Autocompletes _# with game.Selection:Get()[#] in Roblox Studio's command bar.

SelectionGet.lua

--!strict
local ScriptEditorService = game:GetService("ScriptEditorService")
local ALIAS = "game.Selection:Get()[%s]"

type AutocompleteItem = {
	label: string,
	kind: Enum.CompletionItemKind?,
	tags: {Enum.CompletionItemTag}?,
	detail: string?,

	documentation: {
		value: string,
	}?,

	overloads: number?,
	learnMoreLink: string?,
	codeSample: string?,
	preselect: boolean?,

	textEdit: {
		newText: string,
		replace: { 
			start: { line: number, character: number }, 
			["end"]: { line: number, character: number } 
		},
	}?
}

type AutocompleteResponse = {
	items: { AutocompleteItem }
}

type AutocompleteRequest = {
	position: {
		line: number,
		character: number
	},

	textDocument: {
		document: ScriptDocument?,
		script: LuaSourceContainer?
	}
}

local function snippetCallback(request: AutocompleteRequest, response: AutocompleteResponse)
	local textDoc = request.textDocument
	local scriptDoc = textDoc.document
	
	if scriptDoc and scriptDoc:IsCommandBar() then
		local line = scriptDoc:GetLine()
		
		if line:sub(-5) == "for _" then
			-- Typing a placeholder, ignore
			return response
		end
		
		local pos = request.position
		local startAt, endAt, index = line:find("_(%d+)$")
		
		if startAt and endAt and index then
			table.insert(response.items, {
				label = '_' .. index,
				kind = Enum.CompletionItemKind.Snippet,
				detail = "(Snippet)",

				documentation = {
					value = `Alias for {ALIAS:format(index)}`
				},
				
				textEdit = {
					newText = ALIAS:format(index),
					
					replace = {
						start = {
							line = pos.line,
							character = startAt,
						},
						
						["end"] = {
							line = pos.line,
							character = endAt + 1,
						}
					}
				}
			})
		elseif line:sub(-1) == "_" then
			table.insert(response.items, {
				label = "_#",
				kind = Enum.CompletionItemKind.Folder,
				detail = "(Hint)",
				
				documentation = {
					value = `Type a number after an underscore to auto-complete {ALIAS:format('#')}`
				},
				
				textEdit = {
					newText = "",
					
					replace = {
						start = pos,
						["end"] = pos
					}
				}
			})
		end
	end
	
	return response
end

pcall(function ()
	-- If testing, deregister the existing callback.
	ScriptEditorService:DeregisterAutocompleteCallback(ALIAS)
end)

ScriptEditorService:RegisterAutocompleteCallback(ALIAS, 1000, snippetCallback)