Auto merge of #30532 - nikomatsakis:cross-item-dependencies, r=mw

This is roughly the same as my previous PR that created a dependency graph, but that:

1. The dependency graph is only optionally constructed, though this doesn't seem to make much of a difference in terms of overhead (see measurements below).
2. The dependency graph is simpler (I combined a lot of nodes).
3. The dependency graph debugging facilities are much better: you can now use `RUST_DEP_GRAPH_FILTER` to filter the dep graph to just the nodes you are interested in, which is super help.
4. The tests are somewhat more elaborate, including a few known bugs I need to fix in a second pass.

This is potentially a `[breaking-change]` for plugin authors. If you are poking about in tcx state or something like that, you probably want to add `let _ignore = tcx.dep_graph.in_ignore();`, which will cause your reads/writes to be ignored and not affect the dep-graph.

After this, or perhaps as an add-on to this PR in some cases, what I would like to do is the following:

- [x] Write-up a little guide to how to use this system, the debugging options available, and what the possible failure modes are.
- [ ] Introduce read-only and perhaps the `Meta` node
- [x] Replace "memoization tasks" with node from the map itself
- [ ] Fix the shortcomings, obviously! Notably, the HIR map needs to register reads, and there is some state that is not yet tracked. (Maybe as a separate PR.)
- [x] Refactor the dep-graph code so that the actual maintenance of the dep-graph occurs in a parallel thread, and the main thread simply throws things into a shared channel (probably a fixed-size channel). There is no reason for dep-graph construction to be on the main thread. (Maybe as a separate PR.)

Regarding performance: adding this tracking does add some overhead, approximately 2% in my measurements (I was comparing the build times for rustdoc). Interestingly, enabling or disabling tracking doesn't seem to do very much. I want to poke at this some more and gather a bit more data -- in some tests I've seen that 2% go away, but on others it comes back. It's not entirely clear to me if that 2% is truly due to constructing the dep-graph at all.

The next big step after this is write some code to dump the dep-graph to disk and reload it.

r? @michaelwoerister

1 files changed, 8 insertions, 0 deletions

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index a23dc3b8871..c281571305b 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -326,6 +326,14 @@ pub const KNOWN_ATTRIBUTES: &'static [(&'static str, AttributeType, AttributeGat
                                        "the `#[rustc_error]` attribute \
                                         is just used for rustc unit tests \
                                         and will never be stable")),
+    ("rustc_if_this_changed", Whitelisted, Gated("rustc_attrs",
+                                       "the `#[rustc_if_this_changed]` attribute \
+                                        is just used for rustc unit tests \
+                                        and will never be stable")),
+    ("rustc_then_this_would_need", Whitelisted, Gated("rustc_attrs",
+                                       "the `#[rustc_if_this_changed]` attribute \
+                                        is just used for rustc unit tests \
+                                        and will never be stable")),
     ("rustc_move_fragments", Normal, Gated("rustc_attrs",
                                            "the `#[rustc_move_fragments]` attribute \
                                             is just used for rustc unit tests \