Fix #56806 by using delay_span_bug in object safety layout sanity checks

[mikeyhew]

It's possible that is_object_safe is called on a trait method that with an invalid receiver type. This caused an ICE in #56806, because receiver_is_dispatchable returns true for self: Box<dyn Trait>, which causes one of the layout sanity checks in object_safety.rs to fail. Replacing bug! with delay_span_bug solves this.

The fact that receiver_is_dispatchable returns true here could be considered a bug. It passes the check that the method implements, though: Box<dyn Trait> implements DispatchFromDyn<Box<dyn Trait>> because dyn Trait implements Unsize<dyn Trait>. It would be good to hear what @eddyb and @nikomatsakis think.

Note that I only added a test for the case encountered in #56806. I could not come up with a case that triggered an ICE from the other check, bug!("receiver when Self = dyn Trait should be ScalarPair, found Scalar"). There is no way, to my knowledge, that you can make receiver_is_dispatchable return true but still have a Scalar ABI when Self = dyn Trait.

One other case I encountered while debugging #56806 was that if you have a type parameter T that implements Deref<Target=Self> and DispatchFromDyn<T>, and use it as a method receiver, it will cause an ICE during is_object_safe because T has no layout (playground):

trait Trait<T: Deref<Target=Self> + DispatchFromDyn<T>> {
    fn foo(self: T) -> dyn Trait<T>;
}

I don't intend to remove the ICE there because it is a pathological case, especially since there is no way to implement DispatchFromDyn<T> for T — the checks in typeck/coherence/builtin.rs do not allow that.

fixes #56806
r? @varkor

[varkor]

I don't intend to remove the ICE there because it is a pathological case, especially since there is no way to implement DispatchFromDyn for T — the checks in typeck/coherence/builtin.rs do not allow that.

If it's possible for the user to trigger it, it should be addressed, even if it isn't something that's likely to be input. But that can be left for a future PR. It would be good to open an issue to keep track of it, though.

[varkor]

r=me with the copyright notice removed.

[rust-highfive]

The job x86_64-gnu-llvm-6.0 of your PR failed on Travis (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.

travis_time:end:046d1c72:start=1546444275453806109,finish=1546444335251201728,duration=59797395619
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-6.0
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---
[00:59:19] 
[00:59:19] ---- [ui] ui/issues/issue-56806.rs stdout ----
[00:59:19] diff of stderr:
[00:59:19] 
[00:59:19] 1 error[E0307]: invalid method receiver type: std::boxed::Box<(dyn Trait + 'static)>
[00:59:19] +   --> $DIR/issue-56806.rs:2:34
[00:59:19] 3    |
[00:59:19] 3    |
[00:59:19] 4 LL |     fn dyn_instead_of_self(self: Box<dyn Trait>);
[00:59:19] 
[00:59:19] 
[00:59:19] The actual stderr differed from the expected stderr.
[00:59:19] Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/issues/issue-56806/issue-56806.stderr
[00:59:19] Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/issues/issue-56806/issue-56806.stderr
[00:59:19] To update references, rerun the tests and pass the `--bless` flag
[00:59:19] To only update this specific test, also pass `--test-args issues/issue-56806.rs`
[00:59:19] error: 1 errors occurred comparing output.
[00:59:19] status: exit code: 1
[00:59:19] status: exit code: 1
[00:59:19] command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/issues/issue-56806.rs" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "-Zui-testing" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/issues/issue-56806/a" "-Crpath" "-O" "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknorting due to previous error","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to previous error\n\n"}
[00:59:19] {"message":"For more information about this error, try `rustc --explain E0307`.","code":null,"level":"","spans":[],"children":[],"rendered":"For more information about this error, try `rustc --explain E0307`.\n"}
[00:59:19] ------------------------------------------
[00:59:19] 
[00:59:19] thread '[ui] ui/issues/issue-56806.rs' panicked at 'explicit panic', src/tools/compiletest/src/runtest.rs:3245:9
[00:59:19] note: Run with `RUST_BACKTRACE=1` for a backtrace.
---
[00:59:19] 
[00:59:19] thread 'main' panicked at 'Some tests failed', src/tools/compiletest/src/main.rs:495:22
[00:59:19] 
[00:59:19] 
[00:59:19] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/compiletest" "--compile-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" "--run-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib" "--rustc-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "--src-base" "/checkout/src/test/ui" "--build-base" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui" "--stage-id" "stage2-x86_64-unknown-linux-gnu" "--mode" "ui" "--target" "x86_64-unknown-linux-gnu" "--host" "x86_64-unknown-linux-gnu" "--llvm-filecheck" "/usr/lib/ltravis_time:end:0ca3a460:start=1546444343601187202,finish=1546447903148233843,duration=3559547046641
travis_time:start:0de4a138
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Wed Jan  2 16:51:43 UTC 2019
Wed, 02 Jan 2019 16:51:43 GMT

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)

[varkor]

@mikeyhew: sorry for the delay — could you squash the changes? After that, r=me.

[mikeyhew]

@varkor squashed

[varkor]

@bors r+

[bors]

📌 Commit 2433526 has been approved by varkor