Roadmap

go-ruby-erb/erb is grown test-first, each capability differential-tested against MRI rather than built in isolation. The compiler — the deterministic, interpreter-independent core of ERB — is complete; the rendering eval is downstream by design.

Stage	What	Status
Tag scanner & literals	The MRI tag scanner for <% %> / <%= %> / <%# %> and the <%% / %%> literal escapes, split into the exact stags/etags ERB::Compiler produces.	Done
Trim modes	Every MRI trim_mode and its combinations — - explicit trim, > end-of-line, <> start-and-end, % percent-line (%% escaped).	Done
Compiler & text encoding	AST → Ruby source via MRI's put/insert/pre/post command wiring, with literal runs emitted through String#dump on the binary string for byte-exact round-tripping.	Done
Magic comments & ERB::Util	Leading <%# coding: … %> / frozen_string_literal: … (incl. the emacs -*- … -*- form) reflected in the emitted prefix, plus HTMLEscape / URLEncode.	Done
Differential oracle & coverage	A wide corpus compiled both here and by the system ruby, comparing emitted source and rendered output byte-for-byte; 100% coverage, gofmt + go vet clean, green across all six 64-bit Go arches.	Done
Rendering (eval)	eval(compiled_src, binding) against a live binding to produce the rendered string. Downstream — needs a Ruby interpreter; lives in the go-embedded-ruby consumer (rbgo), not this module.	Downstream

Documented out-of-scope boundaries

These are deliberate, recorded so the module's surface is unambiguous:

• No interpreter / no rendering. The library emits the Ruby source and the magic comment; it never runs Ruby. The eval that produces the string is the consumer's job — see Why a compiler, not a renderer.
• Reference is MRI 4.0.5. Byte-for-byte conformance targets that version's ERB::Compiler and ERB::Util; behaviour that changed across MRI releases is matched to this reference.
• String#dump encoding fidelity is to MRI's escaping of the binary string; callers needing a different encoding transcode at the boundary.
• ERB::Util is html_escape + url_encode (the .h / .u helpers); the full ERB object surface (ERB.new, #result, #run) is part of the rendering layer and therefore downstream.

See Usage & API for the surface and Trim modes & literals for the scanning rules these stages build out.