Auto merge of rust-lang#16789 - wyatt-herkamp:cfg_attr, r=Veykril

cfg Attribute Stripping for Proc Macro Expansion

This will attempt to process cfg attributes and cfg_attr attributes for proc macro expansion.
![image](https://github.com/rust-lang/rust-analyzer/assets/11785959/b85ef203-14a5-44c9-9b67-59a65a5f2d96)

Closes rust-lang#8434 , rust-lang#11657, and rust-lang#13904

Author: bors

crates/cfg/Cargo.toml

@@ -31,4 +31,4 @@ mbe.workspace = true
 syntax.workspace = true
 
 [lints]
-workspace = true
+workspace = true

crates/cfg/src/cfg_expr.rs

@@ -47,6 +47,7 @@ impl CfgExpr {
     pub fn parse<S>(tt: &tt::Subtree<S>) -> CfgExpr {
         next_cfg_expr(&mut tt.token_trees.iter()).unwrap_or(CfgExpr::Invalid)
     }
+
     /// Fold the cfg by querying all basic `Atom` and `KeyValue` predicates.
     pub fn fold(&self, query: &dyn Fn(&CfgAtom) -> bool) -> Option<bool> {
         match self {
@@ -62,7 +63,6 @@ impl CfgExpr {
         }
     }
 }
-
 fn next_cfg_expr<S>(it: &mut SliceIter<'_, tt::TokenTree<S>>) -> Option<CfgExpr> {
     let name = match it.next() {
         None => return None,

crates/hir-def/src/macro_expansion_tests/builtin_derive_macro.rs

@@ -528,3 +528,121 @@ impl < > $crate::fmt::Debug for Command< > where {
 }"#]],
     );
 }
+#[test]
+fn test_debug_expand_with_cfg() {
+    check(
+        r#"
+            //- minicore: derive, fmt
+            use core::fmt::Debug;
+
+            #[derive(Debug)]
+            struct HideAndShow {
+                #[cfg(never)]
+                always_hide: u32,
+                #[cfg(not(never))]
+                always_show: u32,
+            }
+            #[derive(Debug)]
+            enum HideAndShowEnum {
+                #[cfg(never)]
+                AlwaysHide,
+                #[cfg(not(never))]
+                AlwaysShow{
+                    #[cfg(never)]
+                    always_hide: u32,
+                    #[cfg(not(never))]
+                    always_show: u32,
+                }
+            }
+        "#,
+        expect![[r#"
+use core::fmt::Debug;
+
+#[derive(Debug)]
+struct HideAndShow {
+    #[cfg(never)]
+    always_hide: u32,
+    #[cfg(not(never))]
+    always_show: u32,
+}
+#[derive(Debug)]
+enum HideAndShowEnum {
+    #[cfg(never)]
+    AlwaysHide,
+    #[cfg(not(never))]
+    AlwaysShow{
+        #[cfg(never)]
+        always_hide: u32,
+        #[cfg(not(never))]
+        always_show: u32,
+    }
+}
+
+impl < > $crate::fmt::Debug for HideAndShow< > where {
+    fn fmt(&self , f: &mut $crate::fmt::Formatter) -> $crate::fmt::Result {
+        match self {
+            HideAndShow {
+                always_show: always_show,
+            }
+            =>f.debug_struct("HideAndShow").field("always_show", &always_show).finish()
+        }
+    }
+}
+impl < > $crate::fmt::Debug for HideAndShowEnum< > where {
+    fn fmt(&self , f: &mut $crate::fmt::Formatter) -> $crate::fmt::Result {
+        match self {
+            HideAndShowEnum::AlwaysShow {
+                always_show: always_show,
+            }
+            =>f.debug_struct("AlwaysShow").field("always_show", &always_show).finish(),
+        }
+    }
+}"#]],
+    );
+}
+#[test]
+fn test_default_expand_with_cfg() {
+    check(
+        r#"
+//- minicore: derive, default
+#[derive(Default)]
+struct Foo {
+    field1: i32,
+    #[cfg(never)]
+    field2: (),
+}
+#[derive(Default)]
+enum Bar {
+    Foo,
+    #[cfg_attr(not(never), default)]
+    Bar,
+}
+"#,
+        expect![[r#"
+#[derive(Default)]
+struct Foo {
+    field1: i32,
+    #[cfg(never)]
+    field2: (),
+}
+#[derive(Default)]
+enum Bar {
+    Foo,
+    #[cfg_attr(not(never), default)]
+    Bar,
+}
+
+impl < > $crate::default::Default for Foo< > where {
+    fn default() -> Self {
+        Foo {
+            field1: $crate::default::Default::default(),
+        }
+    }
+}
+impl < > $crate::default::Default for Bar< > where {
+    fn default() -> Self {
+        Bar::Bar
+    }
+}"#]],
+    );
+}