Fix dead_code_pub_in_binary false positive on library crates

[Kokoro2336]

Fixes #159078

Fix dead_code_pub_in_binary false positive on public items when compiling library crates with --all-targets flag

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[fmease]

Wasn't #74970 (comment) a somewhat important use case for this lint firing under --test? IINM that would no longer be served. cc @RalfJung

After all, crates under tests are binaries with an auto-generated main function. However, this might be considered an implementation detail, so I understand the desire to change this.

Ideally, we would have some replacement for the linked use case, then.

Disclaimer: I've only skimmed everything so I might've missed something obvious.

[RalfJung]

That test I mentioned there was a ui test, i.e. a regular binary crate -- no --test involved.

[Kokoro2336]

All the crates compiled with a --test flag would be considered as CrateType::Executable, making it tricky to recognize the actual type of the compiled crate when running check_mod_deadness. I used opts.crate_types to distinguish it, though I don't think it's an optimal choice since it's read from cmd args.

[Kokoro2336]

Plus, I've supplied comparison tests between normal crate and library crate.

[PaulDance]

The way the added tests look as they are now seems good to me, or at least the most intuitive at first glance. However, would it be possible to also include ones for other targets such as integration tests and examples? That would help ensure that unused public items in these would still get detected by the lint, which would make sense I think, as these effectively are separate binaries.

[mu001999]

I'm afraid this is not enough to fix #159078.

I tried this locally with modified note, and I got:

~/r/p/p/tmp » cargo +dev check --all-targets
info: `cargo` is unavailable for the active toolchain
info: falling back to "/Users/bytedance/.rustup/toolchains/nightly-aarch64-apple-darwin/bin/cargo"
warning: function `hello` is never used
 --> src/lib.rs:5:8
  |
5 | pub fn hello() {}
  |        ^^^^^
  |
  = note: (edited) in libraries, `pub` items can be used by dependent crates; in binaries, they cannot, so this `pub` item is unused
  = note: requested on the command line with `-W dead-code-pub-in-binary`

warning: `tmp` (lib test) generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.00s

What I modified is:

~/r/p/rust @ fix/issue-159078* » git rev-parse HEAD
9e2fe0d6a28c1a870ee1135dde0a7166908c51bb
~/r/p/rust @ fix/issue-159078* » git diff
diff --git a/compiler/rustc_passes/src/diagnostics.rs b/compiler/rustc_passes/src/diagnostics.rs
index 2589941d7da..3a6f5b94517 100644
--- a/compiler/rustc_passes/src/diagnostics.rs
+++ b/compiler/rustc_passes/src/diagnostics.rs
@@ -869,7 +869,7 @@ pub(crate) enum MultipleDeadCodes<'tcx> {
 
 #[derive(Subdiagnostic)]
 #[note(
-    "in libraries, `pub` items can be used by dependent crates; in binaries, they cannot, so this `pub` item is unused"
+    "(edited) in libraries, `pub` items can be used by dependent crates; in binaries, they cannot, so this `pub` item is unused"
 )]
 pub(crate) struct DeadCodePubInBinaryNote;

And cargo +dev test --no-run will also emit this.