Ensure member path assignments memoize independently

Assignment expressions to a member path are a special case because they're the 
only place where a value isn't assigned to a (possibly temporary) variable, 
which is our unit of memoization. #901 demonstrated how this can lead to values 
that can't be independently memoized: 

```javascript 

const x = {a: a} 

x.y = [b, c]; // array recomputed w `x`, even if only `a` changed 

``` 

This PR ensures that assignment expressions where the LHS is a member path lower 
the RHS to a Place. That means the above example is handled as if you wrote: 

```javascript 

const x = {a: a}; 

const tmp1 = [b, c]; 

x.y = tmp1; 

``` 

And we independently memoize the temporary.

compiler/forget/src/HIR/BuildHIR.ts

@@ -946,7 +946,10 @@ function lowerExpression(
       const operator = expr.node.operator;
 
       if (operator === "=") {
-        const right = lowerExpression(builder, expr.get("right"));
+        const right =
+          left.memberPath === null
+            ? lowerExpression(builder, expr.get("right"))
+            : lowerExpressionToPlace(builder, expr.get("right"));
         builder.push({
           id: makeInstructionId(0),
           lvalue: { place: left, kind: InstructionKind.Reassign },

compiler/forget/src/__tests__/fixtures/hir/_bug_independently-memoize-object-property.expect.md

@@ -1,131 +0,0 @@
-
-## Input
-
-```javascript
-function foo(a, b, c) {
-  const x = { a: a };
-  // TODO @josephsavona: this array *should* be memoized independently from `x`,
-  // similar to the behavior if we extract into a variable as with `z` below:
-  x.y = [b, c];
-
-  const y = { a: a };
-  // this array correctly memoizes independently
-  const z = [b, c];
-  y.y = z;
-
-  return [x, y];
-}
-
-```
-
-## HIR
-
-```
-bb0:
-  [1] Const mutate x$11_@0:TObject[1:3] = Object { a: read a$8 }
-  [2] Reassign mutate x$11_@0.y[1:3] = Array [read b$9, read c$10]
-  [3] Const mutate y$12_@1:TObject[3:6] = Object { a: read a$8 }
-  [4] Const mutate z$13_@2 = Array [read b$9, read c$10]
-  [5] Reassign mutate y$12_@1.y[3:6] = read z$13_@2
-  [6] Const mutate t13$14_@3 = Array [read x$11_@0:TObject, read y$12_@1:TObject]
-  [7] Return freeze t13$14_@3
-```
-
-## Reactive Scopes
-
-```
-function foo(
-  a,
-  b,
-  c,
-) {
-  scope @0 [1:3] deps=[read a$8, read b$9, read c$10] out=[x$11_@0] {
-    [1] Const mutate x$11_@0:TObject[1:3] = Object { a: read a$8 }
-    [2] Reassign mutate x$11_@0.y[1:3] = Array [read b$9, read c$10]
-  }
-  scope @1 [3:6] deps=[read a$8, read b$9, read c$10] out=[y$12_@1] {
-    [3] Const mutate y$12_@1:TObject[3:6] = Object { a: read a$8 }
-    scope @2 [4:5] deps=[read b$9, read c$10] out=[z$13_@2] {
-      [4] Const mutate z$13_@2 = Array [read b$9, read c$10]
-    }
-    [5] Reassign mutate y$12_@1.y[3:6] = read z$13_@2
-  }
-  scope @3 [6:7] deps=[read x$11_@0:TObject, read y$12_@1:TObject] out=[$14_@3] {
-    [6] Const mutate $14_@3 = Array [read x$11_@0:TObject, read y$12_@1:TObject]
-  }
-  return freeze $14_@3
-}
-
-```
-
-## Code
-
-```javascript
-function foo$0(a$8, b$9, c$10) {
-  const $ = React.useMemoCache();
-  const c_0 = $[0] !== a$8;
-  const c_1 = $[1] !== b$9;
-  const c_2 = $[2] !== c$10;
-  let x$11;
-  if (c_0 || c_1 || c_2) {
-    x$11 = {
-      a: a$8,
-    };
-    x$11.y = [b$9, c$10];
-    $[0] = a$8;
-    $[1] = b$9;
-    $[2] = c$10;
-    $[3] = x$11;
-  } else {
-    x$11 = $[3];
-  }
-
-  const c_4 = $[4] !== a$8;
-  const c_5 = $[5] !== b$9;
-  const c_6 = $[6] !== c$10;
-  let y$12;
-
-  if (c_4 || c_5 || c_6) {
-    y$12 = {
-      a: a$8,
-    };
-    const c_8 = $[8] !== b$9;
-    const c_9 = $[9] !== c$10;
-    let z$13;
-
-    if (c_8 || c_9) {
-      z$13 = [b$9, c$10];
-      $[8] = b$9;
-      $[9] = c$10;
-      $[10] = z$13;
-    } else {
-      z$13 = $[10];
-    }
-
-    y$12.y = z$13;
-    $[4] = a$8;
-    $[5] = b$9;
-    $[6] = c$10;
-    $[7] = y$12;
-  } else {
-    y$12 = $[7];
-  }
-
-  const c_11 = $[11] !== x$11;
-  const c_12 = $[12] !== y$12;
-  let t13$14;
-
-  if (c_11 || c_12) {
-    t13$14 = [x$11, y$12];
-    $[11] = x$11;
-    $[12] = y$12;
-    $[13] = t13$14;
-  } else {
-    t13$14 = $[13];
-  }
-
-  return t13$14;
-}
-
-```
-      

compiler/forget/src/__tests__/fixtures/hir/_bug_independently-memoize-object-property.js

