fix: Restrict fill_internal_slices pass to acir functions

compiler/noirc_evaluator/src/ssa/opt/fill_internal_slices.rs

@@ -46,7 +46,7 @@ use crate::ssa::{
     ir::{
         basic_block::BasicBlockId,
         dfg::CallStack,
-        function::Function,
+        function::{Function, RuntimeType},
         function_inserter::FunctionInserter,
         instruction::{Instruction, InstructionId, Intrinsic},
         post_order::PostOrder,
@@ -62,8 +62,14 @@ use fxhash::FxHashMap as HashMap;
 impl Ssa {
     pub(crate) fn fill_internal_slices(mut self) -> Ssa {
         for function in self.functions.values_mut() {
-            let mut context = Context::new(function);
-            context.process_blocks();
+            // This pass is only necessary for generating ACIR and thus we should not
+            // process Brillig functions.
+            // The pass is also currently only setup to handle a function with a single flattened block.
+            // For complex Brillig functions we can expect this pass to panic.
+            if function.runtime() == RuntimeType::Acir {
+                let mut context = Context::new(function);
+                context.process_blocks();
+            }
         }
         self
     }

test_programs/execution_success/brillig_set_slice_of_slice/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "brillig_set_slice_of_slice"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.19.4"
+
+[dependencies]

test_programs/execution_success/brillig_set_slice_of_slice/src/main.nr

@@ -0,0 +1,51 @@
+struct Property
+{
+    key : [u8],
+    value : [u8],
+}
+
+struct JSON
+{
+    doc : [Property]
+}
+
+unconstrained fn slice_eq(self: [u8], other: [u8]) -> bool {
+    let mut equal = true;
+    for i in 0..self.len() {
+        if self[i] != other[i] {
+            equal = false;
+        }
+    }
+    equal
+}
+
+// This test acts a regression for issue #3476
+unconstrained fn main() {
+    let mut json = JSON { doc: [] };
+    let mut prop = Property { key: [], value:[] };
+
+    let other_prop = Property { key: [0, 1, 2], value:[10] };
+    json.doc = json.doc.push_back(other_prop);
+
+    for i in 0..3 {
+        prop.key = prop.key.push_back(i as u8);
+    }
+    prop.value = prop.value.push_back(5);
+
+    // add property to json or replace existing
+    let len : Field = json.doc.len();
+    let mut found = false;
+    for i in 0..len 
+    { 
+        if (!found) 
+        { 
+            if (slice_eq(prop.key, json.doc[i].key))
+            { 
+                json.doc[i].value = prop.value; 
+                found = true; 
+            } 
+        } 
+    }
+    assert(found == true);
+    assert(json.doc[0].value[0] == 5);
+}