Performance issues with CompletionsProcessor

[markwpearce]

To get completions in v1, the CompletionsProcessor currently does a scope linking and symbol table lookup for EVERY scope, then checks to see if any resultant completions are valid for every scope.

For a file in many scopes, this can take a bit of time (sometimes greater than 500 ms).

There are probably ways to speed this up, perhaps by creating a temporary scope that is the intersection of all scopes for a file, then linking and doing symbol table lookups once for this temporary scope.

[TwitchBronBron]

In a recent PR to master (and the lsp rewrite branch), we switched to building completions from a subset of all scopes. Not sure why it hasn't made its way into v1, but we should ensure this functionality makes its way into v1.

brighterscript/src/bscPlugin/completions/CompletionsProcessor.ts

Lines 35 to 50 in 963bcf0

	//find the scopes for this file (sort by name so these results are always consistent)
	let scopesForFile = this.event.program.getScopesForFile(this.event.file).sort(firstBy(x => x.name));
	//if there are no scopes, include the global scope so we at least get the built-in functions
	scopesForFile = scopesForFile.length > 0 ? scopesForFile : [this.event.program.globalScope];
	// Only process the first few scopes. This might result in missing completions,
	// but it's better than wasting TONS of cycles building essentially the same completions over and over
	const scopesToProcess = scopesForFile.slice(0, 3);
	// always include the source scope if applicable to this file
	let sourceScope = scopesForFile.find(x => x.name === 'source');
	if (sourceScope && !scopesToProcess.includes(sourceScope)) {
	//replace the first scope with the source scope so we always process exactly 3 scopes
	scopesToProcess[0] = sourceScope;
	}