Looking through the ‘StarterBug’ tag, I started looking into
<https://bugs.swift.org/browse/SR-580 <https://bugs.swift.org/browse/SR-580>>,
in which:
func foo(x: Int) -> Int {
var result = x + 1
#if NOT_ENABLED
_ = result
#endif
return result
}
does not warn that “result" was never written to.
The immediate cause seems to be in lib/Sema/MiscDiagnostics.cpp, in
VarDeclUsageChecker::handleIfConfig(). This code walks the inactive #if blocks
and does:
// If we see a bound reference to a decl in an inactive #if block, then
// conservatively mark it read and written. This will silence "variable
// unused" and "could be marked let" warnings for it.
if (auto *DRE = dyn_cast<DeclRefExpr>(E)) {
VDUC.addMark(DRE->getDecl(), RK_Read|RK_Written);
}
In trying to replace that with something that really walks the Expr (by calling
VDUC.walkToExprPre() instead or something of the like), I run into a bunch of
checks like this:
// Sema leaves some subexpressions null, which seems really unfortunate. It
// should replace them with ErrorExpr.
if (E == nullptr || !E->getType() || E->getType()->is<ErrorType>()) {
sawError = true;
return;
}
And, if I dump the AST for the test case, it has this section with all sorts of
nulls:
(#if_stmt
(#if:
(unresolved_decl_ref_expr type='<null>' name=NOT_ENABLED
specialized=no)
(elements
(sequence_expr type='<null>'
(discard_assignment_expr type='<null>')
(assign_expr
(**NULL EXPRESSION**)
(**NULL EXPRESSION**))
(declref_expr type='<null>' decl=main.(file).func
decl.qqq@/Users/bungi/Desktop/SR-580.swift:2:7 specialized=yes))))
Presumably VarDeclUsageChecker::handleIfConfig() could pass the right subset of
RK_Read|RK_Written if the declref_expr had its accessType set, or maybe a more
complete AST that could be walked.
So, then I started looking into places that look for IfConfig{Stmt,Decl} and
bail, and found ASTWalker.cpp’s Traversal::visitIfConfigStmt(), and
TypeCheckStmt.cpp’s visitIfConfigStmt(). I hacked in a walk() call in the
Traversal case and broke all sorts of stuff (maybe since I didn’t do something
similar in TypeCheckStmt.cpp). That was about the point that I thought I’d ask
if I’m digging in the wrong spot =)
Presumably making more work get done for inactive #if branches will slow down
the compiler some (hopefully the percentage of existing code inside a #if block
at all is pretty low). But, worse, I can imaging there is a ton of other stuff
in them that will cause errors if more of the compiler runs on them (whole
missing types, selectors, …).
Is this direction even worth pursuing? Maybe I should look into whether there
is a quicker way to get the access type of the declref_expr set when it is
being created during the inactive #if parsing?
Thanks!
-tim
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