chore: Remove nested slices SSA and ACIR codegen (#4021)

# Description

## Problem\*

Resolves #4019 

## Summary\*

This removes the relevant code introduced for nested slices. This may be
temporarily removed, but ultimately isn't too bad of a change as most of
the code is contained to the fill internal slices pass with some extra
logic in ACIR gen.

## Additional Context

See issue for extra context.

## Documentation\*

Check one:
- [X] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[Exceptional Case]** Documentation to be submitted in a separate
PR.

# PR Checklist\*

- [X] I have tested the changes locally.
- [X] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

compiler/noirc_evaluator/src/ssa.rs

@@ -45,7 +45,7 @@ pub(crate) fn optimize_into_acir(
 
     let ssa_gen_span = span!(Level::TRACE, "ssa_generation");
     let ssa_gen_span_guard = ssa_gen_span.enter();
-    let ssa_builder = SsaBuilder::new(program, print_ssa_passes)?
+    let ssa = SsaBuilder::new(program, print_ssa_passes)?
         .run_pass(Ssa::defunctionalize, "After Defunctionalization:")
         .run_pass(Ssa::inline_functions, "After Inlining:")
         // Run mem2reg with the CFG separated into blocks
@@ -62,16 +62,11 @@ pub(crate) fn optimize_into_acir(
         // Run mem2reg once more with the flattened CFG to catch any remaining loads/stores
         .run_pass(Ssa::mem2reg, "After Mem2Reg:")
         .run_pass(Ssa::fold_constants, "After Constant Folding:")
-        .run_pass(Ssa::dead_instruction_elimination, "After Dead Instruction Elimination:");
+        .run_pass(Ssa::dead_instruction_elimination, "After Dead Instruction Elimination:")
+        .finish();
 
-    let brillig = ssa_builder.to_brillig(print_brillig_trace);
+    let brillig = ssa.to_brillig(print_brillig_trace);
 
-    // Split off any passes the are not necessary for Brillig generation but are necessary for ACIR generation.
-    // We only need to fill out nested slices as we need to have a known length when dealing with memory operations
-    // in ACIR gen while this is not necessary in the Brillig IR.
-    let ssa = ssa_builder
-        .run_pass(Ssa::fill_internal_slices, "After Fill Internal Slice Dummy Data:")
-        .finish();
     drop(ssa_gen_span_guard);
 
     let last_array_uses = ssa.find_last_array_uses();

compiler/noirc_evaluator/src/ssa/ir/types.rs

@@ -139,7 +139,7 @@ impl Type {
     }
 
     pub(crate) fn is_nested_slice(&self) -> bool {
-        if let Type::Slice(element_types) = self {
+        if let Type::Slice(element_types) | Type::Array(element_types, _) = self {
             element_types.as_ref().iter().any(|typ| typ.contains_slice_element())
         } else {
             false

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

@@ -8,7 +8,6 @@ mod assert_constant;
 mod constant_folding;
 mod defunctionalize;
 mod die;
-mod fill_internal_slices;
 pub(crate) mod flatten_cfg;
 mod inlining;
 mod mem2reg;

test_programs/execution_success/nested_slice_dynamic/Nargo.toml → test_programs/execution_success/nested_array_in_slice/Nargo.toml

@@ -1,5 +1,5 @@
 [package]
-name = "nested_slice_dynamic"
+name = "nested_array_in_slice"
 type = "bin"
 authors = [""]
 [dependencies]