Better diagnostic for fn items in variadic functions

[dev-ardi]

closes #69232

[rustbot]

r? @davidtwco

rustbot has assigned @davidtwco.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[compiler-errors]

This is not how you coerce a function item to a function pointer. Instead, you've suggested to turn something into a pointer-to-a-fn-item (i.e. a pointer to a ZST). You need to use as fn() to coerce a function item to a function pointer.

Please fix this, and also squash this into one commit.

[compiler-errors]

nit: you also don't need to format the suggestion as a snippet. You can use span.shrink_to_{hi,lo}() to get just the beginning or end of the span to add just the part you needed (in this case &, but in the corrected case as ...)

[rust-log-analyzer]

The job x86_64-gnu-llvm-18 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

#16 exporting to docker image format
#16 sending tarball 27.1s done
#16 DONE 33.0s
##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-18]
debug: `DISABLE_CI_RUSTC_IF_INCOMPATIBLE` configured.
---
sccache: Starting the server...
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-18', '--enable-llvm-link-shared', '--set', 'rust.randomize-layout=true', '--set', 'rust.thin-lto-import-instr-limit=10', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'rust.lld=false', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-18/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.randomize-layout := True
configure: rust.thin-lto-import-instr-limit := 10
---
1 error[E0617]: can't pass a function item to a variadic function
-   --> $DIR/issue-69232.rs:12:21
+   --> $DIR/fn-item-diagnostic-issue-69232.rs:12:21
3    |
4 LL |     unsafe { foo(1, test) };
5    |                     ^^^^ help: use a function pointer instead: `&test`

The actual stderr differed from the expected stderr.
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args c-variadic/fn-item-diagnostic-issue-69232.rs`
To only update this specific test, also pass `--test-args c-variadic/fn-item-diagnostic-issue-69232.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/c-variadic/fn-item-diagnostic-issue-69232.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2" "--target=x86_64-unknown-linux-gnu" "--check-cfg" "cfg(FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/c-variadic/fn-item-diagnostic-issue-69232" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers"
--- stderr -------------------------------
error[E0617]: can't pass a function item to a variadic function
##[error]  --> /checkout/tests/ui/c-variadic/fn-item-diagnostic-issue-69232.rs:12:21
   |
   |
LL |     unsafe { foo(1, test) }; //~ ERROR can't pass a function item to a variadic function
   |                     ^^^^ help: use a function pointer instead: `&test`
   = help: a function item is zero-sized and needs to be casted into a pointer-sized function pointer to be used in FFI
   = note: for more information on function items, visit https://doc.rust-lang.org/reference/types/function-item.html

error: aborting due to 1 previous error

[compiler-errors]

also, the past participle of cast is "cast"

Suggested change

	.help = a function item is zero-sized and needs to be casted into a pointer-sized function pointer to be used in FFI
	.help = a function item is zero-sized and needs to be cast into a function pointer to be used in FFI

Also, the fact that it's zero-sized doesn't seem to me to be the most important part of this issue. The issue here is more that the intention is wrong, since you're not passing a function pointer like you expect.