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

@@ -44,6 +44,7 @@ type
     warnUninit, warnGcMem, warnDestructor, warnLockLevel, warnResultShadowed,
     warnInconsistentSpacing, warnCaseTransition, warnCycleCreated,
     warnObservableStores,
+    warnResultUsed,
     warnUser,
     hintSuccess, hintSuccessX, hintCC,
     hintLineTooLong, hintXDeclaredButNotUsed,
@@ -110,6 +111,7 @@ 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`
@@ -160,7 +162,7 @@ const
     "IndexCheck", "GcUnsafe", "GcUnsafe2", "Uninit",
     "GcMem", "Destructor", "LockLevel", "ResultShadowed",
     "Spacing", "CaseTransition", "CycleCreated",
-    "ObservableStores", "User"]
+    "ObservableStores", "ResultUsed", "User"]
 
   HintsToStr* = [
     "Success", "SuccessX", "CC", "LineTooLong",
@@ -191,7 +193,7 @@ type
   TNoteKinds* = set[TNoteKind]
 
 proc computeNotesVerbosity(): array[0..3, TNoteKinds] =
-  result[3] = {low(TNoteKind)..high(TNoteKind)} - {}
+  result[3] = {low(TNoteKind)..high(TNoteKind)} - {warnResultUsed}
   result[2] = result[3] - {hintStackTrace, warnUninit, hintExtendedContext}
   result[1] = result[2] - {warnProveField, warnProveIndex,
     warnGcUnsafe, hintPath, hintDependency, hintCodeBegin, hintCodeEnd,

compiler/options.nim

@@ -310,7 +310,7 @@ proc setNote*(conf: ConfigRef, note: TNoteKind, enabled = true) =
 proc hasHint*(conf: ConfigRef, note: TNoteKind): bool =
   optHints in conf.options and 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

@@ -1207,6 +1207,9 @@ 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: