fix: unknown slice lengths coming from as_slice (#4725)

# Description

## Problem\*

Resolves #4722

## Summary\*



## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

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

---------

Co-authored-by: Tom French <tom@tomfren.ch>

compiler/noirc_evaluator/src/ssa.rs

@@ -53,6 +53,7 @@ pub(crate) fn optimize_into_acir(
         .run_pass(Ssa::inline_functions, "After Inlining:")
         // Run mem2reg with the CFG separated into blocks
         .run_pass(Ssa::mem2reg, "After Mem2Reg:")
+        .run_pass(Ssa::as_slice_optimization, "After `as_slice` optimization")
         .try_run_pass(Ssa::evaluate_assert_constant, "After Assert Constant:")?
         .try_run_pass(Ssa::unroll_loops, "After Unrolling:")?
         .run_pass(Ssa::simplify_cfg, "After Simplifying:")

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

@@ -378,6 +378,30 @@ impl DataFlowGraph {
         value_id
     }
 
+    /// Replaces an instruction result with a fresh id.
+    pub(crate) fn replace_result(
+        &mut self,
+        instruction_id: InstructionId,
+        prev_value_id: ValueId,
+    ) -> ValueId {
+        let typ = self.type_of_value(prev_value_id);
+        let results = self.results.get_mut(&instruction_id).unwrap();
+        let res_position = results
+            .iter()
+            .position(|&id| id == prev_value_id)
+            .expect("Result id not found while replacing");
+
+        let value_id = self.values.insert(Value::Instruction {
+            typ,
+            position: res_position,
+            instruction: instruction_id,
+        });
+
+        // Replace the value in list of results for this instruction
+        results[res_position] = value_id;
+        value_id
+    }
+
     /// Returns the number of instructions
     /// inserted into functions.
     pub(crate) fn num_instructions(&self) -> usize {

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

@@ -0,0 +1,71 @@
+use crate::ssa::{
+    ir::{
+        function::Function,
+        instruction::{Instruction, InstructionId, Intrinsic},
+        types::Type,
+        value::Value,
+    },
+    ssa_gen::Ssa,
+};
+use fxhash::FxHashMap as HashMap;
+
+impl Ssa {
+    /// A simple SSA pass to find any calls to `Intrinsic::AsSlice` and replacing any references to the length of the
+    /// resulting slice with the length of the array from which it was generated.
+    ///
+    /// This allows the length of a slice generated from an array to be used in locations where a constant value is
+    /// necessary when the value of the array is unknown.
+    ///
+    /// Note that this pass must be placed before loop unrolling to be useful.
+    #[tracing::instrument(level = "trace", skip(self))]
+    pub(crate) fn as_slice_optimization(mut self) -> Self {
+        for func in self.functions.values_mut() {
+            let known_slice_lengths = known_slice_lengths(func);
+            replace_known_slice_lengths(func, known_slice_lengths);
+        }
+        self
+    }
+}
+
+fn known_slice_lengths(func: &Function) -> HashMap<InstructionId, usize> {
+    let mut known_slice_lengths = HashMap::default();
+    for block_id in func.reachable_blocks() {
+        let block = &func.dfg[block_id];
+        for instruction_id in block.instructions() {
+            let (target_func, arguments) = match &func.dfg[*instruction_id] {
+                Instruction::Call { func, arguments } => (func, arguments),
+                _ => continue,
+            };
+
+            match &func.dfg[*target_func] {
+                Value::Intrinsic(Intrinsic::AsSlice) => {
+                    let array_typ = func.dfg.type_of_value(arguments[0]);
+                    if let Type::Array(_, length) = array_typ {
+                        known_slice_lengths.insert(*instruction_id, length);
+                    } else {
+                        unreachable!("AsSlice called with non-array {}", array_typ);
+                    }
+                }
+                _ => continue,
+            };
+        }
+    }
+    known_slice_lengths
+}
+
+fn replace_known_slice_lengths(
+    func: &mut Function,
+    known_slice_lengths: HashMap<InstructionId, usize>,
+) {
+    known_slice_lengths.into_iter().for_each(|(instruction_id, known_length)| {
+        let call_returns = func.dfg.instruction_results(instruction_id);
+        let original_slice_length = call_returns[0];
+
+        // We won't use the new id for the original unknown length.
+        // This isn't strictly necessary as a new result will be defined the next time for which the instruction
+        // is reinserted but this avoids leaving the program in an invalid state.
+        func.dfg.replace_result(instruction_id, original_slice_length);
+        let known_length = func.dfg.make_constant(known_length.into(), Type::length_type());
+        func.dfg.set_value_from_id(original_slice_length, known_length);
+    });
+}

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

@@ -4,6 +4,7 @@
 //! simpler form until the IR only has a single function remaining with 1 block within it.
 //! Generally, these passes are also expected to minimize the final amount of instructions.
 mod array_set;
+mod as_slice_length;
 mod assert_constant;
 mod bubble_up_constrains;
 mod constant_folding;

test_programs/execution_success/array_to_slice_constant_length/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "array_to_slice_constant_length"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.26.0"
+
+[dependencies]

test_programs/execution_success/array_to_slice_constant_length/Prover.toml

@@ -0,0 +1 @@
+val = "42"

test_programs/execution_success/array_to_slice_constant_length/src/main.nr

@@ -0,0 +1,10 @@
+// Regression test for https://github.com/noir-lang/noir/issues/4722
+
+unconstrained fn return_array(val: Field) -> [Field; 1] {
+    [val; 1]
+}
+
+fn main(val: Field) {
+    let array = return_array(val);
+    assert_constant(array.as_slice().len());
+}