Thank you for answers.
As people respond on this thread, would you be willing to capture what you learn in a wiki page in the Commentary? I already created such a wiki page some time ago:
Imported functions (eg foldr or build) have strictness and cardinality analysis info in their interface file signatures. That can in turn drive the attachment of one-shot info to binders. See one_shots = argsOneShots (idStrictness fun) n_val_args -- See Note [Use one-shot info] line 1345 of OccurAnal. And if we don't import anything then we're assuming NoOneShotInfo, which means we don't float in
https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/Core2CorePipeline But, since there are things I don't yet understant, this page is still incomplete. past lambdas? Janek