[clang] [LifetimeSafety] Revamp how accesses and uses are handled (PR #225799)

[Utkarsh Saxena via cfe-commits]

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@@ -4318,6 +4319,324 @@ void foo() {
                             // expected-note {{result of call to 'get' aliases the storage of local variable 'o' because the implicit object parameter is inferred as lifetimebound}}
         up = std::move(o);  // expected-note {{potentially moved here}}
     }                       // expected-note {{local variable 'o' is destroyed here}}
-    (void)*p;               // expected-note {{later used here}}
+    use(*p);                // expected-note {{later used here}}
 }
 } // namespace TakeOwnershipTests
+
+//===----------------------------------------------------------------------===//
+// What counts as a use
+//
+// A use is an access through an lvalue: reading it (an lvalue-to-rvalue
+// conversion) or writing through it. The loans of the accessed lvalue say which
+// objects it may name, so a dereference needs no special handling. Taking an
+// address, naming a variable, or copying a pointer out of one is not an access.
+//===----------------------------------------------------------------------===//
+
+namespace std { class type_info; }
+
+namespace what_is_a_use {
+struct Node {
+  int id;
+  Node *next;
+};
+
+void copying_a_pointer_is_not_a_use() {
+  Node *p;
+  {
+    Node local;
+    p = &local;  // expected-warning {{local variable 'local' does not live long enough}}
+  }              // expected-note {{local variable 'local' is destroyed here}}
+  Node *q = p;   // Reads p, not *p.
+                 // expected-note at -1 {{local variable 'p' aliases the storage of local variable 'local'}}
+  use(q);        // expected-note {{later used here}}
+}
+
+void taking_an_address_is_not_a_use() {
+  Node *p;
+  {
+    Node local;
+    p = &local;
+  }
+  Node **pp = &p;          // no-warning: borrows p's storage, never reads it.
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usx95 wrote:

Could you please add a test that `use(pp)` and we issue a warning here. Maybe same for `reborrow` and `pnext` even if it doesn't work right away.

https://github.com/llvm/llvm-project/pull/225799