feat(ssa): Merge slices in if statements with witness conditions (#2347)

Co-authored-by: jfecher <jake@aztecprotocol.com>

crates/nargo/src/ops/foreign_calls.rs

@@ -1,5 +1,5 @@
 use acvm::{
-    acir::brillig::{ForeignCallResult, Value},
+    acir::brillig::{ForeignCallOutput, ForeignCallResult, Value},
     pwg::ForeignCallWaitInfo,
 };
 use iter_extended::vecmap;
@@ -54,13 +54,25 @@ impl ForeignCall {
             }
             Some(ForeignCall::Sequence) => {
                 let sequence_length: u128 = foreign_call.inputs[0][0].to_field().to_u128();
-
-                Ok(vecmap(0..sequence_length, Value::from).into())
+                let sequence = vecmap(0..sequence_length, Value::from);
+
+                Ok(ForeignCallResult {
+                    values: vec![
+                        ForeignCallOutput::Single(sequence_length.into()),
+                        ForeignCallOutput::Array(sequence),
+                    ],
+                })
             }
             Some(ForeignCall::ReverseSequence) => {
                 let sequence_length: u128 = foreign_call.inputs[0][0].to_field().to_u128();
-
-                Ok(vecmap((0..sequence_length).rev(), Value::from).into())
+                let sequence = vecmap((0..sequence_length).rev(), Value::from);
+
+                Ok(ForeignCallResult {
+                    values: vec![
+                        ForeignCallOutput::Single(sequence_length.into()),
+                        ForeignCallOutput::Array(sequence),
+                    ],
+                })
             }
             None => panic!("unexpected foreign call {:?}", foreign_call_name),
         }

crates/nargo_cli/tests/compile_failure/slice_access_failure/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "slice_access_failure"
+type = "bin"
+authors = [""]
+compiler_version = "0.10.2"
+
+[dependencies]

crates/nargo_cli/tests/compile_failure/slice_access_failure/Prover.toml

@@ -0,0 +1,2 @@
+x = "5"
+y = "10"

crates/nargo_cli/tests/compile_failure/slice_access_failure/src/main.nr

@@ -0,0 +1,13 @@
+fn main(x : Field, y : pub Field) {
+    let mut slice = [0; 2];
+    if x == y {
+        slice = slice.push_back(y);
+        slice = slice.push_back(x);
+    } else {
+        slice = slice.push_back(x);
+    }
+    // This constraint should fail as the slice length is 3 and the index is 3
+    // The right hand side AND case ensures that the circuit inputs have not changed
+    // and we always hit the else case in the if statement above.
+    assert((slice[3] == 0) & (slice[2] != y));
+}

crates/nargo_cli/tests/execution_success/brillig_slices/src/main.nr

@@ -68,9 +68,85 @@ unconstrained fn main(x: Field, y: Field) {
     let (empty_slice, should_be_x) = slice.remove(0);
     assert(should_be_x == x);
     assert(empty_slice.len() == 0);
+
+    regression_merge_slices(x, y);
 }
 
 // Tests slice passing to/from functions
 unconstrained fn push_front_to_slice<T>(slice: [T], item: T) -> [T] {
     slice.push_front(item)
 }
+
+// The parameters to this function must come from witness values (inputs to main)
+unconstrained fn regression_merge_slices(x: Field, y: Field) {
+    merge_slices_if(x, y);
+    merge_slices_else(x);
+}
+
+unconstrained fn merge_slices_if(x: Field, y: Field) {
+    let slice = merge_slices_return(x, y);
+    assert(slice[2] == 10);
+    assert(slice.len() == 3);
+
+    let slice = merge_slices_mutate(x, y);
+    assert(slice[3] == 5);
+    assert(slice.len() == 4);
+
+    let slice = merge_slices_mutate_in_loop(x, y);
+    assert(slice[6] == 4);
+    assert(slice.len() == 7);
+}
+
+unconstrained fn merge_slices_else(x: Field) {
+    let slice = merge_slices_return(x, 5);
+    assert(slice[0] == 0);
+    assert(slice[1] == 0);
+    assert(slice.len() == 2);
+
+    let slice = merge_slices_mutate(x, 5);
+    assert(slice[2] == 5);
+    assert(slice.len() == 3);
+
+    let slice = merge_slices_mutate_in_loop(x, 5);
+    assert(slice[2] == 5);
+    assert(slice.len() == 3);
+}
+
+// Test returning a merged slice without a mutation
+unconstrained fn merge_slices_return(x: Field, y: Field) -> [Field] {
+    let slice = [0; 2];
+    if x != y {
+        if x != 20 {
+            slice.push_back(y)
+        } else {
+            slice
+        }
+    } else {
+        slice
+    }
+}
+
+// Test mutating a slice inside of an if statement
+unconstrained fn merge_slices_mutate(x: Field, y: Field) -> [Field] {
+    let mut slice = [0; 2];
+    if x != y {
+        slice = slice.push_back(y);
+        slice = slice.push_back(x);
+    } else {
+        slice = slice.push_back(x);
+    }
+    slice
+}
+
+// Test mutating a slice inside of a loop in an if statement
+unconstrained fn merge_slices_mutate_in_loop(x: Field, y: Field) -> [Field] {
+    let mut slice = [0; 2];
+    if x != y {
+        for i in 0..5 {
+            slice = slice.push_back(i);
+        }
+    } else {
+        slice = slice.push_back(x);
+    }
+    slice
+}

