Feature request: option to disable inlay hints on lines with an active cursor

[raggi]

Sometimes the inlay hints seem to cause refactoring/change troubles, a particularly common case is when adding or removing a type specifier on a let. I suspect this is in part due to my use of the vim plugin at the same time.

I realized today that a great way around this that would work well for me, would be an option to disable inlay hints on the active cursor line.

[SomeoneToIgnore]

First, we need to add the cursor position to the inlay hints LSP request.
Luckily, for inlay hints we use a custom request, so we can adjust it the way we want to.

RA server counterpart is here: https://github.com/rust-analyzer/rust-analyzer/blob/6826dd044a5a08359197b2a006905eb8518494a2/crates/rust-analyzer/src/lsp_ext.rs#L188
VSCode counterpart is here: https://github.com/rust-analyzer/rust-analyzer/blob/6826dd044a5a08359197b2a006905eb8518494a2/editors/code/src/lsp_ext.ts#L91

Position should be enough, similar to the way it's used here: https://github.com/rust-analyzer/rust-analyzer/blob/6826dd044a5a08359197b2a006905eb8518494a2/crates/rust-analyzer/src/lsp_ext.rs#L62

What's very important here is not to break the backward compatibility: we cannot make this new parameter manadatory from the start, because otherwise other editors that use the same command (emacs, vim, etc.) will break and we don't want this.

Then, we need to process this parameter.
After this line, we have the converted inlay hints with their Position calculated:
https://github.com/rust-analyzer/rust-analyzer/blob/7e012ae8b54705bc254154cdff3a503cce3299a7/crates/rust-analyzer/src/handlers.rs#L1078
Keep in mind that there can be multiple cursors in VSCode placed at once, so we might need to pass multiple positions in the request.

Last thing to do would be to add a config entry for this behavior:
server part: https://github.com/rust-analyzer/rust-analyzer/blob/df54561a689a9eb7b1962b69a397a1221200c116/crates/ide/src/inlay_hints.rs#L14
VSCode client part: https://github.com/rust-analyzer/rust-analyzer/blob/816a39cb54766c1dc8960620142659e716f485dc/editors/code/src/config.ts#L99
VSCode defaults & documentation: https://github.com/rust-analyzer/rust-analyzer/blob/d58a3a277a1778ec33e492e958b52869510c1239/editors/code/package.json#L390

[Veetaha]

I think this feature might be less beneficial as it might seem at first glance.
First of all, it means we now need to poll for inlay hints on each cursor move instead of each character typed.
Second, it will be annoying to see inlay hints appear-and-disappear shaking the code around while you are just browsing the editor.
Third, we already have a feature to assign a shortcut for toggling the inlay hints (Rust Analyzer: Toggle inlay hints command), see this PR for more details: #4601

[Veetaha]

cc @matklad

[matklad]

First of all, it means we now need to poll for inlay hints on each cursor move instead of each character typed.

Not really, we always query all inlay hint, but just don't show some of them in the editor. This is purely presentation concern.

I am not sure that "disabling hints on an active line" is the best presentation possible. Some alternatives:

• freeze the hint on the active line (if all the hints update with 100ms delay, make the active line hints update with 500ms delay)
• freeze the size of hints on the active line. Change the text of the hints themselves, but pad then with whitespace/truncate to preserve the overall width to reduce jumps.

All three ideas seem non-trivial to implement though, the state management required seems tricky. But I might be wrong about this :)

[Veetaha]

(if all the hints update with 100ms delay, make the active line hints update with 500ms delay)

There is no delay in updating the inlay hints, as I said we poll for new inlay hints only when the user types a character or opens a new file.

Not really, we always query all inlay hint, but just don't show some of them in the editor. This is purely presentation concern.

My point was not in the amount of inlay hints we query, but the frequency. If we start tracking the cursor, we will need to reevaluate the inlay hints when the user just moves the cursor (currently when the user just moves the cursor the hints are not refetched).

[matklad]

There's no need to refetch the hints in either case. The only difference is in how the client presents the hints to the user, the client-server interaction is exactly the same in both cases.

Client stores a full hints model (which depends only on the contents of the file), and then renders the model, completely client-side, into a view (which depends on the model, the current position of cursors, and the set of timers (if we want to have "draw hints on the current line after a second" behavior)). Only model -> view needs to be recalculated on cursor movements, and, as it is purely client-side, it is cheap.

[saiintbrisson]

I was playing around with the VSCode plugin and came up with this. Should it hide the chaining/params hints too?

[SomeoneToIgnore]

Looks nice!

I've imagined this feature removing all hints entirely, the original issue comment seems to mean the same.
Out of curiosity, does it work with multiple lines selected?

[saiintbrisson]

It does! Multiple selections and cursors.