Support defaults for static EIIs

rust-lang · jhpratt · May 27, 2026 · May 31, 2026 · Jun 1, 2026 · Jun 2, 2026
commit 1e125ceba9a27466b10ed5584b8b0d0a3abc2b19
diff --git a/compiler/rustc_builtin_macros/src/diagnostics.rs b/compiler/rustc_builtin_macros/src/diagnostics.rs
@@ -1134,14 +1134,6 @@ pub(crate) struct EiiStaticMultipleImplementations {
     pub span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag("`#[{$name}]` cannot be used on statics with a value")]
-pub(crate) struct EiiStaticDefault {
-    #[primary_span]
-    pub span: Span,
-    pub name: String,
-}
-
 #[derive(Diagnostic)]
 #[diag("`#[{$name}]` requires the name as an explicit argument when used on a static")]
 pub(crate) struct EiiStaticArgumentRequired {

diff --git a/compiler/rustc_builtin_macros/src/eii.rs b/compiler/rustc_builtin_macros/src/eii.rs
@@ -13,7 +13,7 @@ use crate::diagnostics::{
     EiiAttributeNotSupported, EiiExternTargetExpectedList, EiiExternTargetExpectedMacro,
     EiiExternTargetExpectedUnsafe, EiiMacroExpectedMaxOneArgument, EiiOnlyOnce,
     EiiSharedMacroInStatementPosition, EiiSharedMacroTarget, EiiStaticArgumentRequired,
-    EiiStaticDefault, EiiStaticMultipleImplementations, EiiStaticMutable,
+    EiiStaticMultipleImplementations, EiiStaticMutable,
 };
 
 /// ```rust
@@ -86,14 +86,6 @@ fn eii_(
     let (item_span, foreign_item_name) = match kind {
         ItemKind::Fn(func) => (func.sig.span, func.ident),
         ItemKind::Static(stat) => {
-            // Statics with a default are not supported yet
-            if let Some(stat_body) = &stat.expr {
-                ecx.dcx().emit_err(EiiStaticDefault {
-                    span: stat_body.span,
-                    name: path_to_string(&meta_item.path),
-                });
-                return vec![];
-            }
             // Statics must have an explicit name for the eii
             if meta_item.is_word() {
                 ecx.dcx().emit_err(EiiStaticArgumentRequired {
@@ -139,19 +131,17 @@ fn eii_(
 
     let mut module_items = Vec::new();
 
-    if let ItemKind::Fn(func) = kind
-        && func.body.is_some()
-    {
-        module_items.push(generate_default_func_impl(
-            ecx,
-            &func,
-            impl_unsafe,
-            macro_name,
-            eii_attr_span,
-            item_span,
-            foreign_item_name,
-            default_func_attrs,
-        ))
+    if let Some(default_impl) = generate_default_impl(
+        ecx,
+        kind,
+        impl_unsafe,
+        macro_name,
+        eii_attr_span,
+        item_span,
+        foreign_item_name,
+        default_func_attrs,
+    ) {
+        module_items.push(default_impl);
     }
 
     module_items.push(generate_foreign_item(
@@ -266,18 +256,31 @@ fn filter_attrs_for_multiple_eii_attr(
         .collect()
 }
 
-fn generate_default_func_impl(
+fn generate_default_impl(
     ecx: &mut ExtCtxt<'_>,
-    func: &ast::Fn,
+    item_kind: &ItemKind,
     impl_unsafe: bool,
     macro_name: Ident,
     eii_attr_span: Span,
     item_span: Span,
     foreign_item_name: Ident,
     attrs: ThinVec<Attribute>,
-) -> Box<ast::Item> {
-    let mut default_func = func.clone();
-    default_func.eii_impls.push(EiiImpl {
+) -> Option<Box<ast::Item>> {
+    match item_kind {
+        ItemKind::Fn(func) => {
+            if func.body.is_none() {
+                return None;
+            }
+        }
+        ItemKind::Static(stat) => {
+            if stat.expr.is_none() {
+                return None;
+            }
+        }
+        _ => unreachable!("Target was checked earlier"),
+    };
+
+    let eii_impl = EiiImpl {
         node_id: DUMMY_NODE_ID,
         inner_span: macro_name.span,
         eii_macro_path: ast::Path::from_ident(macro_name),
@@ -297,7 +300,18 @@ fn generate_default_func_impl(
             ),
             impl_unsafe,
         }),
-    });
+    };
+
+    let mut item_kind = item_kind.clone();
+    match &mut item_kind {
+        ItemKind::Fn(func) => {
+            func.eii_impls.push(eii_impl);
+        }
+        ItemKind::Static(stat) => {
+            stat.eii_impls.push(eii_impl);
+        }
+        _ => unreachable!("Target was checked earlier"),
+    };
 
     let anon_mod = |span: Span, stmts: ThinVec<ast::Stmt>| {
         let unit = ecx.ty(item_span, ast::TyKind::Tup(ThinVec::new()));
@@ -311,15 +325,12 @@ fn generate_default_func_impl(
     };
 
