psalm.xml

<psalm>
    <!-- ... -->    
    <plugins>
        <plugin filename="tools/psalm/TidewaysPsalmPlugin.php" />
    </plugins>
</psalm>

TidewaysPsalmPlugin.php

<?php

// This file is BSD-2 licenses
// Copyright 2020-current Tideways GmbH

use Psalm\Plugin\PluginEntryPointInterface;
use Psalm\Plugin\RegistrationInterface;
use Psalm\Plugin\Hook\AfterExpressionAnalysisInterface;
use Psalm\IssueBuffer;
use Psalm\Issue\PluginIssue;
use Psalm\CodeLocation;

class TidewaysPsalmPlugin implements PluginEntryPointInterface, AfterExpressionAnalysisInterface
{
    public function __invoke(RegistrationInterface $psalm, ?SimpleXMLElement $config = null) : void
    {
    }

    static public function afterExpressionAnalysis(
        PhpParser\Node\Expr $expr,
        Psalm\Context $context,
        Psalm\StatementsSource $statements_source,
        Psalm\Codebase $codebase,
        array &$file_replacements = []
    ) : ?bool
    {
        if ($expr instanceof PhpParser\Node\Expr\Isset_ &&
            $expr->vars[0] instanceof PhpParser\Node\Expr\Variable) {
            IssueBuffer::accepts(
                new IssetWithNonComplexVariable(
                    new CodeLocation($statements_source, $expr->vars[0])
                ),
                $statements_source->getSuppressedIssues()
            );
        }

        if ($expr instanceof PHPParser\Node\Expr\Empty_ &&
            $expr->expr instanceof PhpParser\Node\Expr\Variable) {
            $type = $statements_source->getNodeTypeProvider()->getType($expr->expr);

            if ($type !== null && $type->isInt()) {
                IssueBuffer::accepts(
                    new ReplaceEmptyWithMoreSpecificCondition(
                        '$' . $expr->expr->name . ' === 0',
                        new CodeLocation($statements_source, $expr)
                    )
                );
            } elseif ($type !== null && $type->isString()) {
                IssueBuffer::accepts(
                    new ReplaceEmptyWithMoreSpecificCondition(
                        'strlen($' . $expr->expr->name . ') === 0',
                        new CodeLocation($statements_source, $expr)
                    )
                );
            } elseif ($type !== null && $type->hasArray()) {
                IssueBuffer::accepts(
                    new ReplaceEmptyWithMoreSpecificCondition(
                        'count($' . $expr->expr->name . ') === 0',
                        new CodeLocation($statements_source, $expr)
                    )
                );
            } else {
                IssueBuffer::accepts(
                    new ReplaceEmptyWithMoreSpecificCondition(
                        'no suggestion possible for now',
                        new CodeLocation($statements_source, $expr)
                    )
                );
            }
        }

        return null;
    }
}

class IssetWithNonComplexVariable extends PluginIssue
{
    public function __construct(CodeLocation $code_location)
    {
        parent::__construct('Do not use isset on non-complex variables to test for (non-)emptiness.', $code_location);
    }
}

class ReplaceEmptyWithMoreSpecificCondition extends PluginIssue
{
    public function __construct(string $replacement, CodeLocation $code_location)
    {
        parent::__construct('Replace use of empty() with more specific condition: ' . $replacement, $code_location);
    }
}

I also could be '$' . $expr->expr->name . " === ''" instead of 'strlen($' . $expr->expr->name . ') === 0'.

Hi, I'm building a set of psalm plugins to help automatically migrate legacy projects. Do you mind if I publish a fully fleshed plugin based on this idea but with automatic code replacement when it is safe to do so?

@orklah yes please you may take the code, I'll update the gist to indicate a BSD-2 license.

Thanks! In progress at https://github.com/orklah/psalm-not-empty