TSServer: snippet completions

[mjbvz]

Provide a generic, cross editor snippet type that can be used to express complex text insertion operations. At a minimum, these snippets should include placeholder values and indicate where the cursor should end up.

For example, snippets could fill in function call arguments with placeholders. If the user completes ad to add here:

function add(x, y) { return x + y; }

ad|

The inserted snippet would be:

function add(x, y) { return x + y; }

add(«x», «y»)

Where the user can tab from the placeholder values «x» to «y».

Use Cases

• Method completions (currently implemented manually by VSCode under useCodeSnippetsOnMethodSuggest)
• React completions — React support for useCodeSnippetsOnMethodSuggest vscode#48562
• Js doc template completions
• implement overridden method

Why is this needed
Snippets speed up coding and provide additional hints while writing code. VS Code already tries to convert TS's existing response types to snippets but fails to handle many edge cases. In order to improve reliability and support more advanced used cases such as microsoft/vscode#48562, I believe that TS needs to start generating these snippets directly

[mjbvz]

I imagine that the snippet type could similar to SymbolDisplayPart. Just a quick sketch:

interface Snippet {
    parts: SnippetPart[];
}

type SnippetPart = SnippetText | SnippetPlaceholder;

interface SnippetText {
    kind: 'text'
    text: string
}

interface SnippetPlaceholder {
    kind: 'placeholder'
    text: string
}

For completions, the snippet could be new optional additional field on CompletionEntry. That way we don't break clients that do not support snippets.

[mjbvz]

Another possible use case that we discussed: refactorings such as extract constant. We currently implement this by triggering a rename after the extraction but a snippet could be a better fit. Example:

const three = 1 + 2;

Run extract constant on 2. This could generate a placeholder that is duplicated twice in the file:

const «newLocal» = 2
const three = 1 + «newLocal»;

Typing would update both locations

(Note that VS Code's api would not support this currently since the placeholders are at unconnected locations in the file)

[mhegazy]

Typing would update both locations

but is not that what rename does today?

[mjbvz]

Another possible use case: generate complete method signature when overriding a method in a subclass microsoft/vscode#46038

[bpasero]

Inspiration from Eclipse. Notice how the one tab stop for the method name is even linked!