Notes about how to best implement the re!() syntax extension in terms of syntax, behavior, and underlying api.

re_se_api_notes.md

Full syntax:

re!(re_str
    [, engine 
        [(force|error on|warn on) runtime (compile|parse)]*
        [, engine_flags]
    ]
) -> CompiledRe

re!()'s contract: Accept static and non-static strings, any engine implementing either ReAstCompiler or ReStrCompiler, with compiletime guarantees degrading depending on inputs:

• static re_str, build-in engine -> compiletime syntax check, compiletime compiled
• static re_str, user supplied compilation engine -> compiletime syntax check, compiletime parsed, runtime compilation
• static re_str, user supplied parsing engine or non-static re_str, build-in engine or non-static re_str, user supplied compilation engine or non-static re_str, user supplied parsing engine -> runtime parsed re, runtime compilation

parse failure: Could just use conditions (because it's generally unexpected unless enduser provided, and then you might probably want an custom validator anyway.)