implement bitsize pragma for bitfields

compiler/ast.nim

@@ -784,6 +784,7 @@ type
       tab*: TStrTable         # interface table for modules
     of skLet, skVar, skField, skForVar:
       guard*: PSym
+      bitsize*: BiggestInt
     else: nil
     magic*: TMagic
     typ*: PType

compiler/ccgtypes.nim

@@ -441,6 +441,8 @@ proc genRecordFieldsAux(m: BModule, n: PNode,
       elif fieldType.kind == tySequence:
         # we need to use a weak dependency here for trecursive_table.
         addf(result, "$1 $2;$n", [getTypeDescWeak(m, field.loc.t, check), sname])
+      elif field.bitsize != 0:
+        addf(result, "$1 $2:$3;$n", [getTypeDescAux(m, field.loc.t, check), sname, rope($field.bitsize)])
       else:
         # don't use fieldType here because we need the
         # tyGenericInst for C++ template support

compiler/pragmas.nim

@@ -56,7 +56,7 @@ const
     wInheritable, wGensym, wInject, wRequiresInit, wUnchecked, wUnion, wPacked,
     wBorrow, wGcSafe}
   fieldPragmas* = {wImportc, wExportc, wDeprecated, wExtern,
-    wImportCpp, wImportObjC, wError, wGuard}
+    wImportCpp, wImportObjC, wError, wGuard, wBitsize}
   varPragmas* = {wImportc, wExportc, wVolatile, wRegister, wThreadVar, wNodecl,
     wMagic, wHeader, wDeprecated, wCompilerproc, wDynlib, wExtern,
     wImportCpp, wImportObjC, wError, wNoInit, wCompileTime, wGlobal,
@@ -844,6 +844,11 @@ proc singlePragma(c: PContext, sym: PSym, n: PNode, i: int,
         if sym == nil: pragmaLockStmt(c, it)
         elif sym.typ == nil: invalidPragma(it)
         else: sym.typ.lockLevel = pragmaLocks(c, it)
+      of wBitsize:
+        if sym == nil or sym.kind != skField or it.kind != nkExprColonExpr:
+          invalidPragma(it)
+        else:
+          sym.bitsize = expectIntLit(c, it)
       of wGuard:
         if sym == nil or sym.kind notin {skVar, skLet, skField}:
           invalidPragma(it)

compiler/wordrecg.nim

@@ -82,6 +82,7 @@ type
     wStdIn, wStdOut, wStdErr,
 
     wInOut, wByCopy, wByRef, wOneWay,
+    wBitsize,
 
   TSpecialWords* = set[TSpecialWord]
 
@@ -168,6 +169,7 @@ const
     "stdin", "stdout", "stderr",
 
     "inout", "bycopy", "byref", "oneway",
+    "bitsize",
     ]
 
 proc findStr*(a: openArray[string], s: string): int =

tests/pragmas/tbitsize.nim

@@ -0,0 +1,7 @@
+type
+  bits* = object
+    flag* {.bitsize: 1.}: cint
+    opts* {.bitsize: 4.}: cint
+
+var b: bits
+echo b.flag