Note that the server’s support is itself marked experimental. In particular, I can’t seem to get any enum member value hints, despite the existence of the option. (Hence the commented-out case in lsp-javascript-format-inlay.)

Footnotes

1. It didn’t seem useful to have separate parameters for JS and TS, so I clubbed them together. I’m open to the idea that someone else might prefer to distinguish between them. ↩

[yyoncho]

Inlay hints was added to the core spec, can you compare both? If it matches the spec then we should have lsp-inlay-hints.el in lsp-mode core

[shivjm]

There’s spec-related discussion in a few different places:

• Feature: Inlay Hints microsoft/language-server-protocol#956 is a closed issue that mentions a few other PRs
• Add textDocument/inlayHints microsoft/language-server-protocol#1249 is a closed PR with a comment that the draft was added to the spec, but that’s in the not-yet-released 3.17 version
• Add proposed textDocument/inlayHints to protocol & client. microsoft/vscode-languageserver-node#772 is an open PR to add it to the protocol and the client for that server

The TypeScript language server itself puts it under a file with ‘proposed’ in the name. Its implementation is slightly different. I don’t see any mention of the spec in these:

• Support inlay hints typescript-language-server/typescript-language-server#258 (issue about adding hints to the server)
• Implement inlay hints typescript-language-server/typescript-language-server#259 (PR for adding hints to the server)
• Support inlay hints microsoft/TypeScript#42073 (issue about adding hints to TS itself)
• Add inlay hints support microsoft/TypeScript#42089 (PR for adding hints to TS itself)

Maybe for now this should be ts-ls–specific. We could re-evaluate it as and when the server itself adopts the official version. What do you think?

[yyoncho]

We could re-evaluate it as and when the server itself adopts the official version. What do you think?

Sounds good. It will be great if you can take care of this by monitoring ts-ls development so we can drop the ad-hoc protocol implementation in favor of a real one.

[yyoncho]

Looks good to me. I won't be that picky about this implementation since it most likely will be short-lived when the core spec implementation is added. In core implementation I would expect lsp-javascript-inlay-hints-timer to go away in favor of using lsp-on-idle-hook. The benefits are that lsp-on-idle-hook will prevent the refresh of inlay hints when the completion dialog is running (thus making emacs more responsive).

[shivjm]

It will be great if you can take care of this by monitoring ts-ls development so we can drop the ad-hoc protocol implementation in favor of a real one.

Yup, happy to!

Looks good to me. I won't be that picky about this implementation since it most likely will be short-lived when the core spec implementation is added. In core implementation I would expect lsp-javascript-inlay-hints-timer to go away in favor of using lsp-on-idle-hook. The benefits are that lsp-on-idle-hook will prevent the refresh of inlay hints when the completion dialog is running (thus making emacs more responsive).

That makes a lot of sense. I don’t want to unnecessarily bog the interface down. I’ve updated it to use lsp-on-idle-hook instead.

[yyoncho]

Thank you! If you have a Twitter account it will be great if you can tweet about it( I will retweet it).

[shivjm]

Thank you for the gentle and patient guidance. 😊 I unfortunately don’t have a Twitter account, or I would have been happy to.