Remove #[cfg] attributes during cfg-expansion

Currently, we don't remove `#[cfg]` attributes from a target when the
predicates pass. This PR removes all 'passing' `#[cfg]` attributes
during cfg-expansion, which causes derive proc-macros to never see any
`#[cfg]` attributes in the input token stream.

With #82608 merged, we can now do
this without losing spans.

Author: Aaron1011

compiler/rustc_expand/src/config.rs

@@ -246,6 +246,7 @@ impl<'a> StripUnconfigured<'a> {
     pub fn configure<T: AstLike>(&mut self, mut node: T) -> Option<T> {
         self.process_cfg_attrs(&mut node);
         if self.in_cfg(node.attrs()) {
+            node.visit_attrs(|attrs| remove_cfg(attrs));
             self.try_configure_tokens(&mut node);
             Some(node)
         } else {
@@ -267,7 +268,12 @@ impl<'a> StripUnconfigured<'a> {
         mut attrs: Vec<ast::Attribute>,
     ) -> Option<Vec<ast::Attribute>> {
         attrs.flat_map_in_place(|attr| self.process_cfg_attr(attr));
-        if self.in_cfg(&attrs) { Some(attrs) } else { None }
+        if self.in_cfg(&attrs) {
+            remove_cfg(&mut attrs);
+            Some(attrs)
+        } else {
+            None
+        }
     }
 
     /// Performs cfg-expansion on `stream`, producing a new `AttrAnnotatedTokenStream`.
@@ -294,9 +300,10 @@ impl<'a> StripUnconfigured<'a> {
                 AttrAnnotatedTokenTree::Attributes(mut data) => {
                     let mut attrs: Vec<_> = std::mem::take(&mut data.attrs).into();
                     attrs.flat_map_in_place(|attr| self.process_cfg_attr(attr));
-                    data.attrs = attrs.into();
 
-                    if self.in_cfg(&data.attrs) {
+                    if self.in_cfg(&attrs) {
+                        remove_cfg(&mut attrs);
+                        data.attrs = attrs.into();
                         data.tokens = LazyTokenStream::new(
                             self.configure_tokens(&data.tokens.create_token_stream()),
                         );
@@ -535,3 +542,7 @@ impl<'a> StripUnconfigured<'a> {
 fn is_cfg(sess: &Session, attr: &Attribute) -> bool {
     sess.check_name(attr, sym::cfg)
 }
+
+fn remove_cfg(attrs: &mut Vec<Attribute>) {
+    attrs.retain(|a| !a.has_name(sym::cfg));
+}

src/test/ui/conditional-compilation/cfg-attr-syntax-validation.rs

@@ -29,7 +29,6 @@ macro_rules! generate_s10 {
     ($expr: expr) => {
         #[cfg(feature = $expr)]
         //~^ ERROR expected unsuffixed literal or identifier, found `concat!("nonexistent")`
-        //~| ERROR expected unsuffixed literal or identifier, found `concat!("nonexistent")`
         struct S10;
     }
 }

src/test/ui/conditional-compilation/cfg-attr-syntax-validation.stderr

@@ -63,18 +63,7 @@ LL | generate_s10!(concat!("nonexistent"));
    |
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: expected unsuffixed literal or identifier, found `concat!("nonexistent")`
-  --> $DIR/cfg-attr-syntax-validation.rs:30:25
-   |
-LL |         #[cfg(feature = $expr)]
-   |                         ^^^^^
-...
-LL | generate_s10!(concat!("nonexistent"));
-   | -------------------------------------- in this macro invocation
-   |
-   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error: aborting due to 11 previous errors
+error: aborting due to 10 previous errors
 
 Some errors have detailed explanations: E0537, E0565.
 For more information about an error, try `rustc --explain E0537`.

src/test/ui/proc-macro/cfg-eval.stdout

@@ -1,4 +1,4 @@
-PRINT-ATTR INPUT (DISPLAY): struct S1 { #[cfg(all())] #[allow()] field_true : u8, }
+PRINT-ATTR INPUT (DISPLAY): struct S1 { #[allow()] field_true : u8, }
 PRINT-ATTR INPUT (DEBUG): TokenStream [
     Ident {
         ident: "struct",
@@ -11,36 +11,6 @@ PRINT-ATTR INPUT (DEBUG): TokenStream [
     Group {
         delimiter: Brace,
         stream: TokenStream [
-            Punct {
-                ch: '#',
-                spacing: Alone,
-                span: $DIR/cfg-eval.rs:20:5: 20:6 (#0),
-            },
-            Group {
-                delimiter: Bracket,
-                stream: TokenStream [
-                    Ident {
-                        ident: "cfg",
-                        span: $DIR/cfg-eval.rs:20:7: 20:10 (#0),
-                    },
-                    Group {
-                        delimiter: Parenthesis,
-                        stream: TokenStream [
-                            Ident {
-                                ident: "all",
-                                span: $DIR/cfg-eval.rs:20:11: 20:14 (#0),
-                            },
-                            Group {
-                                delimiter: Parenthesis,
-                                stream: TokenStream [],
-                                span: $DIR/cfg-eval.rs:20:14: 20:24 (#0),
-                            },
-                        ],
-                        span: $DIR/cfg-eval.rs:20:10: 20:25 (#0),
-                    },
-                ],
-                span: $DIR/cfg-eval.rs:20:6: 20:26 (#0),
-            },
             Punct {
                 ch: '#',
                 spacing: Alone,
@@ -83,7 +53,7 @@ PRINT-ATTR INPUT (DEBUG): TokenStream [
         span: $DIR/cfg-eval.rs:17:11: 24:2 (#0),
     },
 ]
-PRINT-ATTR INPUT (DISPLAY): #[rustc_dummy] (#[cfg(all())] 1,)
+PRINT-ATTR INPUT (DISPLAY): #[rustc_dummy] (1,)
 PRINT-ATTR INPUT (DEBUG): TokenStream [
     Punct {
         ch: '#',
@@ -103,36 +73,6 @@ PRINT-ATTR INPUT (DEBUG): TokenStream [
     Group {
         delimiter: Parenthesis,
         stream: TokenStream [
-            Punct {
-                ch: '#',
-                spacing: Alone,
-                span: $DIR/cfg-eval.rs:36:23: 36:24 (#0),
-            },
-            Group {
-                delimiter: Bracket,
-                stream: TokenStream [
-                    Ident {
-                        ident: "cfg",
-                        span: $DIR/cfg-eval.rs:36:25: 36:28 (#0),
-                    },
-                    Group {
-                        delimiter: Parenthesis,
-                        stream: TokenStream [
-                            Ident {
-                                ident: "all",
-                                span: $DIR/cfg-eval.rs:36:29: 36:32 (#0),
-                            },
-                            Group {
-                                delimiter: Parenthesis,
-                                stream: TokenStream [],
-                                span: $DIR/cfg-eval.rs:36:32: 36:42 (#0),
-                            },
-                        ],
-                        span: $DIR/cfg-eval.rs:36:28: 36:43 (#0),
-                    },
-                ],
-                span: $DIR/cfg-eval.rs:36:24: 36:44 (#0),
-            },
             Literal {
                 kind: Integer,
                 symbol: "1",