feat: add Igniter.Code.Pattern for ExAST-powered pattern matching

[dannote]

Adds Igniter.Code.Pattern — pattern-based AST navigation and rewriting powered by ExAST as an optional dependency.

Motivation

Writing zipper predicates for code search/replace is verbose:

Igniter.Code.Common.move_to(zipper, fn zipper ->
  Igniter.Code.Function.function_call?(zipper, :use, 2) &&
    Igniter.Code.Function.argument_equals?(zipper, 0, GenServer)
end)

With Igniter.Code.Pattern:

Pattern.move_to(zipper, "use GenServer")

Particularly useful for upgrade tasks and broad API migrations.

API

Zipper-level (follows existing Igniter.Code.* conventions):

• matches?(zipper, pattern) — predicate, for use in Common.move_to callbacks
• move_to(zipper, pattern, opts) → {:ok, zipper} | :error
• find_all(zipper, pattern, opts) → [zipper]
• replace(zipper, pattern, replacement, opts) → {:ok, zipper} | :error
• replace_all(zipper, pattern, replacement, opts) → {:ok, zipper} | :error

Project-level:

• replace_in_file(igniter, path, pattern, replacement, opts) → Igniter.t()
• replace_in_all_files(igniter, pattern, replacement, opts) → Igniter.t()

All accept :inside / :not_inside options for ancestor context filtering.

Pattern syntax

Patterns are valid Elixir — strings, quoted expressions, or ~p sigil:

Syntax	Meaning
_ or _name	Wildcard — matches anything, not captured
expr, name	Variable — matches anything, captured by name
...	Ellipsis — matches zero or more nodes
Everything else	Literal — must match exactly

# String patterns
Pattern.move_to(zipper, "use GenServer")
Pattern.find_all(zipper, "Repo.get!(...)")
Pattern.replace_all(zipper, "Logger.debug(msg, ...)", "Logger.warning(msg, ...)")

# Quoted patterns
Pattern.matches?(zipper, quote(do: Enum.map(_, _)))

# ~p sigil — compile-time parsing, no runtime overhead
import Igniter.Code.Pattern
Pattern.find_all(zipper, ~p"Repo.get!(...)")
Pattern.replace_all(zipper, ~p"Enum.map(list, f)", ~p"Enum.flat_map(list, f)")

Dependency

ExAST is optional — all functions gracefully return :error or [] when not available:

{:ex_ast, "~> 0.5", optional: true}

[zachdaniel]

Lets just make it a required dependency 😄 Easier to wrangle since igniter itself is typically an optional dependency used only in dev. This is awesome! ❤️

[zachdaniel]

I believe there is a better way to check the source. There is like a source type or a source implementation.

[dannote]

True, fixed

[zachdaniel]

Okay, only one last small comment, just a nitpick really.