No limit on the depth of the return path analysis (#6435)

## Description

Fixes #6369.

The return path analysis performs a breadth-first traversal of the
control flow graph to determine whether every path returns a value.

So far the traversal has been limited to 50 iterations of the outer
while loop, meaning that if the graph has a depth of more than 50 there
are paths that would not be checked. The limit was introduced to ensure
that that the analysis terminated.

This PR removes this limit. Termination is ensured by keeping track of
which nodes have already been visited so that they are not visited
again.


## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [ ] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

---------

Co-authored-by: Sophie Dankel <47993817+sdankel@users.noreply.github.com>

sway-core/src/control_flow_analysis/analyze_return_paths.rs

@@ -81,14 +81,14 @@ impl<'cfg> ControlFlowGraph<'cfg> {
         return_ty: &TypeInfo,
     ) -> Vec<CompileError> {
         let mut rovers = vec![entry_point];
+        let mut visited = vec![];
         let mut errors = vec![];
-        let mut max_iterations = 50;
-        while !rovers.is_empty() && rovers[0] != exit_point && max_iterations > 0 {
-            max_iterations -= 1;
+        while !rovers.is_empty() && rovers[0] != exit_point {
             rovers.retain(|idx| *idx != exit_point);
             let mut next_rovers = vec![];
             let mut last_discovered_span;
             for rover in rovers {
+                visited.push(rover);
                 last_discovered_span = match &self.graph[rover] {
                     ControlFlowGraphNode::ProgramNode { node, .. } => Some(node.span.clone()),
                     ControlFlowGraphNode::MethodDeclaration { span, .. } => Some(span.clone()),
@@ -122,6 +122,7 @@ impl<'cfg> ControlFlowGraph<'cfg> {
                 }
                 next_rovers.append(&mut neighbors);
             }
+            next_rovers.retain(|idx| !visited.contains(idx));
             rovers = next_rovers;
         }
 

test/src/e2e_vm_tests/test_programs/should_fail/return_path_analysis/Forc.lock

@@ -0,0 +1,16 @@
+[[package]]
+name = "core"
+source = "path+from-root-EA042F5351E6BA99"
+
+[[package]]
+name = "return_path_analysis"
+source = "member"
+dependencies = [
+    "core",
+    "std",
+]
+
+[[package]]
+name = "std"
+source = "path+from-root-EA042F5351E6BA99"
+dependencies = ["core"]

test/src/e2e_vm_tests/test_programs/should_fail/return_path_analysis/Forc.toml

@@ -0,0 +1,9 @@
+[project]
+authors = ["Fuel Labs <contact@fuel.sh>"]
+entry = "main.sw"
+license = "Apache-2.0"
+name = "return_path_analysis"
+
+[dependencies]
+core = { path = "../../../../../../sway-lib-core" }
+std = { path = "../../../reduced_std_libs/sway-lib-std-assert" }

test/src/e2e_vm_tests/test_programs/should_fail/return_path_analysis/json_abi_oracle.json

@@ -0,0 +1 @@
+[]