LifetimeDependenceDiagnostics: find the correct variable introducer.

Author: atrick

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/LifetimeDependenceDiagnostics.swift

@@ -236,32 +236,22 @@ private struct LifetimeVariable {
       self = Self(accessBase: value.accessBase, context)
       return
     }
-    if let firstIntroducer = getFirstBorrowIntroducer(of: value, context) {
+    if let firstIntroducer = getFirstVariableIntroducer(of: value, context) {
       self = Self(introducer: firstIntroducer)
       return
     }
     self.varDecl = nil
     self.sourceLoc = nil
   }
 
-  // FUTURE: consider diagnosing multiple variable introducers. It's
-  // unclear how more than one can happen.
-  private func getFirstBorrowIntroducer(of value: Value,
-                                        _ context: some Context)
-    -> Value? {
-    var introducers = Stack<Value>(context)
-    gatherBorrowIntroducers(for: value, in: &introducers, context)
-    return introducers.pop()
-  }
-
-  private func getFirstLifetimeIntroducer(of value: Value,
-                                          _ context: some Context)
-    -> Value? {
+  private func getFirstVariableIntroducer(of value: Value, _ context: some Context) -> Value? {
     var introducer: Value?
-    _ = visitLifetimeIntroducers(for: value, context) {
+    var useDefVisitor = VariableIntroducerUseDefWalker(context) {
       introducer = $0
       return .abortWalk
     }
+    defer { useDefVisitor.deinitialize() }
+    _ = useDefVisitor.walkUp(valueOrAddress: value)
     return introducer
   }
 
@@ -283,12 +273,11 @@ private struct LifetimeVariable {
   private init(accessBase: AccessBase, _ context: some Context) {
     switch accessBase {
     case .box(let projectBox):
-      if let box = getFirstLifetimeIntroducer(of: projectBox.box, context) {
-        self = Self(introducer: box)
-      }
-      // We should always find an introducer since boxes are nontrivial.
-      self.varDecl = nil
-      self.sourceLoc = nil
+      // Note: referenceRoot looks through `begin_borrow [var_decl]` and `move_value [var_decl]`. But the box should
+      // never be produced by one of these, except when it is redundant with the `alloc_box` VarDecl. It does not seem
+      // possible for a box to be moved/borrowed directly into another variable's box. Reassignment always loads/stores
+      // the value.
+      self = Self(introducer: projectBox.box.referenceRoot)
     case .stack(let allocStack):
       self = Self(introducer: allocStack)
     case .global(let globalVar):