"expected struct `std::boxed::Box`, found struct `std::string::String`", scan-rules 0.2.0, Rust 1.17 nightly

[brson]

brian@ip-10-145-43-250:~/dev/rust-scan-rules⟫ git remote -v
origin  https://github.com/DanielKeep/rust-scan-rules (fetch)
origin  https://github.com/DanielKeep/rust-scan-rules (push)
brian@ip-10-145-43-250:~/dev/rust-scan-rules⟫ git log -1
commit 151b4d822c33dcb8a04d9fa55d1084c948e5b76d
Author: Daniel Keep <daniel.keep@gmail.com>
Date:   Mon Nov 7 16:27:19 2016 +1100

    chore: updated docs for release.

brian@ip-10-145-43-250:/mnt2/dev⟫ rustc +stable -Vv
rustc 1.15.1 (021bd294c 2017-02-08)
binary: rustc
commit-hash: 021bd294c039bd54aa5c4aa85bcdffb0d24bc892
commit-date: 2017-02-08
host: x86_64-unknown-linux-gnu
release: 1.15.1
LLVM version: 3.9

101 brian@ip-10-145-43-250:~/dev/rust-scan-rules⟫ cargo +nightly test
   Compiling bitflags v0.4.0
   Compiling semver v0.1.20
   Compiling unicode-segmentation v0.1.3
   Compiling lazy_static v0.1.16
   Compiling itertools v0.4.19
warning[E0122]: trait bounds are not (yet) enforced in type definitions
   --> /home/brian/.cargo/registry/src/github.com-1ecc6299db9ec823/itertools-0.4.19/src/lib.rs:123:1
    |
123 | pub type MapFn<I, B> where I: Iterator = iter::Map<I, fn(I::Item) -> B>;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

   Compiling rustc_version v0.1.7
   Compiling strcursor v0.2.3
   Compiling scan-rules v0.2.0 (file:///mnt2/dev/rust-scan-rules)
error[E0308]: mismatched types
   --> /home/brian/.cargo/registry/src/github.com-1ecc6299db9ec823/strcursor-0.2.3/src/grapheme.rs:525:15
    |
525 |         GcBuf(s)
    |               ^ expected struct `std::boxed::Box`, found struct `std::string::String`
    |
    = note: expected type `std::boxed::Box<str>`
               found type `std::string::String`
    = help: here are some functions which might fulfill your needs:
            - .into_boxed_str()

error: aborting due to previous error

error: Could not compile `strcursor`.

To learn more, run the command again with --verbose.

Not on 1.15. Different from 1.16 #39825

cc @DanielKeep

[arielb1]

libs team - does this look like XIB (expected inference breakage) to you?

[brson]

Also in strcursor-0.2.3

[nikomatsakis]

cc @rust-lang/libs -- it sure smells like expected inference breakage of some kind or another (as @arielb1 noted, but didn't cc the alias).

[alexcrichton]

I believe the crate in the title, scan-rules, is a red herring. The original log (in the OP) and @brson's most recent comment lead me to believe that this is only a problem with the strcursor crate.

In that crate the type in question is conditionally compiled based off a #[cfg] value. If the cfg is set, the crate will not compile (as one would expect). On 1.16.0 the build script is not picked up (there's no build = "build.rs") but on 1.17.0 we're picking it up by default (this changed in Cargo). That means that on Rust 1.16.0 we're not running a build script, the cfg isn't set, and the crate compiles just fine. On Rust 1.17.0 the build script is auto-detected, the cfg is set, and the crate fails to compile.

Looking at the history of the crate it looks like build = "build.rs" has never been present, so I presume that this commit accidentally forgot to add it, and ever since the crate has accidentally been compiling.

@DanielKeep does all that sound right to you?

In any case I don't believe this is a regression, so I'm going to close in the meantime. @DanielKeep if you need any help updating though please let me know!