Replace state_for_location with DataflowResultsCursor

[ecstatic-morse]

These are two different ways of getting the same data from the result of a dataflow analysis. However, state_for_location goes quadratic if you try to call it for every statement in the body.

[rust-highfive]

r? @cramertj

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[rust-highfive]

The job mingw-check of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

2019-09-17T00:14:17.3888468Z ##[command]git remote add origin https://github.com/rust-lang/rust
2019-09-17T00:14:17.4076267Z ##[command]git config gc.auto 0
2019-09-17T00:14:17.4164414Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2019-09-17T00:14:17.4234969Z ##[command]git config --get-all http.proxy
2019-09-17T00:14:17.4383276Z ##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --force --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/* +refs/pull/64532/merge:refs/remotes/pull/64532/merge
---
2019-09-17T00:24:21.1640857Z     Checking rustc_plugin v0.0.0 (/checkout/src/librustc_plugin/deprecated)
2019-09-17T00:24:21.2618948Z error: method is never used: `get`
2019-09-17T00:24:21.2619329Z    --> src/librustc_mir/dataflow/mod.rs:457:5
2019-09-17T00:24:21.2619721Z     |
2019-09-17T00:24:21.2620119Z 457 |     pub fn get(&self) -> &BitSet<BD::Idx> {
2019-09-17T00:24:21.2630640Z 
2019-09-17T00:24:21.6701530Z error: aborting due to previous error
2019-09-17T00:24:21.6702746Z 
2019-09-17T00:24:21.7960453Z error: Could not compile `rustc_mir`.
---
2019-09-17T00:24:21.8060651Z == clock drift check ==
2019-09-17T00:24:21.8079464Z   local time: Tue Sep 17 00:24:21 UTC 2019
2019-09-17T00:24:22.0946960Z   network time: Tue, 17 Sep 2019 00:24:22 GMT
2019-09-17T00:24:22.0947137Z == end clock drift check ==
2019-09-17T00:24:23.2605709Z ##[error]Bash exited with code '1'.
2019-09-17T00:24:23.2645365Z ##[section]Starting: Checkout
2019-09-17T00:24:23.2647834Z ==============================================================================
2019-09-17T00:24:23.2647921Z Task         : Get sources
2019-09-17T00:24:23.2647975Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[pnkfelix]

This change looks good to me.

I had a brief desire to attempt to find out if any open PR's are using state_for_location, but I don't think Github makes it easy to do a search like that...?

[ecstatic-morse]

#64207 is exporting most of rustc_mir::dataflow for use in clippy. I believe the author is currently using state_for_location but I can help them switch to DataflowResultsCursor. No other open PRs show up when searching for "dataflow" that use this method.

[ecstatic-morse]

The author of #64207 has already changed their clippy PR to use the cursor.

[tmandry]

This also looks good to me. r+'ing so we can get it together with #64207.
@bors r+

[bors]

📌 Commit 73c7a68 has been approved by tmandry