The GHC-driven front end, part one: an in-process GHC session that drives parse -> rename -> typecheck -> desugar over the whole home module graph and hands back the (-O0, pre-simplifier) desugared Core, one ModGuts per home module in dependency order.

Wrinkles this module exists to handle:

• The desugarer runs GHC's simple optimizer unconditionally, and its occurrence analysis drops top-level binds unreachable from an exported binder -- even at -O0. A file with no module header is implicitly module Main (main) where, so everything but main would vanish. ensureLiveExports rewrites the parsed module before typechecking: headerless modules become export-all.
• mg_binds is strictly per-module: the target's guts contain no code from imported modules (including all of rewire-user, compiled from source via the loadpath). So every home module is desugared, not just the target.
• GHC reports errors as SourceError exceptions (from the per-module phases) or through the log action (during load); warnings only via the log action. Both are re-routed through ReWire's AstError / warnAt machinery so -w/-Werror and the usual error formatting (filename included) keep working.