implement Error::source for Box<T: Error>

[s3bk]

fixes #61899

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @withoutboats (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[Centril]

@s3bk Looks like you are closing the wrong issue in the description?

[Arnavion]

Should be #61899

[s3bk]

@Centril, @Arnavion indeed. I had to fetch the rust repo locally to fix it. Slow connection, so it took a very long time…

[awaitlink]

@s3bk I think you should remove the incorrect striked through issue reference, since it prevents GitHub from parsing the keywords to automatically close the correct issue when this PR is merged, because the issue number has to immediatly follow fixes (see help.github.com/en/articles/closing-issues-using-keywords), and GitHub preserves edit history anyway.

[s3bk]

@u32i64 noted and fixed. Thanks!

[sfackler]

@bors r+ rollup

[bors]

📌 Commit 8951cb5 has been approved by sfackler

[bors]

⌛ Testing commit 8951cb5 with merge 15ead0c2584b23b9b1737103436843985811708c...

[Centril]

@bors retry rolled up.

[rust-highfive]

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.

Cloning into 'rust-lang/rust'...
travis_time:end:0191af41:start=1561063788750788870,finish=1561063794750202661,duration=5999413791
$ cd rust-lang/rust
$ git checkout -qf 15ead0c2584b23b9b1737103436843985811708c
fatal: reference is not a tree: 15ead0c2584b23b9b1737103436843985811708c
The command "git checkout -qf 15ead0c2584b23b9b1737103436843985811708c" failed and exited with 128 during .
Your build has been stopped.

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)

[bors]

⌛ Testing commit 8951cb5 with merge 6306ea8b8eea727e58db96683f989e82409b70ba...

[Centril]

@bors retry rolled up.

[rust-highfive]

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.

Cloning into 'rust-lang/rust'...
travis_time:end:00186390:start=1561064109329128462,finish=1561064115493925057,duration=6164796595
$ cd rust-lang/rust
$ git checkout -qf 6306ea8b8eea727e58db96683f989e82409b70ba
fatal: reference is not a tree: 6306ea8b8eea727e58db96683f989e82409b70ba
The command "git checkout -qf 6306ea8b8eea727e58db96683f989e82409b70ba" failed and exited with 128 during .
Your build has been stopped.

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)