Reject multiple EII implementations on one static

Author: JonathanBrouwer

compiler/rustc_builtin_macros/src/eii.rs

@@ -12,7 +12,8 @@ use thin_vec::{ThinVec, thin_vec};
 use crate::errors::{
     EiiExternTargetExpectedList, EiiExternTargetExpectedMacro, EiiExternTargetExpectedUnsafe,
     EiiMacroExpectedMaxOneArgument, EiiOnlyOnce, EiiSharedMacroInStatementPosition,
-    EiiSharedMacroTarget, EiiStaticArgumentRequired, EiiStaticDefault, EiiStaticMutable,
+    EiiSharedMacroTarget, EiiStaticArgumentRequired, EiiStaticDefault,
+    EiiStaticMultipleImplementations, EiiStaticMutable,
 };
 
 /// ```rust
@@ -512,7 +513,14 @@ pub(crate) fn eii_shared_macro(
 
     let eii_impls = match &mut i.kind {
         ItemKind::Fn(func) => &mut func.eii_impls,
-        ItemKind::Static(stat) => &mut stat.eii_impls,
+        ItemKind::Static(stat) => {
+            if !stat.eii_impls.is_empty() {
+                // Reject multiple implementations on one static item
+                // because it might be unintuitive for libraries defining statics the defined statics may alias
+                ecx.dcx().emit_err(EiiStaticMultipleImplementations { span });
+            }
+            &mut stat.eii_impls
+        }
         _ => {
             ecx.dcx()
                 .emit_err(EiiSharedMacroTarget { span, name: path_to_string(&meta_item.path) });

compiler/rustc_builtin_macros/src/errors.rs

@@ -1124,6 +1124,16 @@ pub(crate) struct EiiSharedMacroTarget {
     pub name: String,
 }
 
+#[derive(Diagnostic)]
+#[diag("static cannot implement multiple EIIs")]
+#[note(
+    "this is not allowed because multiple externally implementable statics that alias may be unintuitive"
+)]
+pub(crate) struct EiiStaticMultipleImplementations {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag("`#[{$name}]` cannot be used on statics with a value")]
 pub(crate) struct EiiStaticDefault {

tests/ui/eii/static/multiple_decls.rs

@@ -0,0 +1,11 @@
+#![feature(extern_item_impls)]
+
+const A: () = ();
+#[eii(A)]
+static A: u64;
+//~^ ERROR the name `A` is defined multiple times
+
+#[A]
+static A_IMPL: u64 = 5;
+
+fn main() {}

tests/ui/eii/static/multiple_decls.stderr

@@ -0,0 +1,14 @@
+error[E0428]: the name `A` is defined multiple times
+  --> $DIR/multiple_decls.rs:5:1
+   |
+LL | const A: () = ();
+   | ----------------- previous definition of the value `A` here
+LL | #[eii(A)]
+LL | static A: u64;
+   | ^^^^^^^^^^^^^^ `A` redefined here
+   |
+   = note: `A` must be defined only once in the value namespace of this module
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0428`.

tests/ui/eii/static/multiple_impls.rs

@@ -1,5 +1,3 @@
-//@ run-pass
-//@ check-run-results
 //@ ignore-backends: gcc
 // FIXME: linking on windows (specifically mingw) not yet supported, see tracking issue #125418
 //@ ignore-windows
@@ -14,6 +12,7 @@ static B: u64;
 
 #[a]
 #[b]
+//~^ ERROR static cannot implement multiple EIIs
 static IMPL: u64 = 5;
 
 fn main() {

tests/ui/eii/static/multiple_impls.stderr

@@ -0,0 +1,10 @@
+error: static cannot implement multiple EIIs
+  --> $DIR/multiple_impls.rs:14:1
+   |
+LL | #[b]
+   | ^^^^
+   |
+   = note: this is not allowed because multiple externally implementable statics that alias may be unintuitive
+
+error: aborting due to 1 previous error
+