DCA now detects dead code when all branches return explicitly. (FuelL…

…abs#4169)

Fixing `If` was straightforward. But that didn't automatically the fix
`match` scenario (see test case). Turns out that `match` always has a
covering "ImplicitReturn" expression, and with implicit returns in DCA,
we add an edge from the beginning to the end, which I couldn't figure
out why. Just removing that got it working. I request the reviewers to
validate this part of the change carefully.

Closes FuelLabs#2313

## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [ ] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [ ] 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.

sway-core/src/control_flow_analysis/dead_code_analysis.rs

@@ -230,9 +230,6 @@ fn connect_node<'eng: 'cfg, 'cfg>(
                 options,
             )?;
 
-            for leaf in return_contents.clone() {
-                graph.add_edge(this_index, leaf, "".into());
-            }
             // connect return to the exit node
             if let Some(exit_node) = exit_node {
                 graph.add_edge(this_index, exit_node, "return".into());
@@ -1040,7 +1037,7 @@ fn connect_expression<'eng: 'cfg, 'cfg>(
                 engines,
                 &then.expression,
                 graph,
-                leaves,
+                &condition_expr,
                 exit_node,
                 "then branch",
                 tree_type,
@@ -1053,18 +1050,18 @@ fn connect_expression<'eng: 'cfg, 'cfg>(
                     engines,
                     &else_expr.expression,
                     graph,
-                    leaves,
+                    &condition_expr,
                     exit_node,
                     "else branch",
                     tree_type,
                     else_expr.clone().span,
                     options,
                 )?
             } else {
-                vec![]
+                condition_expr
             };
 
-            Ok([condition_expr, then_expr, else_expr].concat())
+            Ok([then_expr, else_expr].concat())
         }
         CodeBlock(a @ ty::TyCodeBlock { .. }) => {
             connect_code_block(engines, a, graph, leaves, exit_node, tree_type, options)

test/src/e2e_vm_tests/test_programs/should_pass/dca/all_paths_return/Forc.lock

@@ -0,0 +1,8 @@
+[[package]]
+name = 'all_paths_return'
+source = 'member'
+dependencies = ['core']
+
+[[package]]
+name = 'core'
+source = 'path+from-root-F8249D11C4071B26'

test/src/e2e_vm_tests/test_programs/should_pass/dca/all_paths_return/Forc.toml

@@ -0,0 +1,8 @@
+[project]
+authors = ["Fuel Labs <[email protected]>"]
+entry = "main.sw"
+license = "Apache-2.0"
+name = "all_paths_return"
+
+[dependencies]
+core = { path = "../../../../../../../sway-lib-core" }

test/src/e2e_vm_tests/test_programs/should_pass/dca/all_paths_return/src/main.sw

@@ -0,0 +1,28 @@
+script;
+
+fn if_f() -> u64 {
+    if true {
+        return 0;
+    } else {
+        return 1;
+    }
+    // should trigger a warning
+    return 2;
+}
+
+fn match_f() -> u64 {
+   match 10 {
+     1 => return 8,
+     _ => return 3,
+   }
+   // should trigger a warning
+   return 21;
+}
+
+fn main() -> u64 {
+   if true {
+      return if_f();
+   } else {
+     return match_f();
+   }
+}

test/src/e2e_vm_tests/test_programs/should_pass/dca/all_paths_return/test.toml

@@ -0,0 +1,7 @@
+category = "compile"
+
+# check: $()return 2;
+# nextln: $()This code is unreachable.
+
+# check: $()return 21;
+# nextln: $()This code is unreachable.