Contexts

A context is passed basically everywhere in dotc. This is convenient but carries the risk of captured contexts in objects that turn into space leaks. To combat this risk, here are some conventions to follow:

• Never let an implicit context be an argument of a class whose instances live longer than the context.
• Classes that need contexts for their initialization take an explicit parameter named initctx. They pass initctx to all positions where it is needed (and these positions should all be part of the intialization sequence of the class).
• Classes that need contexts that survive initialization are instead passed a "condensed context", typically named cctx (or they create one). Condensed contexts just add some basic information to the context base without the risk of capturing complete trees.
• To make sure these rules are kept, it would be good to do a sanity check using bytecode inspection with javap or scalap: Keep track of all class fields of type context; allow them only in whitelisted classes (which should be short-lived).

A context base defines state and associated methods that exist once per compiler run.

The essential mutable state of a context base, collected into a common class

A fresh context allows selective modification of its attributes using the with... methods.

Execute op at given period

Execute op at given phase id

Execute op at given phase

Execute op at the current phase if it's before the first transform phase, otherwise at the last phase before the first transform phase.

Note: this should be used instead of atPhaseNoLater(ctx.picklerPhase) because the later won't work if the Pickler phase is not present (for example, when using QuoteCompiler).

The current context

Run op with given context

Run op in current context, with a mode is temporarily set as specified.