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Reject cfg on expressions that cannot be safely removed #159580

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7 changes: 7 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 @@ -2427,6 +2427,13 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
Some(sym::cfg) => {
let span = attr.span;
if self.expand_cfg_true(node, attr, pos).as_bool() {
match Node::KIND {
AstFragmentKind::Expr | AstFragmentKind::MethodReceiverExpr => {
self.cx.dcx().emit_err(RemoveExprNotSupported { span });
}
AstFragmentKind::Crate => {}
_ => unreachable!(),
}
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petrochenkov marked this conversation as resolved.
continue;
}
Comment on lines 2429 to 2438

@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)

@petrochenkov petrochenkov Aug 5, 2026

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

The false version is reported in expand_cfg_false.

This seems like an ok place to do this check for the true case, if it's moved to expand_cfg_true, the match #159580 (comment) will be much larger at least.

@petrochenkov petrochenkov Aug 5, 2026

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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.

If expand_cfg_false is inlined and replaced with a similar match then it will be more clear.


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27 changes: 27 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
@@ -1,11 +1,38 @@
#![feature(stmt_expr_attributes)]
#![feature(custom_test_frameworks)]

#[derive(Clone)]
enum E {
V1 = #[cfg(true)] 1,
//~^ ERROR removing an expression is not supported in this position
//~| ERROR removing an expression is not supported in this position
V2 = #[cfg_attr(true, cfg(true))] 2,
//~^ ERROR removing an expression is not supported in this position
}

macro_rules! mac {
($expr:expr) => { $expr.clone() }
}

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;
//~^ ERROR removing an expression is not supported in this position
let _ = [1, 2, 3][#[cfg(false)] 1];
//~^ ERROR removing an expression is not supported in this position
let _ = mac!(#[cfg(true)] 10);
//~^ ERROR removing an expression is not supported in this position
let _ = #[cfg(true)] ();
//~^ ERROR removing an expression is not supported in this position
let _ = 1 + 2 + #[cfg(true)] 3;
//~^ ERROR removing an expression is not supported in this position
let _ = [1, 2, 3][#[cfg(true)] 1];
//~^ ERROR removing an expression is not supported in this position
}
70 changes: 66 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 +1,82 @@
error: removing an expression is not supported in this position
--> $DIR/cfg-non-opt-expr.rs:5:13
--> $DIR/cfg-non-opt-expr.rs:18:17
|
LL | let _ = 1 + #[cfg(unix)] 2;
| ^^^^^^^^^^^^

error: removing an expression is not supported in this position
--> $DIR/cfg-non-opt-expr.rs:20:17
|
LL | let _ = 1 + #[cfg(windows)] 2;
| ^^^^^^^^^^^^^^^

error: removing an expression is not supported in this position
--> $DIR/cfg-non-opt-expr.rs:22:17
|
LL | let _ = 1 + #[cfg(all())] 2;
| ^^^^^^^^^^^^^

error: removing an expression is not supported in this position
--> $DIR/cfg-non-opt-expr.rs:24:13
|
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:26:21
|
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:28:23
|
LL | let _ = [1, 2, 3][#[cfg(false)] 1];
| ^^^^^^^^^^^^^

error: aborting due to 3 previous errors
error: removing an expression is not supported in this position
--> $DIR/cfg-non-opt-expr.rs:32:13
|
LL | let _ = #[cfg(true)] ();
| ^^^^^^^^^^^^

error: removing an expression is not supported in this position
--> $DIR/cfg-non-opt-expr.rs:34:21
|
LL | let _ = 1 + 2 + #[cfg(true)] 3;
| ^^^^^^^^^^^^

error: removing an expression is not supported in this position
--> $DIR/cfg-non-opt-expr.rs:36:23
|
LL | let _ = [1, 2, 3][#[cfg(true)] 1];
| ^^^^^^^^^^^^

error: removing an expression is not supported in this position
--> $DIR/cfg-non-opt-expr.rs:6:10
|
LL | V1 = #[cfg(true)] 1,
| ^^^^^^^^^^^^

error: removing an expression is not supported in this position
--> $DIR/cfg-non-opt-expr.rs:6:10
|
LL | V1 = #[cfg(true)] 1,
| ^^^^^^^^^^^^
|
= note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error: removing an expression is not supported in this position
--> $DIR/cfg-non-opt-expr.rs:9:27
|
LL | V2 = #[cfg_attr(true, cfg(true))] 2,
| ^^^^^^^^^

error: removing an expression is not supported in this position
--> $DIR/cfg-non-opt-expr.rs:30:18
|
LL | let _ = mac!(#[cfg(true)] 10);
| ^^^^^^^^^^^^

error: aborting due to 13 previous errors

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