     // const _: () = {
-    //     <orig fn>
+    //     <orig item>
     // }
-    anon_mod(
+    Some(anon_mod(
         item_span,
-        thin_vec![ecx.stmt_item(
-            item_span,
-            ecx.item(item_span, attrs, ItemKind::Fn(Box::new(default_func)))
-        ),],
-    )
+        thin_vec![ecx.stmt_item(item_span, ecx.item(item_span, attrs, item_kind))],
+    ))
 }
 
 /// Generates a foreign item, like
@@ -405,6 +416,8 @@ fn generate_foreign_static(mut stat: Box<ast::StaticItem>) -> ast::ForeignItemKi
         stat.safety = ast::Safety::Safe(stat.ident.span);
     }
 
+    stat.expr = None;
+
     ast::ForeignItemKind::Static(stat)
 }
 

diff --git a/tests/ui/eii/static/auxiliary/decl_with_default.rs b/tests/ui/eii/static/auxiliary/decl_with_default.rs
@@ -0,0 +1,5 @@
+#![crate_type = "rlib"]
+#![feature(extern_item_impls)]
+
+#[eii(eii1)]
+pub static DECL1: u64 = 5;
diff --git a/tests/ui/eii/static/auxiliary/impl_default_override.rs b/tests/ui/eii/static/auxiliary/impl_default_override.rs
@@ -0,0 +1,8 @@
+//@ aux-build: decl_with_default.rs
+#![crate_type = "rlib"]
+#![feature(extern_item_impls)]
+
+extern crate decl_with_default as decl;
+
+#[decl::eii1]
+pub static EII1_IMPL: u64 = 10;
diff --git a/tests/ui/eii/static/default.rs b/tests/ui/eii/static/default.rs
@@ -0,0 +1,15 @@
+//@ run-pass
+//@ check-run-results
+//@ ignore-backends: gcc
+// FIXME: linking on windows (specifically mingw) not yet supported, see tracking issue #125418
+//@ ignore-windows
+// Tests static EIIs with default implementations.
+
+#![feature(extern_item_impls)]
+
+#[eii(eii1)]
+pub static DECL1: u64 = 5;
+
+fn main() {
+    println!("{DECL1}");
+}
diff --git a/tests/ui/eii/static/default.run.stdout b/tests/ui/eii/static/default.run.stdout
@@ -0,0 +1 @@
+5
diff --git a/tests/ui/eii/static/default_cross_crate.rs b/tests/ui/eii/static/default_cross_crate.rs
@@ -0,0 +1,13 @@
+//@ aux-build: decl_with_default.rs
+//@ run-pass
+//@ check-run-results
+//@ ignore-backends: gcc
+// FIXME: linking on windows (specifically mingw) not yet supported, see tracking issue #125418
+//@ ignore-windows
+// Tests that a static EII default can be used from another crate.
+
+extern crate decl_with_default;
+
+fn main() {
+    println!("{}", decl_with_default::DECL1);
+}
diff --git a/tests/ui/eii/static/default_cross_crate.run.stdout b/tests/ui/eii/static/default_cross_crate.run.stdout
@@ -0,0 +1 @@
+5
diff --git a/tests/ui/eii/static/default_cross_crate_explicit.rs b/tests/ui/eii/static/default_cross_crate_explicit.rs
@@ -0,0 +1,15 @@
+//@ aux-build: decl_with_default.rs
+//@ aux-build: impl_default_override.rs
+//@ run-pass
+//@ check-run-results
+//@ ignore-backends: gcc
+// FIXME: linking on windows (specifically mingw) not yet supported, see tracking issue #125418
+//@ ignore-windows
+// Tests that an explicit static EII implementation overrides a cross-crate default.
+
+extern crate decl_with_default;
+extern crate impl_default_override;
+
+fn main() {
+    println!("{}", decl_with_default::DECL1);
+}
diff --git a/tests/ui/eii/static/default_cross_crate_explicit.run.stdout b/tests/ui/eii/static/default_cross_crate_explicit.run.stdout
@@ -0,0 +1 @@
+10
diff --git a/tests/ui/eii/static/default_explicit.rs b/tests/ui/eii/static/default_explicit.rs
@@ -0,0 +1,19 @@
+//@ run-pass
+//@ check-run-results
+//@ ignore-backends: gcc
+// FIXME: linking on windows (specifically mingw) not yet supported, see tracking issue #125418
+//@ ignore-windows
+// Tests that an explicit static EII implementation overrides a local default.
+
+#![feature(extern_item_impls)]
+#![allow(dead_code)]
+
+#[eii(eii1)]
+pub static DECL1: u64 = 5;
+
+#[eii1]
+pub static EII1_IMPL: u64 = 10;
+
+fn main() {
+    println!("{DECL1}");
+}
diff --git a/tests/ui/eii/static/default_explicit.run.stdout b/tests/ui/eii/static/default_explicit.run.stdout
@@ -0,0 +1 @@
+10