crates/nargo_cli/tests/execution_success/brillig_to_le_bytes/src/main.nr

@@ -4,7 +4,6 @@ unconstrained fn main(x : Field) -> pub [u8; 31] {
     // The result of this byte array will be little-endian
     let byte_array = x.to_le_bytes(31);
     assert(byte_array.len() == 31);
-
     let mut bytes = [0; 31];
     for i in 0..31 {
         bytes[i] = byte_array[i];

crates/nargo_cli/tests/execution_success/slices/src/main.nr

@@ -1,9 +1,6 @@
 use dep::std::slice;
 use dep::std;
 fn main(x : Field, y : pub Field) {
-    /// TODO(#1889): Using slices in if statements where the condition is a witness
-    /// is not yet supported
-
     let mut slice = [0; 2];
     assert(slice[0] == 0);
     assert(slice[0] != 1);
@@ -49,6 +46,8 @@ fn main(x : Field, y : pub Field) {
     assert(append[4] == 5);
 
     regression_2083();
+    // The parameters to this function must come from witness values (inputs to main)
+    regression_merge_slices(x, y);
 }
 
 // Ensure that slices of struct/tuple values work.
@@ -83,3 +82,77 @@ fn regression_2083() {
     assert(x.0 == 5);
     assert(x.1 == 6);
 }
+
+// The parameters to this function must come from witness values (inputs to main)
+fn regression_merge_slices(x: Field, y: Field) {
+    merge_slices_if(x, y);
+    merge_slices_else(x);
+}
+
+fn merge_slices_if(x: Field, y: Field) {
+    let slice = merge_slices_return(x, y);
+    assert(slice[2] == 10);
+    assert(slice.len() == 3);
+
+    let slice = merge_slices_mutate(x, y);
+    assert(slice[3] == 5);
+    assert(slice.len() == 4);
+
+    let slice = merge_slices_mutate_in_loop(x, y);
+    assert(slice[6] == 4);
+    assert(slice.len() == 7);
+}
+
+fn merge_slices_else(x: Field) {
+    let slice = merge_slices_return(x, 5);
+    assert(slice[0] == 0);
+    assert(slice[1] == 0);
+    assert(slice.len() == 2);
+
+    let slice = merge_slices_mutate(x, 5);
+    assert(slice[2] == 5);
+    assert(slice.len() == 3);
+
+    let slice = merge_slices_mutate_in_loop(x, 5);
+    assert(slice[2] == 5);
+    assert(slice.len() == 3);
+}
+
+// Test returning a merged slice without a mutation
+fn merge_slices_return(x: Field, y: Field) -> [Field] {
+    let slice = [0; 2];
+    if x != y {
+        if x != 20 {
+            slice.push_back(y)
+        } else {
+            slice
+        }
+    } else {
+        slice
+    }
+}
+
+// Test mutating a slice inside of an if statement
+fn merge_slices_mutate(x: Field, y: Field) -> [Field] {
+    let mut slice = [0; 2];
+    if x != y {
+        slice = slice.push_back(y);
+        slice = slice.push_back(x);
+    } else {
+        slice = slice.push_back(x);
+    }
+    slice
+}
+
+// Test mutating a slice inside of a loop in an if statement
+fn merge_slices_mutate_in_loop(x: Field, y: Field) -> [Field] {
+    let mut slice = [0; 2];
+    if x != y {
+        for i in 0..5 {
+            slice = slice.push_back(i);
+        }
+    } else {
+        slice = slice.push_back(x);
+    }
+    slice
+}