LifetimeDependence type check: infer trivial _read accessor

This fixes a small oversight in the type checker's LifetimeDependence
inference. Allow inference on _read accessors even when 'self' is a trivial
type. This is needed because the compiler synthesizes a _read accessor even when
the user defines a getter (this is probably a mistake, but it's easire to just
fix inference at this point). There is no workaround because it defining both a
getter and '_read' is illegal!

    extension UnsafeMutableRawBufferPointer {
      var mutableBytes: MutableRawSpan {
        @_lifetime(borrow self)
        get {
          unsafe MutableRawSpan(_unsafeBytes: self)
        }
      }
    }

Fixes rdar://153346478 (Can't compile the
UnsafeMutableRawBufferPointer.mutableBytes property)

(cherry picked from commit 125a086)

Author: atrick

lib/AST/LifetimeDependence.cpp

@@ -932,16 +932,19 @@ class LifetimeDependenceChecker {
     if (!useLazyInference() && afd->getParameters()->size() > 0) {
       return;
     }
-    if (!nonEscapableSelf && isBitwiseCopyable(selfTypeInContext, ctx)) {
-      diagnose(returnLoc,
-               diag::lifetime_dependence_cannot_infer_bitwisecopyable,
-               diagnosticQualifier(), "self");
-      return;
-    }
-    if (!useLazyInference()) {
-      // Do not infer LifetimeDependenceKind::Inherit unless this is an implicit
-      // getter, which simply returns a stored property.
-      if (nonEscapableSelf && !isImplicitOrSIL()) {
+    // Allow inference for implicit getters, which simply return a stored,
+    // property, and for implicit _read/_modify, which cannot be defined
+    // explicitly alongside a regular getter.
+    if (!useLazyInference() && !isImplicitOrSIL()) {
+      // Require explicit @_lifetime(borrow self) for UnsafePointer-like self.
+      if (!nonEscapableSelf && isBitwiseCopyable(selfTypeInContext, ctx)) {
+        diagnose(returnLoc,
+                 diag::lifetime_dependence_cannot_infer_bitwisecopyable,
+                 diagnosticQualifier(), "self");
+        return;
+      }
+      // Require explicit @_lifetime(copy or borrow) for non-Escapable self.
+      if (nonEscapableSelf) {
         diagnose(returnLoc, diag::lifetime_dependence_cannot_infer_kind,
                  diagnosticQualifier(), "self");
         return;

test/Sema/lifetime_depend_infer.swift

@@ -318,6 +318,36 @@ struct Accessors {
   }
 }
 
+struct TrivialAccessors {
+  let p: UnsafeRawPointer
+
+  // The implicit _read/_modify accessors must be inferred. They cannot be written explicitly because a getter is
+  // already defined.
+  var neComputed: NEImmortal {
+    @_lifetime(borrow self)
+    get { // OK
+      NEImmortal()
+    }
+
+    @_lifetime(&self)
+    set { // OK (no dependency)
+    }
+  }
+
+  var neYielded: NEImmortal {
+    @_lifetime(borrow self)
+    _read { // OK
+      yield NEImmortal()
+    }
+
+    @_lifetime(&self)
+    _modify { // OK
+      var ne = NEImmortal()
+      yield &ne
+    }
+  }
+}
+
 struct NonescapableSelfAccessors: ~Escapable {
   var ne: NE