feat: add diagnostic for E0596

crates/hir-ty/src/infer.rs

@@ -405,6 +405,10 @@ pub enum InferenceDiagnostic {
         expr: ExprId,
         found: StoredTy,
     },
+    MutRefInImmRefPat {
+        #[type_visitable(ignore)]
+        pat: PatId,
+    },
     CannotImplicitlyDerefTraitObject {
         #[type_visitable(ignore)]
         pat: PatId,

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

@@ -916,7 +916,7 @@ impl<'a, 'db> InferenceContext<'a, 'db> {
         if matches!(bm.0, ByRef::Yes(Mutability::Mut))
             && let MutblCap::WeaklyNot = pat_info.max_ref_mutbl
         {
-            // FIXME: Emit an error: cannot borrow as mutable inside an `&` pattern.
+            self.push_diagnostic(InferenceDiagnostic::MutRefInImmRefPat { pat });
         }
 
         // ...and store it in a side table:

crates/hir/src/diagnostics.rs

@@ -133,6 +133,7 @@ diagnostics![AnyDiagnostic<'db> ->
     MissingMatchArms,
     MissingUnsafe,
     MovedOutOfRef<'db>,
+    MutRefInImmRefPat,
     MutableRefBinding,
     NeedMut,
     NonExhaustiveLet,
@@ -335,6 +336,11 @@ pub struct CannotBeDereferenced<'db> {
     pub found: Type<'db>,
 }
 
+#[derive(Debug)]
+pub struct MutRefInImmRefPat {
+    pub pat: InFile<ExprOrPatPtr>,
+}
+
 #[derive(Debug)]
 pub struct CannotImplicitlyDerefTraitObject<'db> {
     pub pat: InFile<ExprOrPatPtr>,
@@ -980,6 +986,10 @@ impl<'db> AnyDiagnostic<'db> {
                 let expr = expr_syntax(*expr)?;
                 CannotBeDereferenced { expr, found: new_ty(found.as_ref()) }.into()
             }
+            InferenceDiagnostic::MutRefInImmRefPat { pat } => {
+                let pat = pat_syntax(*pat)?.map(Into::into);
+                MutRefInImmRefPat { pat }.into()
+            }
             InferenceDiagnostic::CannotImplicitlyDerefTraitObject { pat, found } => {
                 let pat = pat_syntax(*pat)?.map(Into::into);
                 CannotImplicitlyDerefTraitObject { pat, found: new_ty(found.as_ref()) }.into()

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

@@ -0,0 +1,37 @@
+use crate::{Diagnostic, DiagnosticCode, DiagnosticsContext};
+
+// Diagnostic: mut-ref-in-imm-ref-pat
+//
+// This diagnostic is triggered when a binding tries to mutably borrow through
+// an `&` pattern.
+pub(crate) fn mut_ref_in_imm_ref_pat(
+    ctx: &DiagnosticsContext<'_, '_>,
+    d: &hir::MutRefInImmRefPat,
+) -> Diagnostic {
+    Diagnostic::new_with_syntax_node_ptr(
+        ctx,
+        DiagnosticCode::RustcHardError("E0596"),
+        "cannot borrow as mutable inside an `&` pattern",
+        d.pat.map(Into::into),
+    )
+    .stable()
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::check_diagnostics;
+
+    #[test]
+    fn mut_ref_in_imm_ref_pat() {
+        check_diagnostics(
+            r#"
+#![feature(ref_pat_eat_one_layer_2024)]
+
+fn main() {
+    let &ref mut _x = &mut 0;
+       //^^^^^^^^^^ error: cannot borrow as mutable inside an `&` pattern
+}
+"#,
+        );
+    }
+}

crates/ide-diagnostics/src/lib.rs

@@ -64,6 +64,7 @@ mod handlers {
     pub(crate) mod missing_match_arms;
     pub(crate) mod missing_unsafe;
     pub(crate) mod moved_out_of_ref;
+    pub(crate) mod mut_ref_in_imm_ref_pat;
     pub(crate) mod mutability_errors;
     pub(crate) mod mutable_ref;
     pub(crate) mod no_such_field;
@@ -477,6 +478,7 @@ 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::MutRefInImmRefPat(d) => handlers::mut_ref_in_imm_ref_pat::mut_ref_in_imm_ref_pat(&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,