[analyzer] Report violations of the "returns_nonnull" attribute

Make sure code respects the GNU-extension
__attribute__((returns_nonnull)).

Extend the NullabilityChecker to check that a function returns_nonnull
does not return a nullptr.

CPP-4741

Author: necto

clang/lib/StaticAnalyzer/Checkers/NullabilityChecker.cpp

@@ -692,6 +692,14 @@ void NullabilityChecker::checkPreStmt(const ReturnStmt *S,
   NullConstraint Nullness = getNullConstraint(*RetSVal, State);
 
   Nullability RequiredNullability = getNullabilityAnnotation(RequiredRetType);
+  if (const auto *FunDecl = C.getLocationContext()->getDecl();
+      FunDecl && FunDecl->getAttr<ReturnsNonNullAttr>() &&
+      (RequiredNullability == Nullability::Unspecified ||
+       RequiredNullability == Nullability::Nullable)) {
+    // If a function is marked with the returns_nonnull attribute,
+    // the return value must be non-null.
+    RequiredNullability = Nullability::Nonnull;
+  }
 
   // If the returned value is null but the type of the expression
   // generating it is nonnull then we will suppress the diagnostic.

clang/test/Analysis/nullability.c

@@ -1,4 +1,6 @@
-// RUN: %clang_analyze_cc1 -fblocks -analyzer-checker=core,nullability -Wno-deprecated-non-prototype -verify %s
+// RUN: %clang_analyze_cc1 -fblocks -analyzer-checker=core,nullability,debug.ExprInspection -Wno-deprecated-non-prototype -verify %s
+
+void clang_analyzer_warnIfReached();
 
 void it_takes_two(int a, int b);
 void function_pointer_arity_mismatch() {
@@ -10,3 +12,42 @@ void block_arity_mismatch() {
   void(^b)() = ^(int a, int b) { };
   b(1);  // no-crash expected-warning {{Block taking 2 arguments is called with fewer (1)}}
 }
+
+int *nonnull_return_annotation_indirect() __attribute__((returns_nonnull));
+int *nonnull_return_annotation_indirect() {
+  int *x = 0;
+  return x; // expected-warning {{Null returned from a function that is expected to return a non-null value}}
+}
+
+int *nonnull_return_annotation_direct() __attribute__((returns_nonnull));
+int *nonnull_return_annotation_direct() {
+  return 0; // expected-warning {{Null returned from a function that is expected to return a non-null value}}
+} // expected-warning@-1 {{null returned from function that requires a non-null return value}}
+
+int *nonnull_return_annotation_assumed() __attribute__((returns_nonnull));
+int *nonnull_return_annotation_assumed(int* ptr) {
+  if (ptr) {
+    return ptr;
+  }
+  return ptr; // expected-warning {{Null returned from a function that is expected to return a non-null value}}
+}
+
+int *produce_nonnull_ptr() __attribute__((returns_nonnull));
+
+__attribute__((returns_nonnull))
+int *cannot_return_null() {
+  int *x = produce_nonnull_ptr();
+  if (!x) {
+    clang_analyzer_warnIfReached();
+    // Incorrect: expected-warning@-1 {{REACHABLE}}
+    // According to produce_nonnull_ptr contract, x cannot be null.
+  }
+  // Regardless of the potential state split above, x cannot be nullptr
+  // according to the produce_nonnull_ptr annotation.
+  return x;
+  // False positive: expected-warning@-1 {{Null returned from a function that is expected to return a non-null value}}
+}
+
+__attribute__((returns_nonnull)) int *passthrough(int *p) {
+  return p; // no-warning: we have no evidence that `p` is null, i.e., violating the contract
+}