@@ -1,13 +0,0 @@
-function foo(a, b, c) {
-  const x = { a: a };
-  // TODO @josephsavona: this array *should* be memoized independently from `x`,
-  // similar to the behavior if we extract into a variable as with `z` below:
-  x.y = [b, c];
-
-  const y = { a: a };
-  // this array correctly memoizes independently
-  const z = [b, c];
-  y.y = z;
-
-  return [x, y];
-}

compiler/forget/src/__tests__/fixtures/hir/assignment-variations.expect.md

@@ -62,11 +62,12 @@ function f$0() {
 
 ```
 bb0:
-  [1] Const mutate $5:TPrimitive = 1
-  [2] Reassign mutate a$4_@0.b.c[0:5] = Binary read a$4_@0.b.c + read $5:TPrimitive
-  [3] Const mutate $6:TPrimitive = 2
-  [4] Reassign mutate a$4_@0.b.c[0:5] = Binary read a$4_@0.b.c * read $6:TPrimitive
-  [5] Return
+  [1] Const mutate $6:TPrimitive = 1
+  [2] Const mutate $7:TPrimitive = Binary read a$5_@0.b.c + read $6:TPrimitive
+  [3] Reassign read a$5_@0.b.c[0:6] = read $7:TPrimitive
+  [4] Const mutate $8:TPrimitive = 2
+  [5] Reassign mutate a$5_@0.b.c[0:6] = Binary read a$5_@0.b.c * read $8:TPrimitive
+  [6] Return
 ```
 
 ## Reactive Scopes
@@ -75,10 +76,11 @@ bb0:
 function g(
   a,
 ) {
-  [1] Const mutate $5:TPrimitive = 1
-  [2] Reassign mutate a$4_@0.b.c[0:5] = Binary read a$4_@0.b.c + read $5:TPrimitive
-  [3] Const mutate $6:TPrimitive = 2
-  [4] Reassign mutate a$4_@0.b.c[0:5] = Binary read a$4_@0.b.c * read $6:TPrimitive
+  [1] Const mutate $6:TPrimitive = 1
+  [2] Const mutate $7:TPrimitive = Binary read a$5_@0.b.c + read $6:TPrimitive
+  [3] Reassign read a$5_@0.b.c[0:6] = read $7:TPrimitive
+  [4] Const mutate $8:TPrimitive = 2
+  [5] Reassign mutate a$5_@0.b.c[0:6] = Binary read a$5_@0.b.c * read $8:TPrimitive
   return
 }
 
@@ -87,9 +89,9 @@ function g(
 ## Code
 
 ```javascript
-function g$0(a$4) {
-  a$4.c.b = a$4.b.c + 1;
-  a$4.c.b = a$4.b.c * 2;
+function g$0(a$5) {
+  a$5.c.b = a$5.b.c + 1;
+  a$5.c.b = a$5.b.c * 2;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/hir/independently-memoize-object-property.expect.md

@@ -0,0 +1,84 @@
+
+## Input
+
+```javascript
+function foo(a, b, c) {
+  const x = { a: a };
+  // NOTE: this array should memoize independently from x, w only b,c as deps
+  x.y = [b, c];
+
+  return x;
+}
+
+```
+
+## HIR
+
+```
+bb0:
+  [1] Const mutate x$9_@0:TObject[1:4] = Object { a: read a$6 }
+  [2] Const mutate t6$10_@1 = Array [read b$7, read c$8]
+  [3] Reassign mutate x$9_@0.y[1:4] = read t6$10_@1
+  [4] Return freeze x$9_@0:TObject
+```
+
+## Reactive Scopes
+
+```
+function foo(
+  a,
+  b,
+  c,
+) {
+  scope @0 [1:4] deps=[read a$6, read b$7, read c$8] out=[x$9_@0] {
+    [1] Const mutate x$9_@0:TObject[1:4] = Object { a: read a$6 }
+    scope @1 [2:3] deps=[read b$7, read c$8] out=[$10_@1] {
+      [2] Const mutate $10_@1 = Array [read b$7, read c$8]
+    }
+    [3] Reassign mutate x$9_@0.y[1:4] = read $10_@1
+  }
+  return freeze x$9_@0:TObject
+}
+
+```
+
+## Code
+
+```javascript
+function foo$0(a$6, b$7, c$8) {
+  const $ = React.useMemoCache();
+  const c_0 = $[0] !== a$6;
+  const c_1 = $[1] !== b$7;
+  const c_2 = $[2] !== c$8;
+  let x$9;
+  if (c_0 || c_1 || c_2) {
+    x$9 = {
+      a: a$6,
+    };
+    const c_4 = $[4] !== b$7;
+    const c_5 = $[5] !== c$8;
+    let t6$10;
+
+    if (c_4 || c_5) {
+      t6$10 = [b$7, c$8];
+      $[4] = b$7;
+      $[5] = c$8;
+      $[6] = t6$10;
+    } else {
+      t6$10 = $[6];
+    }
+
+    x$9.y = t6$10;
+    $[0] = a$6;
+    $[1] = b$7;
+    $[2] = c$8;
+    $[3] = x$9;
+  } else {
+    x$9 = $[3];
+  }
+
+  return x$9;
+}
+
+```
+      

compiler/forget/src/__tests__/fixtures/hir/independently-memoize-object-property.js

@@ -0,0 +1,7 @@
+function foo(a, b, c) {
+  const x = { a: a };
+  // NOTE: this array should memoize independently from x, w only b,c as deps
+  x.y = [b, c];
+
+  return x;
+}