feat: Check the number of direct dependants while tree shaking (#8473)

### Description

<img width="285" alt="image" src="https://github.com/vercel/turbo/assets/29931815/c37d81fb-ab6e-482a-8e6f-3784a62e923b">

In the graph above, `x` and `y` can both reach `item 5`, but grouping from `item 5` to `item 9` is fine

crates/turbopack-ecmascript/src/tree_shake/graph.rs

@@ -473,14 +473,22 @@ impl DepGraph {
  continue;
  }
 
- let count = global_done
+ // The number of nodes that this node is dependent on.
+ let dependant_count = self
+ .g
+ .idx_graph
+ .neighbors_directed(ix as _, petgraph::Direction::Incoming)
+ .count();
+
+ // The number of starting points that can reach to this node.
+ let count_of_startings = global_done
  .iter()
  .filter(|&&staring_point| {
  has_path_connecting(&self.g.idx_graph, staring_point, ix as _, None)
  })
  .count();
 
- if count >= 2 {
+ if dependant_count >= 2 && count_of_startings >= 2 {
  groups.push((vec![id.clone()], FxHashSet::default()));
  global_done.insert(ix as u32);
  }

crates/turbopack-ecmascript/tests/tree-shaker/analyzer/1/output.md

@@ -332,31 +332,29 @@ graph TD
  N4["Items: [ItemId(Export(("external2", #2), "external2")), ItemId(11, Normal)]"];
  N5["Items: [ItemId(1, VarDeclarator(0))]"];
  N6["Items: [ItemId(2, VarDeclarator(0))]"];
- N7["Items: [ItemId(3, VarDeclarator(0))]"];
- N8["Items: [ItemId(4, Normal)]"];
- N9["Items: [ItemId(5, VarDeclarator(0))]"];
- N10["Items: [ItemId(6, Normal)]"];
- N11["Items: [ItemId(0, ImportBinding(0)), ItemId(9, Normal)]"];
- N0 --> N9;
- N0 --> N11;
+ N7["Items: [ItemId(3, VarDeclarator(0)), ItemId(4, Normal)]"];
+ N8["Items: [ItemId(5, VarDeclarator(0))]"];
+ N9["Items: [ItemId(6, Normal)]"];
+ N10["Items: [ItemId(0, ImportBinding(0)), ItemId(9, Normal)]"];
+ N0 --> N8;
  N0 --> N10;
+ N0 --> N9;
  N0 --> N6;
- N1 --> N10;
+ N1 --> N9;
  N2 --> N6;
- N3 --> N11;
  N3 --> N10;
+ N3 --> N9;
  N4 --> N6;
+ N4 --> N8;
  N4 --> N9;
- N4 --> N10;
  N6 --> N5;
+ N7 --> N5;
+ N7 --> N6;
  N8 --> N7;
- N8 --> N5;
- N8 --> N6;
+ N9 --> N7;
+ N9 --> N6;
  N9 --> N8;
- N10 --> N8;
- N10 --> N6;
  N10 --> N9;
- N11 --> N10;
 ```
 # Entrypoints
 
@@ -383,13 +381,13 @@ graph TD
 ## Part 0
 ```js
 import { foobarCopy } from "__TURBOPACK_PART__" assert {
- __turbopack_part__: 9
+ __turbopack_part__: 8
 };
 import "__TURBOPACK_PART__" assert {
- __turbopack_part__: 11
+ __turbopack_part__: 10
 };
 import "__TURBOPACK_PART__" assert {
- __turbopack_part__: 10
+ __turbopack_part__: 9
 };
 import "__TURBOPACK_PART__" assert {
  __turbopack_part__: 6
@@ -403,7 +401,7 @@ console.log(foobarCopy);
 ## Part 1
 ```js
 import { foobar } from "__TURBOPACK_PART__" assert {
- __turbopack_part__: 10
+ __turbopack_part__: 9
 };
 export { foobar };
 
@@ -419,10 +417,10 @@ export { foo };
 ## Part 3
 ```js
 import { internal } from "__TURBOPACK_PART__" assert {
- __turbopack_part__: 11
+ __turbopack_part__: 10
 };
 import { foobar } from "__TURBOPACK_PART__" assert {
- __turbopack_part__: 10
+ __turbopack_part__: 9
 };
 export { external1 };
 function external1() {
@@ -439,10 +437,10 @@ import "__TURBOPACK_PART__" assert {
  __turbopack_part__: 6
 };
 import "__TURBOPACK_PART__" assert {
- __turbopack_part__: 9
+ __turbopack_part__: 8
 };
 import { foobar } from "__TURBOPACK_PART__" assert {
- __turbopack_part__: 10
+ __turbopack_part__: 9
 };
 export { external2 };
 function external2() {
@@ -474,61 +472,54 @@ export { foo } from "__TURBOPACK_VAR__" assert {
 ```
 ## Part 7
 ```js
-const bar = "bar";
-export { bar } from "__TURBOPACK_VAR__" assert {
