feature: the compiler can warn when you use the implicit 'result' variable (#17988) [backport:1.2]

* implements #17855

(cherry picked from commit 378ee7f888)

compiler/lineinfos.nim

@@ -55,6 +55,7 @@ type
     warnLockLevel = "LockLevel", warnResultShadowed = "ResultShadowed",
     warnInconsistentSpacing = "Spacing",  warnCaseTransition = "CaseTransition",
     warnCycleCreated = "CycleCreated", warnObservableStores = "ObservableStores",
+    warnResultUsed = "ResultUsed",
     warnUser = "User",
 
     hintSuccess = "Success", hintSuccessX = "SuccessX", hintCC = "CC",
@@ -125,6 +126,7 @@ const
     warnCaseTransition: "Potential object case transition, instantiate new object instead",
     warnCycleCreated: "$1",
     warnObservableStores: "observable stores to '$1'",
+    warnResultUsed: "used 'result' variable",
     warnUser: "$1",
     hintSuccess: "operation successful: $#",
     # keep in sync with `testament.isSuccess`
@@ -176,7 +178,7 @@ type
   TNoteKinds* = set[TNoteKind]
 
 proc computeNotesVerbosity(): array[0..3, TNoteKinds] =
-  result[3] = {low(TNoteKind)..high(TNoteKind)} - {warnObservableStores}
+  result[3] = {low(TNoteKind)..high(TNoteKind)} - {warnObservableStores, warnResultUsed}
   result[2] = result[3] - {hintStackTrace, warnUninit, hintExtendedContext}
   result[1] = result[2] - {warnProveField, warnProveIndex,
     warnGcUnsafe, hintPath, hintDependency, hintCodeBegin, hintCodeEnd,

compiler/options.nim

@@ -356,7 +356,7 @@ proc hasHint*(conf: ConfigRef, note: TNoteKind): bool =
     note in conf.mainPackageNotes
   else: note in conf.notes
 
-proc hasWarn*(conf: ConfigRef, note: TNoteKind): bool =
+proc hasWarn*(conf: ConfigRef, note: TNoteKind): bool {.inline.} =
   optWarns in conf.options and note in conf.notes
 
 proc hcrOn*(conf: ConfigRef): bool = return optHotCodeReloading in conf.globalOptions

compiler/semexprs.nim

@@ -1216,6 +1216,9 @@ proc semSym(c: PContext, n: PNode, sym: PSym, flags: TExprFlags): PNode =
     # not sure the symbol really ends up being used:
     # var len = 0 # but won't be called
     # genericThatUsesLen(x) # marked as taking a closure?
+    if hasWarn(c.config, warnResultUsed):
+      message(c.config, n.info, warnResultUsed)
+
   of skGenericParam:
     onUse(n.info, s)
     if s.typ.kind == tyStatic: