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Reject cfg on expressions that cannot be safely removed #159580

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Reject cfg on expressions that cannot be safely removed #159580

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6 changes: 6 additions & 0 deletions compiler/rustc_expand/src/expand.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2410,6 +2410,12 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
Some(sym::cfg) => {
let span = attr.span;
if self.expand_cfg_true(node, attr, pos).as_bool() {
if matches!(
Node::KIND,
AstFragmentKind::Expr | AstFragmentKind::MethodReceiverExpr
) {
self.cx.dcx().emit_err(RemoveExprNotSupported { span });
}
continue;
}
Comment on lines 2412 to 2420

@mejrs mejrs Jul 20, 2026 •
edited by rustbot
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What this code does it emits this error if the cfg predicate is true, then if it's false we call expand_cfg_false and (if it's not the crate root) that emits the same error. Not only isn't that immediately obvious, it also suggests that there are still places where #[cfg(true)] is allowed but #[cfg(false)] is not.

Can you change it so there's only one place we emit this error (whether the predicate is true or not)?

Can you also change the error name and message to say that cfg is not supported in these positions?

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@mejrs mejrs Jul 20, 2026 •
edited
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(this check could also be moved elsewhere, I'm not sure what the best place for it is)


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7 changes: 7 additions & 0 deletions tests/ui/conditional-compilation/cfg-non-opt-expr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,13 @@
#![feature(custom_test_frameworks)]

fn main() {
let _ = 1 + #[cfg(unix)] 2;
//~^ ERROR removing an expression is not supported in this position
let _ = 1 + #[cfg(windows)] 2;
//~^ ERROR removing an expression is not supported in this position

let _ = 1 + #[cfg(all())] 2;
//~^ ERROR removing an expression is not supported in this position
let _ = #[cfg(false)] ();
//~^ ERROR removing an expression is not supported in this position
let _ = 1 + 2 + #[cfg(false)] 3;
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26 changes: 22 additions & 4 deletions tests/ui/conditional-compilation/cfg-non-opt-expr.stderr
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@mejrs mejrs Jul 20, 2026 •
edited by rustbot
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Can this file have some #[cfg(true)] as well?

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Original file line number Diff line number Diff line change
@@ -1,20 +1,38 @@
error: removing an expression is not supported in this position
--> $DIR/cfg-non-opt-expr.rs:5:13
--> $DIR/cfg-non-opt-expr.rs:5:17
|
LL | let _ = 1 + #[cfg(unix)] 2;
| ^^^^^^^^^^^^

error: removing an expression is not supported in this position
--> $DIR/cfg-non-opt-expr.rs:7:17
|
LL | let _ = 1 + #[cfg(windows)] 2;
| ^^^^^^^^^^^^^^^

error: removing an expression is not supported in this position
--> $DIR/cfg-non-opt-expr.rs:10:17
|
LL | let _ = 1 + #[cfg(all())] 2;
| ^^^^^^^^^^^^^

error: removing an expression is not supported in this position
--> $DIR/cfg-non-opt-expr.rs:12:13
|
LL | let _ = #[cfg(false)] ();
| ^^^^^^^^^^^^^

error: removing an expression is not supported in this position
--> $DIR/cfg-non-opt-expr.rs:7:21
--> $DIR/cfg-non-opt-expr.rs:14:21
|
LL | let _ = 1 + 2 + #[cfg(false)] 3;
| ^^^^^^^^^^^^^

error: removing an expression is not supported in this position
--> $DIR/cfg-non-opt-expr.rs:9:23
--> $DIR/cfg-non-opt-expr.rs:16:23
|
LL | let _ = [1, 2, 3][#[cfg(false)] 1];
| ^^^^^^^^^^^^^

error: aborting due to 3 previous errors
error: aborting due to 6 previous errors

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