Extract rule: template-no-redundant-fn

[NullVoxPopuli]

Split from #2371.

[NullVoxPopuli-ai-agent]

Review: template-no-redundant-fn

Compared against the original ember-template-lint rule no-redundant-fn.

What looks good

• Core detection logic faithfully matches the original: checks for fn helper with exactly 1 param and no hash pairs.
• Covers both GlimmerSubExpression (e.g., (fn this.handleClick)) and GlimmerMustacheStatement (e.g., {{fn this.handleClick}}), matching the original's SubExpression and MustacheStatement visitors.
• The error message includes a helpful suggestion showing the direct function reference.
• Test cases match the original's test suite exactly (same valid and invalid cases).
• Both .gjs and .hbs parsers are tested.
• originallyFrom metadata is present.

Items worth attention

1. Original restricts to PathExpression params only: The original rule checks params[0].type !== 'PathExpression' and returns early if the single param is not a path expression. The ESLint implementation does not enforce this restriction. This means it would flag (fn "some-string") or (fn 42) which the original would not. In practice these are unlikely inputs, but it's a behavioral difference. The paramText fallback to context.sourceCode.getText(param) handles non-path params gracefully.

2. Original has autofix support: The original rule supports fix mode, replacing (fn this.action) with this.action (for SubExpressions) and {{fn this.action}} with {{this.action}} (for MustacheStatements). The ESLint rule has fixable: null. Given that the fix is straightforward, this could be a nice enhancement in a follow-up.

3. Error message style difference: The original uses a terse message: `fn` helpers without additional arguments are not allowed. The ESLint version is more verbose but more helpful: "Unnecessary use of (fn) helper. Pass the function directly instead: {{suggestion}}". This is an improvement.

Summary

This is a clean, faithful port. The test coverage matches the original well. The only meaningful behavioral difference is the missing PathExpression type check on params[0], which is very minor in practice. Nice work.

🤖 Automated review comparing with ember-template-lint source