feat(diagnostics): mut_ref binding feature diagnostic

crates/hir-ty/src/infer.rs

@@ -482,6 +482,10 @@ pub enum InferenceDiagnostic {
         #[type_visitable(ignore)]
         kind: ExplicitDropMethodUseKind,
     },
+    MutableRefBinding {
+        #[type_visitable(ignore)]
+        pat: PatId,
+    },
 }
 
 #[derive(Debug, PartialEq, Eq, Clone)]

crates/hir-ty/src/infer/pat.rs

@@ -893,14 +893,14 @@ impl<'a, 'db> InferenceContext<'a, 'db> {
         let user_bind_annot = BindingMode::from_annotation(binding_data.mode);
         let bm = match user_bind_annot {
             BindingMode(ByRef::No, Mutability::Mut) if let ByRef::Yes(_) = def_br => {
-                // Only mention the experimental `mut_ref` feature if if we're in edition 2024 and
+                // Only mention the experimental `mut_ref` feature if we're in edition 2024 and
                 // using other experimental matching features compatible with it.
                 if self.edition.at_least_2024()
                     && (self.features.ref_pat_eat_one_layer_2024
                         || self.features.ref_pat_eat_one_layer_2024_structural)
                 {
                     if !self.features.mut_ref {
-                        // FIXME: Emit an error: binding cannot be both mutable and by-reference.
+                        self.push_diagnostic(InferenceDiagnostic::MutableRefBinding { pat });
                     }
 
                     BindingMode(def_br, Mutability::Mut)

crates/hir/src/diagnostics.rs

@@ -131,6 +131,7 @@ diagnostics![AnyDiagnostic<'db> ->
     MissingMatchArms,
     MissingUnsafe,
     MovedOutOfRef<'db>,
+    MutableRefBinding,
     NeedMut,
     NonExhaustiveLet,
     NonExhaustiveRecordExpr,
@@ -633,6 +634,11 @@ pub struct UnimplementedTrait<'db> {
     pub root_trait_predicate: Option<crate::TraitPredicate<'db>>,
 }
 
+#[derive(Debug)]
+pub struct MutableRefBinding {
+    pub pat: InFile<ExprOrPatPtr>,
+}
+
 impl<'db> AnyDiagnostic<'db> {
     pub(crate) fn body_validation_diagnostic(
         db: &'db dyn HirDatabase,
@@ -1070,6 +1076,10 @@ impl<'db> AnyDiagnostic<'db> {
                 };
                 ExplicitDropMethodUse { expr_or_path }.into()
             }
+            InferenceDiagnostic::MutableRefBinding { pat } => {
+                let pat = pat_syntax(*pat)?.map(Into::into);
+                MutableRefBinding { pat }.into()
+            }
         })
     }
 

crates/ide-diagnostics/src/handlers/mutable_ref.rs

@@ -0,0 +1,64 @@
+use crate::{Diagnostic, DiagnosticCode, DiagnosticsContext};
+
+// Diagnostic: mutable-ref
+//
+// This diagnostic is triggered when binding is taken that is both mutable and by-reference.
+pub(crate) fn mutable_ref_binding(
+    ctx: &DiagnosticsContext<'_, '_>,
+    d: &hir::MutableRefBinding,
+) -> Diagnostic {
+    Diagnostic::new_with_syntax_node_ptr(
+        ctx,
+        DiagnosticCode::RustcHardError("E0658"),
+        "bindings cannot be both mutable and by-reference by default in 2024 edition. add experimental #![feature(mut_ref)] for this functionality",
+        d.pat.map(Into::into),
+    )
+    .stable()
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::check_diagnostics;
+
+    #[test]
+    fn mutable_ref_binding_missing_feature() {
+        check_diagnostics(
+            r#"
+//- minicore: option
+#![feature(ref_pat_eat_one_layer_2024)]
+struct TestStruct {
+   val: i32
+}
+fn main() {
+    let opt_ref = &Some(TestStruct {val: 1});
+
+    if let Some(mut x) = opt_ref {
+              //^^^^^ error: bindings cannot be both mutable and by-reference by default in 2024 edition. add experimental #![feature(mut_ref)] for this functionality
+        x = &TestStruct{val: 5};
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn mutable_ref_binding_with_feature() {
+        check_diagnostics(
+            r#"
+//- minicore: option
+#![feature(ref_pat_eat_one_layer_2024)]
+#![feature(mut_ref)]
+struct TestStruct {
+    val: i32
+}
+fn main() {
+    let opt_ref = &Some(TestStruct{val: 1});
+
+    if let Some(mut x) = opt_ref {
+        x = &TestStruct{val: 5};
+    }
+}
+"#,
+        );
+    }
+}

crates/ide-diagnostics/src/lib.rs

@@ -62,6 +62,7 @@ mod handlers {
     pub(crate) mod missing_unsafe;
     pub(crate) mod moved_out_of_ref;
     pub(crate) mod mutability_errors;
+    pub(crate) mod mutable_ref;
     pub(crate) mod no_such_field;
     pub(crate) mod non_exhaustive_let;
     pub(crate) mod non_exhaustive_record_expr;
@@ -466,6 +467,7 @@ pub fn semantic_diagnostics(
             AnyDiagnostic::MissingMatchArms(d) => handlers::missing_match_arms::missing_match_arms(&ctx, &d),
             AnyDiagnostic::MissingUnsafe(d) => handlers::missing_unsafe::missing_unsafe(&ctx, &d),
             AnyDiagnostic::MovedOutOfRef(d) => handlers::moved_out_of_ref::moved_out_of_ref(&ctx, &d),
+            AnyDiagnostic::MutableRefBinding(d) => handlers::mutable_ref::mutable_ref_binding(&ctx, &d),
             AnyDiagnostic::NeedMut(d) => match handlers::mutability_errors::need_mut(&ctx, &d) {
                 Some(it) => it,
                 None => continue,