[WIP] Bring rust-semverver in-tree and test that it builds.

[eddyb]

What is rust-semverver?

rust-semverver is a tool to check semver-compliance in Rust library crates. The core of the tool has been developed as a student project during the Google Summer of Code 2017.

Details on the work done during GSoC 2017 can be found here.

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This PR initializes src/tools/rust-semverver to rust-lang/rust-semverver@0bbdabf using git subtree add, akin to what @oli-obk did to bring clippy in-tree. Only the last 3 commits are mine.

I was also able to make ./x.py check src/tools/semverver and ./x.py build src/tools/semverver work, but for now I've kept ./x.py test src/tools/semverver equivalent to build (i.e. not actually running any tests), pending actual test integration.

To get tests working I suspect we'll have to rewrite some of the test driving logic in src/tools/semverver/tests to rely more on environment variables than relative paths (akin to what clippy does).

In order to get rust-semverver to build successfully in-tree, I had to work around the fact that it depends on some Cargo crates which do not build with SourceType::InTree (i.e. Cargo doesn't build with -Drust_2018_idioms), so for now it's using SourceType::Submodule despite that being incorrect for src/tools/semverver itself.

I also updated it to the latest rustc and Cargo, but I haven't tested that it still works afterwards.

r? @Mark-Simulacrum cc @ibabushkin @rust-lang/compiler @rust-lang/dev-tools @Manishearth @JohnTitor

[eddyb]

mingw-check passes now, after I changed it from Ubuntu 16.04 to 18.04, thanks @mati865 and @ehuss!

[eddyb]

@bors rollup=never (in case the tools builder fails)

[nikomatsakis]

I am a strong supporter of us shipping this tool, but I'd like to understand the maintenance plan -- can we create a team for it under the dev-tools team, for example?

[Mark-Simulacrum]

Yes, that's one of the questions I was hoping an MCP (or similar, perhaps RFC or so) would answer.

To be clear, I've not been planning on reviewing this until someone with "T-compiler" hat on says that we're going to merge this in and we have some sort of plan for maintenance etc.

[Manishearth]

The dev tools team has been "maintenance mode" maintaining it, doing rustups. We can continue keeping an eye on it, in part one of the things we do is keep an eye out for these smaller tools. We'd need maintainers to do more

[Manishearth]

@bors rollup=never

[bors]

☔ The latest upstream changes (presumably #73441) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #73265) made this pull request unmergeable. Please resolve the merge conflicts.

[Dylan-DPC-zz]

Closing this as it's blocked for a long time