Skip unreachable code when early return can be statically analysed (v…

…ercel/turborepo#8890)

### Description

* tracks early returns and their conditions
* replaces unreachable code with `"TURBOPACK unreachable"`
* finds hoisted declarations in unreachable code and keeps them

### Testing Instructions

<!--
  Give a quick description of steps to test your changes.
-->

crates/turbopack-ecmascript/src/analyzer/mod.rs

@@ -3866,6 +3866,11 @@ mod tests {
                                         queue
                                             .extend(then.effects.into_iter().rev().map(|e| (i, e)));
                                     }
+                                    ConditionalKind::Else { r#else } => {
+                                        queue.extend(
+                                            r#else.effects.into_iter().rev().map(|e| (i, e)),
+                                        );
+                                    }
                                     ConditionalKind::IfElse { then, r#else }
                                     | ConditionalKind::Ternary { then, r#else } => {
                                         queue.extend(
@@ -3874,6 +3879,18 @@ mod tests {
                                         queue
                                             .extend(then.effects.into_iter().rev().map(|e| (i, e)));
                                     }
+                                    ConditionalKind::IfElseMultiple { then, r#else } => {
+                                        for then in then {
+                                            queue.extend(
+                                                then.effects.into_iter().rev().map(|e| (i, e)),
+                                            );
+                                        }
+                                        for r#else in r#else {
+                                            queue.extend(
+                                                r#else.effects.into_iter().rev().map(|e| (i, e)),
+                                            );
+                                        }
+                                    }
                                     ConditionalKind::And { expr }
                                     | ConditionalKind::Or { expr }
                                     | ConditionalKind::NullishCoalescing { expr } => {
@@ -3904,6 +3921,12 @@ mod tests {
                                     JsValue::member_call(Box::new(obj), Box::new(prop), new_args),
                                 ));
                             }
+                            Effect::Unreachable { .. } => {
+                                resolved.push((
+                                    format!("{parent} -> {i} unreachable"),
+                                    JsValue::unknown_empty(true, "unreachable"),
+                                ));
+                            }
                             _ => {}
                         }
                         let time = start.elapsed();

crates/turbopack-ecmascript/src/path_visitor.rs

@@ -153,6 +153,8 @@ impl VisitMutAstPath for ApplyVisitors<'_, '_> {
     method!(visit_mut_call_expr, CallExpr);
     method!(visit_mut_lit, Lit);
     method!(visit_mut_str, Str);
+    method!(visit_mut_block_stmt, BlockStmt);
+    method!(visit_mut_switch_case, SwitchCase);
 }
 
 #[cfg(test)]

crates/turbopack-ecmascript/src/references/mod.rs

@@ -134,6 +134,7 @@ use crate::{
         type_issue::SpecifiedModuleTypeIssue,
     },
     tree_shake::{find_turbopack_part_id_in_asserts, part_of_module, split},
+    utils::AstPathRange,
     EcmascriptInputTransforms, EcmascriptModuleAsset, SpecifiedModuleType, TreeShakingMode,
 };
 
@@ -861,6 +862,11 @@ pub(crate) async fn analyse_ecmascript_module_internal(
         };
 
         match effect {
+            Effect::Unreachable { start_ast_path } => {
+                analysis.add_code_gen(Unreachable::new(
+                    AstPathRange::StartAfter(start_ast_path.to_vec()).cell(),
+                ));
+            }
             Effect::Conditional {
                 condition,
                 kind,
@@ -872,7 +878,7 @@ pub(crate) async fn analyse_ecmascript_module_internal(
 
                 macro_rules! inactive {
                     ($block:ident) => {
-                        analysis.add_code_gen(Unreachable::new(Vc::cell($block.ast_path.to_vec())));
+                        analysis.add_code_gen(Unreachable::new($block.range.clone().cell()));
                     };
                 }
                 macro_rules! condition {
@@ -904,6 +910,19 @@ pub(crate) async fn analyse_ecmascript_module_internal(
                             active!(then);
                         }
                     },
+                    ConditionalKind::Else { r#else } => match condition.is_truthy() {
+                        Some(true) => {
+                            condition!(ConstantConditionValue::Truthy);
+                            inactive!(r#else);
+                        }
+                        Some(false) => {
+                            condition!(ConstantConditionValue::Falsy);
+                            active!(r#else);
+                        }
+                        None => {
+                            active!(r#else);
+                        }
+                    },
                     ConditionalKind::IfElse { then, r#else }
                     | ConditionalKind::Ternary { then, r#else } => match condition.is_truthy() {
                         Some(true) => {
@@ -921,6 +940,35 @@ pub(crate) async fn analyse_ecmascript_module_internal(
                             active!(r#else);
                         }
                     },
+                    ConditionalKind::IfElseMultiple { then, r#else } => match condition.is_truthy()
+                    {
+                        Some(true) => {
+                            condition!(ConstantConditionValue::Truthy);
+                            for then in then {
+                                active!(then);
+                            }
+                            for r#else in r#else {
+                                inactive!(r#else);
+                            }
+                        }
+                        Some(false) => {
+                            condition!(ConstantConditionValue::Falsy);
+                            for then in then {
+                                inactive!(then);
+                            }
+                            for r#else in r#else {
+                                active!(r#else);
+                            }
+                        }
+                        None => {
+                            for then in then {
+                                active!(then);
+                            }
+                            for r#else in r#else {
+                                active!(r#else);
+                            }
+                        }
+                    },
                     ConditionalKind::And { expr } => match condition.is_truthy() {
                         Some(true) => {
                             condition!(ConstantConditionValue::Truthy);