Reject cfg on expressions that cannot be safely removed

Previously, expressions behind stmt_expr_attributes were only rejected
when the cfg condition evaluated to false. If the condition was true,
the code compiled successfully.

This meant code like an attributed binary operand could compile on one
platform but fail on another, even though removing the operand would leave
an invalid expression.

This change always rejects cfg in expression positions where removing the
expression would produce invalid code. Expression positions where removal is
safe continue to work as before.

Signed-off-by: Usman Akinyemi <usmanakinyemi202@gmail.com>

Author: Unique-Usman

compiler/rustc_expand/src/expand.rs

@@ -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;
                         }
 

tests/ui/conditional-compilation/cfg-non-opt-expr.rs

@@ -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;

tests/ui/conditional-compilation/cfg-non-opt-expr.stderr

@@ -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