new language feature: explicit 'import system' statements are allowed

compiler/modules.nim

@@ -184,8 +184,8 @@ proc importModule*(s: PSym, fileIdx: int32): PSym {.procvar.} =
   # this is called by the semantic checking phase
   result = compileModule(fileIdx, {})
   if optCaasEnabled in gGlobalOptions: addDep(s, fileIdx)
-  if sfSystemModule in result.flags:
-    localError(result.info, errAttemptToRedefine, result.name.s)
+  #if sfSystemModule in result.flags:
+  #  localError(result.info, errAttemptToRedefine, result.name.s)
   # restore the notes for outer module:
   gNotes = if s.owner.id == gMainPackageId: gMainPackageNotes
            else: ForeignPackageNotes

compiler/sem.nim

@@ -418,9 +418,6 @@ proc myOpen(module: PSym): PPassContext =
   c.importTable.addSym(module) # a module knows itself
   if sfSystemModule in module.flags:
     magicsys.systemModule = module # set global variable!
-  else:
-    c.importTable.addSym magicsys.systemModule # import the "System" identifier
-    importAllSymbols(c, magicsys.systemModule)
   c.topLevelScope = openScope(c)
   # don't be verbose unless the module belongs to the main package:
   if module.owner.id == gMainPackageId:
@@ -434,7 +431,29 @@ proc myOpenCached(module: PSym, rd: PRodReader): PPassContext =
   result = myOpen(module)
   for m in items(rd.methods): methodDef(m, true)
 
+proc isImportSystemStmt(n: PNode): bool =
+  if magicsys.systemModule == nil: return false
+  case n.kind
+  of nkImportStmt:
+    for x in n:
+      let f = checkModuleName(x)
+      if f == magicsys.systemModule.info.fileIndex:
+        return true
+  of nkImportExceptStmt, nkFromStmt:
+    let f = checkModuleName(n[0])
+    if f == magicsys.systemModule.info.fileIndex:
+      return true
+  else: discard
+
 proc semStmtAndGenerateGenerics(c: PContext, n: PNode): PNode =
+  if c.topStmts == 0 and not isImportSystemStmt(n):
+    if sfSystemModule notin c.module.flags and
+        n.kind notin {nkEmpty, nkCommentStmt}:
+      c.importTable.addSym magicsys.systemModule # import the "System" identifier
+      importAllSymbols(c, magicsys.systemModule)
+      inc c.topStmts
+  else:
+    inc c.topStmts
   if sfNoForward in c.module.flags:
     result = semAllTypeSections(c, n)
   else:

compiler/semdata.nim

@@ -99,6 +99,7 @@ type
     unknownIdents*: IntSet     # ids of all unknown identifiers to prevent
                                # naming it multiple times
     generics*: seq[TInstantiationPair] # pending list of instantiated generics to compile
+    topStmts*: int # counts the number of encountered top level statements
     lastGenericIdx*: int      # used for the generics stack
     hloLoopDetector*: int     # used to prevent endless loops in the HLO
     inParallelStmt*: int

tests/modules/texplicit_system_import.nim

@@ -0,0 +1,9 @@
+##.
+import system except `+`
+discard """
+  errormsg: "undeclared identifier: '+'"
+  line: 9
+"""
+# Testament requires that the initial """ occurs before the 40th byte
+# in the file. No kidding...
+echo 4+5

web/news/version_0_15_released.rst

@@ -54,6 +54,9 @@ Language Additions
 
 - Added ``{.intdefine.}`` and ``{.strdefine.}`` macros to make use of
   (optional) compile time defines.
+- If the first statement is an ``import system`` statement then ``system``
+  is not imported implicitly anymore. This allows for code like
+  ``import system except echo`` or ``from system import nil``.
 
 Bugfixes
 --------