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fixes #19833 #19833; transform a discardable expression into a discard statement #23516

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fixes #19833 #19833; transform a discardable expression into a discard statement #23516

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4 changes: 2 additions & 2 deletions compiler/semexprs.nim
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Original file line number Diff line number Diff line change
Expand Up @@ -1186,7 +1186,7 @@ proc semExprNoType(c: PContext, n: PNode): PNode =
let isPush = c.config.hasHint(hintExtendedContext)
if isPush: pushInfoContext(c.config, n.info)
result = semExpr(c, n, {efWantStmt})
discardCheck(c, result, {})
result = discardCheck(c, result, {})
if isPush: popInfoContext(c.config)

proc isTypeExpr(n: PNode): bool =
Expand Down Expand Up @@ -2022,7 +2022,7 @@ proc semProcBody(c: PContext, n: PNode; expectedType: PType = nil): PNode =
a[1] = result
result = semAsgn(c, a)
else:
discardCheck(c, result, {})
result = discardCheck(c, result, {})

if c.p.owner.kind notin {skMacro, skTemplate} and
c.p.resultSym != nil and c.p.resultSym.typ.isMetaType:
Expand Down
65 changes: 50 additions & 15 deletions compiler/semstmts.nim
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Original file line number Diff line number Diff line change
Expand Up @@ -137,12 +137,25 @@ const
nkElifBranch, nkElifExpr, nkElseExpr, nkBlockStmt, nkBlockExpr,
nkHiddenStdConv, nkHiddenDeref}

proc implicitlyDiscardable(n: PNode): bool =
const skipForDiscardableStmt = skipForDiscardable - {nkHiddenStdConv, nkHiddenDeref}

type
DiscardableKind = enum
No, LastBlock, Discardable

proc implicitlyDiscardableClassifier(n: PNode): DiscardableKind =
var n = n
while n.kind in skipForDiscardable: n = n.lastSon
result = n.kind in nkLastBlockStmts or
(isCallExpr(n) and n[0].kind == nkSym and
sfDiscardable in n[0].sym.flags)
if n.kind in nkLastBlockStmts:
result = LastBlock
elif isCallExpr(n) and n[0].kind == nkSym and
sfDiscardable in n[0].sym.flags:
result = Discardable
else:
result = No

proc implicitlyDiscardable(n: PNode): bool =
result = implicitlyDiscardableClassifier(n) in {LastBlock, Discardable}

proc fixNilType(c: PContext; n: PNode) =
if isAtom(n):
Expand All @@ -153,13 +166,31 @@ proc fixNilType(c: PContext; n: PNode) =
for it in n: fixNilType(c, it)
n.typ = nil

proc discardCheck(c: PContext, result: PNode, flags: TExprFlags) =
proc wrapDiscardableExpr(n: PNode): PNode =
result = n
var n = n
var parent = n
var hasWork = false
while n.kind in skipForDiscardableStmt:
parent = n
n = n.lastSon
if n.kind notin skipForDiscardableStmt:
parent[^1] = newTreeI(nkDiscardStmt, n.info, n)
hasWork = true
break
if not hasWork:
result = newTreeI(nkDiscardStmt, result.info, result)

proc discardCheck(c: PContext, n: PNode, flags: TExprFlags): PNode =
result = n
if c.matchedConcept != nil or efInTypeof in flags: return

if result.typ != nil and result.typ.kind notin {tyTyped, tyVoid}:
if implicitlyDiscardable(result):
var n = newNodeI(nkDiscardStmt, result.info, 1)
n[0] = result
let kind = implicitlyDiscardableClassifier(result)
if kind == Discardable:
result = wrapDiscardableExpr(result)
elif kind == LastBlock:
discard
elif result.typ.kind != tyError and c.config.cmd != cmdInteractive:
if result.typ.kind == tyNone:
localError(c.config, result.info, "expression has no type: " &
Expand Down Expand Up @@ -207,7 +238,8 @@ proc semIf(c: PContext, n: PNode; flags: TExprFlags; expectedType: PType = nil):
else: illFormedAst(it, c.config)
if isEmptyType(typ) or typ.kind in {tyNil, tyUntyped} or
(not hasElse and efInTypeof notin flags):
for it in n: discardCheck(c, it.lastSon, flags)
for it in n:
it[^1] = discardCheck(c, it[^1], flags)
result.transitionSonsKind(nkIfStmt)
# propagate any enforced VoidContext:
if typ == c.enforceVoidContext: result.typ = c.enforceVoidContext
Expand Down Expand Up @@ -314,12 +346,14 @@ proc semTry(c: PContext, n: PNode; flags: TExprFlags; expectedType: PType = nil)
closeScope(c)

if isEmptyType(typ) or typ.kind in {tyNil, tyUntyped}:
discardCheck(c, n[0], flags)
for i in 1..<n.len: discardCheck(c, n[i].lastSon, flags)
n[0] = discardCheck(c, n[0], flags)
for i in 1..<n.len:
n[i][^1] = discardCheck(c, n[i][^1], flags)
if typ == c.enforceVoidContext:
result.typ = c.enforceVoidContext
else:
if n.lastSon.kind == nkFinally: discardCheck(c, n.lastSon.lastSon, flags)
if n.lastSon.kind == nkFinally:
n[^1][^1] = discardCheck(c, n[^1][^1], flags)
if not endsInNoReturn(n[0]):
n[0] = fitNode(c, typ, n[0], n[0].info)
for i in 1..last:
Expand Down Expand Up @@ -1035,7 +1069,7 @@ proc semForVars(c: PContext, n: PNode; flags: TExprFlags): PNode =
openScope(c)
n[^1] = semExprBranch(c, n[^1], flags)
if efInTypeof notin flags:
discardCheck(c, n[^1], flags)
n[^1] = discardCheck(c, n[^1], flags)
closeScope(c)
c.p.breakInLoop = oldBreakInLoop
dec(c.p.nestedLoopCounter)
Expand Down Expand Up @@ -1244,7 +1278,8 @@ proc semCase(c: PContext, n: PNode; flags: TExprFlags; expectedType: PType = nil
closeScope(c)
if isEmptyType(typ) or typ.kind in {tyNil, tyUntyped} or
(not hasElse and efInTypeof notin flags):
for i in 1..<n.len: discardCheck(c, n[i].lastSon, flags)
for i in 1..<n.len:
n[i][^1] = discardCheck(c, n[i][^1], flags)
# propagate any enforced VoidContext:
if typ == c.enforceVoidContext:
result.typ = c.enforceVoidContext
Expand Down Expand Up @@ -2733,7 +2768,7 @@ proc semStmtList(c: PContext, n: PNode, flags: TExprFlags, expectedType: PType =
n.typ = n[i].typ
if not isEmptyType(n.typ): n.transitionSonsKind(nkStmtListExpr)
elif i != last or voidContext:
discardCheck(c, n[i], flags)
n[i] = discardCheck(c, n[i], flags)
else:
n.typ = n[i].typ
if not isEmptyType(n.typ): n.transitionSonsKind(nkStmtListExpr)
Expand Down