fixes #26062; ResultUsed warning behaves inconsistently with manual c… (#26087)

…haracterization with--warning:ResultUsed:on

fixes #26062

> A return statement with no expression is shorthand for return result.

> ResultUsed: Warn about the usage of the built-in result variable.

> A procedure that does not have any return statement and does not use
the special result variable returns the value of its last expression.

(cherry picked from commit 0ec8682abe)

compiler/semexprs.nim

@@ -1409,7 +1409,11 @@ 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):
+    # Lowered returns use resolved symbol nodes internally; warn only for
+    # source-level references to the implicit result variable.
+    if s.kind == skResult and
+        (n.kind != nkSym or nfFromTemplate in n.flags) and
+        hasWarn(c.config, warnResultUsed):
       message(c.config, n.info, warnResultUsed)
 
   of skGenericParam:
@@ -2089,6 +2093,8 @@ proc semReturn(c: PContext, n: PNode): PNode =
       # optimize away ``result = result``:
       if result[0][1].kind == nkSym and result[0][1].sym == c.p.resultSym:
         result[0] = c.graph.emptyNode
+    elif c.p.resultSym != nil and hasWarn(c.config, warnResultUsed):
+      message(c.config, n.info, warnResultUsed)
   else:
     localError(c.config, n.info, "'return' not allowed here")
 

tests/msgs/tresultused.nim

@@ -0,0 +1,42 @@
+discard """
+  action: compile
+  cmd: "nim check --hints:off --warning:ResultUsed:on $file"
+  nimout: '''
+tresultused.nim(34, 3) Warning: used 'result' variable [ResultUsed]
+tresultused.nim(37, 3) Warning: used 'result' variable [ResultUsed]
+'''
+"""
+
+let value = 0
+discard value
+
+for item in 0 .. 0:
+  discard item
+
+proc usesLocal() =
+  var local = 0
+  discard local
+
+block:
+  let result = 0
+  discard result
+
+proc voidReturn() =
+  return
+
+proc implicitReturn(): int =
+  0
+
+proc explicitReturn(): int =
+  return 0
+
+proc usesResult(): int =
+  result = 0
+
+proc usesBareReturn(): int =
+  return
+
+doAssert implicitReturn() == 0
+doAssert explicitReturn() == 0
+doAssert usesResult() == 0
+doAssert usesBareReturn() == 0