add experimental support for "phaseless" modules

The intent is to support phase-crossing data such as the `exn:fail:syntax'
structure type that is instantiaed by macros and recognized by contexts
that use `eval' or `expand'. Phaseless modules are highly constrained,
however, to avoid new cross-phase channels, and a module is inferred to
be phaseless when it fits syntactic constraints.

I've adjusted `racket/kernel' and improved its documentation a little
so that it can be used to implement a phaseless module (which can
import only from other phaseless modules).

This change also adds a `flags' field to the `mod' structure type
from `compiler/zo-structs'.

original commit: 899a3279c2

collects/compiler/decompile.rkt

@@ -185,11 +185,12 @@
 (define (decompile-module mod-form orig-stack stx-ht mod-name)
   (match mod-form
     [(struct mod (name srcname self-modidx prefix provides requires body syntax-bodies unexported 
-                       max-let-depth dummy lang-info internal-context pre-submodules post-submodules))
+                       max-let-depth dummy lang-info internal-context flags pre-submodules post-submodules))
      (let-values ([(globs defns) (decompile-prefix prefix stx-ht)]
                   [(stack) (append '(#%modvars) orig-stack)]
                   [(closed) (make-hasheq)])
        `(,mod-name ,(if (symbol? name) name (last name)) ....
+           ,@(if (null? flags) '() (list `(quote ,@flags)))
            ,@(let ([l (apply
                        append
                        (for/list ([req (in-list requires)]

collects/compiler/demodularizer/merge.rkt

@@ -114,7 +114,7 @@
   (match mod-form
     [(struct mod (name srcname self-modidx mod-prefix provides requires body syntax-bodies
                        unexported mod-max-let-depth dummy lang-info internal-context
-                       pre-submodules post-submodules))
+                       flags pre-submodules post-submodules))
      (define toplevel-offset (length (prefix-toplevels top-prefix)))
      (define topsyntax-offset (length (prefix-stxs top-prefix)))
      (define lift-offset (prefix-num-lifts top-prefix))

collects/compiler/demodularizer/module.rkt

@@ -34,6 +34,7 @@
                 lang-info
                 #t
                 empty
+                empty
                 empty))]))
 
 (provide/contract

collects/compiler/demodularizer/nodep.rkt

@@ -137,7 +137,7 @@
   (match mod-form
     [(struct mod (name srcname self-modidx prefix provides requires body syntax-bodies
                        unexported max-let-depth dummy lang-info internal-context
-                       pre-submodules post-submodules))
+                       flags pre-submodules post-submodules))
      (define new-prefix prefix)
      ; Cache all the mpi paths
      (for-each (match-lambda
@@ -154,7 +154,7 @@
                          (begin (log-debug (format "[~S] lang-info : ~S" name lang-info)) ; XXX Seems to always be #f now
                                 (list (make-mod name srcname self-modidx new-prefix provides requires body empty
                                                 unexported max-let-depth dummy lang-info internal-context
-                                                empty empty)))
+                                                empty empty empty)))
                          (begin (log-debug (format "[~S] Dropping module @ ~S" name phase))
                                 empty))))]              
     [else (error 'nodep-module "huh?: ~e" mod-form)]))

collects/compiler/zo-marshal.rkt

@@ -995,7 +995,7 @@
 (define (convert-module mod-form)
   (match mod-form
     [(struct mod (name srcname self-modidx prefix provides requires body syntax-bodies unexported 
-                       max-let-depth dummy lang-info internal-context pre-submodules post-submodules))
+                       max-let-depth dummy lang-info internal-context flags pre-submodules post-submodules))
      (let* ([lookup-req (lambda (phase)
                           (let ([a (assq phase requires)])
                             (if a
@@ -1091,6 +1091,7 @@
             [l (cons lang-info l)] ; lang-info
             [l (cons (map convert-module post-submodules) l)]
             [l (cons (map convert-module pre-submodules) l)]
+            [l (cons (if (memq 'phaseless flags) #t #f) l)]
             [l (cons self-modidx l)]
             [l (cons srcname l)]
             [l (cons (if (pair? name) (car name) name) l)]

collects/compiler/zo-parse.rkt

@@ -251,7 +251,7 @@
 (define (read-module v)
   (match v
     [`(,submod-path 
-       ,name ,srcname ,self-modidx 
+       ,name ,srcname ,self-modidx ,phaseless?
        ,pre-submods ,post-submods
        ,lang-info ,functional? ,et-functional?
        ,rename ,max-let-depth ,dummy
@@ -337,6 +337,7 @@
                     dummy
                     lang-info
                     rename
+                    (if phaseless? '(phaseless) '())
                     (map read-module pre-submods)
                     (map read-module post-submods))]))]))
 (define (read-module-wrap v)

collects/compiler/zo-structs.rkt

@@ -139,6 +139,7 @@
                                 [dummy toplevel?]
                                 [lang-info (or/c #f (vector/c module-path? symbol? any/c))]
                                 [internal-context (or/c #f #t stx? (vectorof stx?))]
+                                [flags (listof (or/c 'phaseless))]
                                 [pre-submodules (listof mod?)]
                                 [post-submodules (listof mod?)]))