Deny (by default) transmuting from fn item types to pointer-sized types.

[eddyb]

This sets the #19925 lint (transmute from zero-sized fn item type) to deny by default.
Technically a [breaking-change], but will not affect dependent crates because of --cap-lints.

[rust-highfive]

r? @Aatch

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[eddyb]

@nikomatsakis This supersedes #34198, for now. I've started a three-way crater build on both PRs, to measure the impact for both changes.

[eddyb]

Argh, the report came out half-baked. All of the cases I checked out are just timeouts, while updating the registry. cc @brson

[eddyb]

The good crater report shows 3 regressions, td_rlua, hlua_master (both of which seem abandoned), and zmq-rs, which needs a new release (cc @Ginurx).

I've also done a crater run for #34198, for which the report shows 3 more regressions (because --cap-lints is not in effect there): glutin_cocoa, carto and syslog-ng-common, all of which are using outdated versions of fixed crates.

[nikomatsakis]

cc @rust-lang/compiler @rust-lang/core -- another question of making warnings into a hard error, though in this case we opted for a more gradual approach by making the lint become Deny By Default, since the impact of this change was initially large.

@eddyb's measurements show 3 regressions:

The good crater report shows 3 regressions, td_rlua, hlua_master (both of which seem abandoned), and zmq-rs, which needs a new release (cc @Ginurx).

My vote is let's go for it.

[alexcrichton]

🍈

(I'm in favor)

[steveklabnik]

👍

[eddyb]

@nikomatsakis Are we go?

[nikomatsakis]

@eddyb interestingly, @wycats was just telling me that he thinks the lint may be missing some scenarios in this code. He's going to investigate a bit.

Seems worth knowing about before we pull the trigger.

[nikomatsakis]

@wycats any updates here? Do you have examples where you think the lint is failing to fire or be reported?

[nikomatsakis]

OK, I think we should land this. It's not irreversible.

[nikomatsakis]

@bors r+

[bors]

📌 Commit 7de83f0 has been approved by nikomatsakis

[bors]

⌛ Testing commit 7de83f0 with merge 71bdeea...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-no-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-no-mir, auto-win-msvc-64-opt-rustbuild
Approved by: nikomatsakis
Pushing 71bdeea to master...