Rollup merge of #40038 - nikomatsakis:issue-39828, r=michaelwoerister

detect "bootstrap outputs" when serializing the dep-graph

Fixes #39828.

r? @michaelwoerister

src/test/incremental/issue-39828/auxiliary/generic.rs

@@ -0,0 +1,18 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// revisions:rpass1 rpass2
+// compile-flags: -Z query-dep-graph
+
+#![rustc_partition_reused(module="__rustc_fallback_codegen_unit", cfg="rpass2")]
+#![feature(rustc_attrs)]
+
+#![crate_type="rlib"]
+pub fn foo<T>() { }

src/test/incremental/issue-39828/issue-39828.rs

@@ -0,0 +1,22 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Regression test for #39828. If you make use of a module that
+// consists only of generics, no code is generated, just a dummy
+// module. The reduced graph consists of a single node (for that
+// module) with no inputs. Since we only serialize edges, when we
+// reload, we would consider that node dirty since it is not recreated
+// (it is not the target of any edges).
+
+// revisions:rpass1 rpass2
+// aux-build:generic.rs
+
+extern crate generic;
+fn main() { }