- __turbopack_var__: true
-};
-
-```
-## Part 8
-```js
-import { bar } from "__TURBOPACK_PART__" assert {
- __turbopack_part__: 7
-};
 import { foobar } from "__TURBOPACK_PART__" assert {
  __turbopack_part__: 5
 };
 import "__TURBOPACK_PART__" assert {
  __turbopack_part__: 6
 };
+const bar = "bar";
 foobar += bar;
+export { bar } from "__TURBOPACK_VAR__" assert {
+ __turbopack_var__: true
+};
 export { foobar } from "__TURBOPACK_VAR__" assert {
  __turbopack_var__: true
 };
 
 ```
-## Part 9
+## Part 8
 ```js
 import { foobar } from "__TURBOPACK_PART__" assert {
- __turbopack_part__: 8
+ __turbopack_part__: 7
 };
 let foobarCopy = foobar;
 export { foobarCopy } from "__TURBOPACK_VAR__" assert {
  __turbopack_var__: true
 };
 
 ```
-## Part 10
+## Part 9
 ```js
 import { foobar } from "__TURBOPACK_PART__" assert {
- __turbopack_part__: 8
+ __turbopack_part__: 7
 };
 import "__TURBOPACK_PART__" assert {
  __turbopack_part__: 6
 };
 import "__TURBOPACK_PART__" assert {
- __turbopack_part__: 9
+ __turbopack_part__: 8
 };
 foobar += "foo";
 export { foobar } from "__TURBOPACK_VAR__" assert {
  __turbopack_var__: true
 };
 
 ```
-## Part 11
+## Part 10
 ```js
 import { foobar } from "__TURBOPACK_PART__" assert {
- __turbopack_part__: 10
+ __turbopack_part__: 9
 };
 import { upper } from "module";
 function internal() {
@@ -542,13 +533,13 @@ export { internal } from "__TURBOPACK_VAR__" assert {
 ## Merged (module eval)
 ```js
 import { foobarCopy } from "__TURBOPACK_PART__" assert {
- __turbopack_part__: 9
+ __turbopack_part__: 8
 };
 import "__TURBOPACK_PART__" assert {
- __turbopack_part__: 11
+ __turbopack_part__: 10
 };
 import "__TURBOPACK_PART__" assert {
- __turbopack_part__: 10
+ __turbopack_part__: 9
 };
 import "__TURBOPACK_PART__" assert {
  __turbopack_part__: 6
@@ -584,7 +575,7 @@ console.log(foobarCopy);
 ## Part 0
 ```js
 import { foobar } from "__TURBOPACK_PART__" assert {
- __turbopack_part__: 7
+ __turbopack_part__: 6
 };
 "module evaluation";
 import "module";
@@ -598,7 +589,7 @@ export { foobarCopy } from "__TURBOPACK_VAR__" assert {
 ## Part 1
 ```js
 import { foobar } from "__TURBOPACK_PART__" assert {
- __turbopack_part__: 8
+ __turbopack_part__: 7
 };
 export { foobar };
 
@@ -618,7 +609,7 @@ export { foo } from "__TURBOPACK_VAR__" assert {
 ## Part 3
 ```js
 import { foobar } from "__TURBOPACK_PART__" assert {
- __turbopack_part__: 8
+ __turbopack_part__: 7
 };
 export { external1 };
 import { upper } from "module";
@@ -657,30 +648,23 @@ export { foobar } from "__TURBOPACK_VAR__" assert {
 ```
 ## Part 6
 ```js
-const bar = "bar";
-export { bar } from "__TURBOPACK_VAR__" assert {
- __turbopack_var__: true
-};
-
-```
-## Part 7
-```js
-import { bar } from "__TURBOPACK_PART__" assert {
- __turbopack_part__: 6
-};
 import { foobar } from "__TURBOPACK_PART__" assert {
  __turbopack_part__: 5
 };
+const bar = "bar";
 foobar += bar;
+export { bar } from "__TURBOPACK_VAR__" assert {
+ __turbopack_var__: true
+};
 export { foobar } from "__TURBOPACK_VAR__" assert {
  __turbopack_var__: true
 };
 
 ```
-## Part 8
+## Part 7
 ```js
 import { foobar } from "__TURBOPACK_PART__" assert {
- __turbopack_part__: 7
+ __turbopack_part__: 6
 };
 foobar += "foo";
 export { foobar } from "__TURBOPACK_VAR__" assert {
@@ -691,7 +675,7 @@ export { foobar } from "__TURBOPACK_VAR__" assert {
 ## Merged (module eval)
 ```js
 import { foobar } from "__TURBOPACK_PART__" assert {
- __turbopack_part__: 7
+ __turbopack_part__: 6
 };
 import "module";
 "module evaluation";