ensure generics are still properly reported on EII *implementations*, and test this

compiler/rustc_hir_analysis/messages.ftl

@@ -165,6 +165,11 @@ hir_analysis_drop_impl_reservation = reservation `Drop` impls are not supported
 hir_analysis_duplicate_precise_capture = cannot capture parameter `{$name}` twice
     .label = parameter captured again here
 
+hir_analysis_eii_with_generics =
+    `{$impl_name}` cannot have generic parameters other than lifetimes
+    .label = required by this attribute
+    .help = `#[{$eii_name}]` marks the implementation of an "externally implementable item"
+
 hir_analysis_empty_specialization = specialization impl does not specialize any associated items
     .note = impl is a specialization of this impl
 

compiler/rustc_hir_analysis/src/check/compare_eii.rs

@@ -8,8 +8,9 @@
 
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::{Applicability, E0806, struct_span_code_err};
+use rustc_hir::attrs::{AttributeKind, EiiImplResolution};
 use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_hir::{self as hir, FnSig, HirId, ItemKind};
+use rustc_hir::{self as hir, FnSig, HirId, ItemKind, find_attr};
 use rustc_infer::infer::{self, InferCtxt, TyCtxtInferExt};
 use rustc_infer::traits::{ObligationCause, ObligationCauseCode};
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
@@ -24,7 +25,7 @@
 use crate::check::compare_impl_item::{
     CheckNumberOfEarlyBoundRegionsError, check_number_of_early_bound_regions,
 };
-use crate::errors::LifetimesOrBoundsMismatchOnEii;
+use crate::errors::{EiiWithGenerics, LifetimesOrBoundsMismatchOnEii};
 
 /// Checks whether the signature of some `external_impl`, matches
 /// the signature of `declaration`, which it is supposed to be compatible
@@ -154,11 +155,44 @@ fn check_is_structurally_compatible<'tcx>(
     eii_name: Symbol,
     eii_attr_span: Span,
 ) -> Result<(), ErrorGuaranteed> {
+    check_no_generics(tcx, external_impl, declaration, eii_name, eii_attr_span)?;
     check_number_of_arguments(tcx, external_impl, declaration, eii_name, eii_attr_span)?;
     check_early_region_bounds(tcx, external_impl, declaration, eii_attr_span)?;
     Ok(())
 }
 
+/// externally implementable items can't have generics
+fn check_no_generics<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    external_impl: LocalDefId,
+    _declaration: DefId,
+    eii_name: Symbol,
+    eii_attr_span: Span,
+) -> Result<(), ErrorGuaranteed> {
+    let generics = tcx.generics_of(external_impl);
+    if generics.own_requires_monomorphization()
+        // When an EII implementation is automatically generated by the `#[eii]` macro,
+        // it will directly refer to the foreign item, not through a macro.
+        // We don't want to emit this error if it's an implementation that's generated by the `#[eii]` macro,
+        // since in that case it looks like a duplicate error: the declaration of the EII already can't contain generics.
+        // So, we check here if at least one of the eii impls has ImplResolution::Macro, which indicates it's
+        // not generated as part of the declaration.
+        && find_attr!(
+            tcx.get_all_attrs(external_impl),
+            AttributeKind::EiiImpls(impls) if impls.iter().any(|i| matches!(i.resolution, EiiImplResolution::Macro(_)))
+        )
+    {
+        tcx.dcx().emit_err(EiiWithGenerics {
+            span: tcx.def_span(external_impl),
+            attr: eii_attr_span,
+            eii_name,
+            impl_name: tcx.item_name(external_impl),
+        });
+    }
+
+    Ok(())
+}
+
 fn check_early_region_bounds<'tcx>(
     tcx: TyCtxt<'tcx>,
     external_impl: LocalDefId,

compiler/rustc_hir_analysis/src/errors.rs

@@ -1652,3 +1652,15 @@ pub(crate) struct LifetimesOrBoundsMismatchOnEii {
     pub bounds_span: Vec<Span>,
     pub ident: Symbol,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_eii_with_generics)]
+#[help]
+pub(crate) struct EiiWithGenerics {
+    #[primary_span]
+    pub span: Span,
+    #[label]
+    pub attr: Span,
+    pub eii_name: Symbol,
+    pub impl_name: Symbol,
+}

tests/ui/eii/type_checking/generic_implementation.rs

@@ -0,0 +1,13 @@
+//@ check-fail
+// Check that type parameters on EIIs are properly rejected.
+// Specifically a regression test for https://github.com/rust-lang/rust/issues/149983.
+#![feature(extern_item_impls)]
+
+#[eii]
+fn foo();
+
+#[foo]
+fn foo_impl<T>() {}
+//~^ ERROR `foo_impl` cannot have generic parameters other than lifetimes
+
+fn main() {}

tests/ui/eii/type_checking/generic_implementation.stderr

@@ -0,0 +1,12 @@
+error: `foo_impl` cannot have generic parameters other than lifetimes
+  --> $DIR/generic_implementation.rs:10:1
+   |
+LL | #[foo]
+   | ------ required by this attribute
+LL | fn foo_impl<T>() {}
+   | ^^^^^^^^^^^^^^^^
+   |
+   = help: `#[foo]` marks the implementation of an "externally implementable item"
+
+error: aborting due to 1 previous error
+