[beta] Give a better error message for unknown derive messages

[keeperofdakeys]

This PR improves the error message for unknown derive macros.

Currently unknown derives give the following error, which is very misleading:

`#[derive]` for custom traits is not stable enough for use. It is deprecated and will be removed in v1.15 (see issue #29644)

I'm currently working on a PR that will change this (#39442) to the following:

cannot find derive macro `Foo` in this scope

This PR backports the (as yet unmerged) error message to beta/1.16 (it's a pity that this is probably too late for 1.15).

r? @jseyfried

[rust-highfive]

Warning

• Pull requests are usually filed against the master branch for this repo, but this one is against beta. Please double check that you specified the right target!

[alexcrichton]

As a note to reviewers on this PR, we're not able to use @bors to approve branches going to beta, and it's in fact highly recommended to do so!

(just to avoid hitting the merge button)

[nikomatsakis]

@alexcrichton

You wrote (emphasis mine):

As a note to reviewers on this PR, we're not able to use @bors to approve branches going to beta, and it's in fact highly recommended to do so!

Did you mean: "we're NOW able"?

[alexcrichton]

Ooops, yes! To clarify

@bors should be used to approve this PR

To be fair the "t" and "w" keys are ... not at all close to each other.

[nikomatsakis]

I'm a bit confused, what is this a backport of? And which beta does it have to land in?

[keeperofdakeys]

On 1.15, the code #[derive(Foo)] struct A; with an unrecognised derive macro Foo gets the error message '#[derive]' for custom traits is not stable enough for use. It is deprecated and will be removed in v1.15 (see issue #29644). As part of PR #39442 the error message was updated. I intended this PR to update the error message to match that PR for the 1.16 beta/stable release.

[jseyfried]

@bors r+

[bors]

📌 Commit c3f9075 has been approved by jseyfried

[alexcrichton]

@bors: p=1

(PR against beta branch)

[bors]

⌛ Testing commit c3f9075 with merge 8fc0085...

[bors]

💔 Test failed - status-travis

[alexcrichton]

This'll need to hold off on using @bors to merge until #39546 lands unfortunately :(

[brson]

It might be prudent to wait still for patches to land on master before letting bors try to land them on beta, in case there are problems with the patch.

[alexcrichton]

@bors r=jseyfried

[bors]

📌 Commit 6519e8b has been approved by jseyfried

[bors]

⌛ Testing commit 6519e8b with merge 762a037...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: jseyfried
Pushing 762a037 to beta...