new feature: .used pragma to suppress declared-but-not-used warning

compiler/pragmas.nim

@@ -25,19 +25,19 @@ const
     wBorrow, wExtern, wImportCompilerProc, wThread, wImportCpp, wImportObjC,
     wAsmNoStackFrame, wError, wDiscardable, wNoInit, wDestructor, wCodegenDecl,
     wGensym, wInject, wRaises, wTags, wLocks, wDelegator, wGcSafe,
-    wOverride, wConstructor, wExportNims}
+    wOverride, wConstructor, wExportNims, wUsed}
   converterPragmas* = procPragmas
   methodPragmas* = procPragmas+{wBase}-{wImportCpp}
   templatePragmas* = {wImmediate, wDeprecated, wError, wGensym, wInject, wDirty,
-    wDelegator, wExportNims}
+    wDelegator, wExportNims, wUsed}
   macroPragmas* = {FirstCallConv..LastCallConv, wImmediate, wImportc, wExportc,
     wNodecl, wMagic, wNosideeffect, wCompilerproc, wDeprecated, wExtern,
     wImportCpp, wImportObjC, wError, wDiscardable, wGensym, wInject, wDelegator,
-    wExportNims}
+    wExportNims, wUsed}
   iteratorPragmas* = {FirstCallConv..LastCallConv, wNosideeffect, wSideeffect,
     wImportc, wExportc, wNodecl, wMagic, wDeprecated, wBorrow, wExtern,
     wImportCpp, wImportObjC, wError, wDiscardable, wGensym, wInject, wRaises,
-    wTags, wLocks, wGcSafe, wExportNims}
+    wTags, wLocks, wGcSafe, wExportNims, wUsed}
   exprPragmas* = {wLine, wLocks, wNoRewrite, wGcSafe}
   stmtPragmas* = {wChecks, wObjChecks, wFieldChecks, wRangechecks,
     wBoundchecks, wOverflowchecks, wNilchecks, wAssertions, wWarnings, wHints,
@@ -55,16 +55,16 @@ const
     wPure, wHeader, wCompilerproc, wFinal, wSize, wExtern, wShallow,
     wImportCpp, wImportObjC, wError, wIncompleteStruct, wByCopy, wByRef,
     wInheritable, wGensym, wInject, wRequiresInit, wUnchecked, wUnion, wPacked,
-    wBorrow, wGcSafe, wExportNims, wPartial}
+    wBorrow, wGcSafe, wExportNims, wPartial, wUsed}
   fieldPragmas* = {wImportc, wExportc, wDeprecated, wExtern,
-    wImportCpp, wImportObjC, wError, wGuard, wBitsize}
+    wImportCpp, wImportObjC, wError, wGuard, wBitsize, wUsed}
   varPragmas* = {wImportc, wExportc, wVolatile, wRegister, wThreadVar, wNodecl,
     wMagic, wHeader, wDeprecated, wCompilerproc, wDynlib, wExtern,
     wImportCpp, wImportObjC, wError, wNoInit, wCompileTime, wGlobal,
-    wGensym, wInject, wCodegenDecl, wGuard, wGoto, wExportNims}
+    wGensym, wInject, wCodegenDecl, wGuard, wGoto, wExportNims, wUsed}
   constPragmas* = {wImportc, wExportc, wHeader, wDeprecated, wMagic, wNodecl,
     wExtern, wImportCpp, wImportObjC, wError, wGensym, wInject, wExportNims,
-    wIntDefine, wStrDefine}
+    wIntDefine, wStrDefine, wUsed}
   letPragmas* = varPragmas
   procTypePragmas* = {FirstCallConv..LastCallConv, wVarargs, wNosideeffect,
                       wThread, wRaises, wLocks, wTags, wGcSafe}
@@ -961,6 +961,10 @@ proc singlePragma(c: PContext, sym: PSym, n: PNode, i: int,
         sym.magic = mIntDefine
       of wStrDefine:
         sym.magic = mStrDefine
+      of wUsed:
+        noVal(it)
+        if sym == nil: invalidPragma(it)
+        else: sym.flags.incl sfUsed
       else: invalidPragma(it)
     else: invalidPragma(it)
 

compiler/wordrecg.nim

@@ -45,7 +45,7 @@ type
     wImportc, wExportc, wExportNims, wIncompleteStruct, wRequiresInit,
     wAlign, wNodecl, wPure, wSideeffect, wHeader,
     wNosideeffect, wGcSafe, wNoreturn, wMerge, wLib, wDynlib,
-    wCompilerproc, wProcVar, wBase,
+    wCompilerproc, wProcVar, wBase, wUsed,
     wFatal, wError, wWarning, wHint, wLine, wPush, wPop, wDefine, wUndef,
     wLinedir, wStacktrace, wLinetrace, wLink, wCompile,
     wLinksys, wDeprecated, wVarargs, wCallconv, wBreakpoint, wDebugger,
@@ -131,7 +131,7 @@ const
     "incompletestruct",
     "requiresinit", "align", "nodecl", "pure", "sideeffect",
     "header", "nosideeffect", "gcsafe", "noreturn", "merge", "lib", "dynlib",
-    "compilerproc", "procvar", "base",
+    "compilerproc", "procvar", "base", "used",
     "fatal", "error", "warning", "hint", "line",
     "push", "pop", "define", "undef", "linedir", "stacktrace", "linetrace",
     "link", "compile", "linksys", "deprecated", "varargs",

doc/manual/pragmas.txt

@@ -503,6 +503,26 @@ identifier that can be used to enable or disable it:
 This is often better than disabling all warnings at once.
 
 
+used pragma
+-----------
+
+Nim produces a warning for symbols that are not exported and not used either.
+The ``used`` pragma can be attached to a symbol to suppress this warning. This
+is particularly useful when the symbol was generated by a macro:
+
+.. code-block:: nim
+  template implementArithOps(T) =
+    proc echoAdd(a, b: T) {.used.} =
+      echo a + b
+    proc echoSub(a, b: T) {.used.} =
+      echo a - b
+
+  # no warning produced for the unused 'echoSub'
+  implementArithOps(int)
+  echoAdd 3, 5
+
+
+
 experimental pragma
 -------------------
 

tests/pragmas/tused.nim

@@ -0,0 +1,13 @@
+discard """
+  output: '''8'''
+"""
+
+template implementArithOps(T) =
+  proc echoAdd(a, b: T) {.used.} =
+    echo a + b
+  proc echoSub(a, b: T) {.used.} =
+    echo a - b
+
+# no warning produced for the unused 'echoSub'
+implementArithOps(int)
+echoAdd 3, 5

web/news/e031_version_0_16_2.rst

@@ -66,6 +66,10 @@ these procedures.
 In the near future we will be converting all exception types to refs to
 remove the need for the ``newException`` template.
 
+- A new pragma ``.used`` can be used for symbols to prevent
+the "declared but not used" warning. More details can be found `here <http://nim-lang.org/docs/manual.html#pragmas-used-pragma>`_.
+
+
 Bugfixes
 --------