Provide extension hooks to generate debugger configuration when clicking Add Configuration button

[DonJayamanne]

Currently clicking the Configure or fix launch.json button will invoke the DebugConfigurationProvider of the extension.

We (Python Extension) have used this to generate a leaner launch.json file (#62851 & microsoft/vscode-python#3321 (comment)).
However, when the user clicks the Add Configuration button (either in the debugger dropdown or button displayed on the bottom right of launch.json, the corresponding extension hooks do not get triggered. Instead VSC merely displays a pick list of configuration snippets.

We were hoping VSC could provide some hook (similar to DebugConfigurationProvider) that would get invoked when clicking the Add Configuration button.

[isidorn]

@DonJayamanne this makes good sense and is somewhat a duplicate of #63121
As stated there:

Dynamic snippets should be shown on top of the add configuration suggest box and they should get configurations via provideDebugConfigurations call.
One dynamic snippet per debug type.

So basically the add configuration would call into the same exisiting DebugConfigurationProvider, we would not add any hooks.
Assigning to this milestone to investigate, and closing the other issue as dup of this one

fyi @weinand

[isidorn]

Note to self:
debugConfigurationManager should CompletionProviderRegistry.register for launch.json and return kind: snippet for CompletionItem. One CompletionItem per each debug type that has a DebugConfigurationProvider. Title should be just ${adapter.label}.
Code pointer https://github.com/Microsoft/vscode/blob/f7fa42107e8d37149410bcbe67a5cf4043390634/src/vs/workbench/parts/debug/electron-browser/debugConfigurationManager.ts#L278

@aeschli is there some way for me to provide a dynamic JSON schema snippet like this but that I compute it on the fly and return a promise. Or I have to go thorugh the CompletionProviderRegistry as I propose here.

[isidorn]

@DonJayamanne we tried implementing this and we believe that all the pieces are there already and this could be done by extensions. Here's what we recommend:

1. An extension should register a CompletionItemProvider for the **/launch.json files
2. As a result to provide suggestions the extension should for each configuration it wants to show return kind: snippet and return a proper snippet

Here's a very rough sketch of this

CompletionProviderRegistry.register({ pattern: '**/launch.json' }, {
	provideCompletionItems: (model, position, context, token) => {
		return this.provideDebugConfigurations().then(configurations => {	
			// TODO map configurations to suggestions
		});
	}
});

Here's the actual vscode api for this https://github.com/Microsoft/vscode/blob/275d2a02ca922738b58cea7dbf9dce229ea75f21/src/vs/vscode.d.ts#L7838

Can you please try this out and let us know if you find any missing pieces?
What might be a bit pesky is to try to figure out when you shuold provide suggetsions and when not. For that you need to look at the passed model and position and by parsing the text figure out if it is ok to show suggestions.
For example if inside the configurations array it is ok to do that.

Here's a sample for completions https://github.com/Microsoft/vscode-extension-samples/tree/master/completions-sample