fix: allow empty loop headers (#6736)

Co-authored-by: jfecher <jake@aztecprotocol.com>
Co-authored-by: Akosh Farkash <aakoshh@gmail.com>
Co-authored-by: Tom French <tom@tomfren.ch>

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

@@ -302,6 +302,11 @@ impl Loop {
     pub(super) fn get_const_upper_bound(&self, function: &Function) -> Option<FieldElement> {
         let block = &function.dfg[self.header];
         let instructions = block.instructions();
+        if instructions.is_empty() {
+            // If the loop condition is constant time, the loop header will be
+            // simplified to a simple jump.
+            return None;
+        }
         assert_eq!(
             instructions.len(),
             1,

test_programs/execution_success/regression_6734/Nargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "regression_6734"
+type = "bin"
+authors = [""]
+
+[dependencies]

test_programs/execution_success/regression_6734/src/main.nr

@@ -0,0 +1,24 @@
+// https://github.com/noir-lang/noir/issues/6734
+
+pub fn sub_array_extended<let SRC_LEN: u32, let DST_LEN: u32>(
+    src: [Field; SRC_LEN],
+    offset: u32,
+) -> [Field; DST_LEN] {
+    let available_elements_to_copy = SRC_LEN - offset;
+    let elements_to_copy = if DST_LEN > available_elements_to_copy {
+        available_elements_to_copy
+    } else {
+        DST_LEN
+    };
+
+    let mut dst: [Field; DST_LEN] = std::mem::zeroed();
+    for i in 0..elements_to_copy {
+        dst[i] = src[i + offset];
+    }
+
+    dst
+}
+
+unconstrained fn main() {
+    assert_eq(sub_array_extended([], 0), []);
+}