rustc incorrectly warn #[no_mangle] is not used when doing incremental build

[wangbj]

Steps to reproduce:

1. make an new crate, set crate type to cdylib
2. put a simple function in src/lib.rs:

#[no_mangle]
pub extern "C" fn rust_no_mangle() -> i32{
    42
}

3. build with cargo build --verbose
4. copy the rustc command line used from above step, and run it as-is

example:

$ cargo build --verbose
   Compiling nm v0.1.0 (/tmp/nm)
     Running `rustc --edition=2018 --crate-name nm src/lib.rs --color always --crate-type cdylib --emit=dep-info,link -C debuginfo=2 -C metadata=38c165a3ed3ed429 --out-dir /tmp/nm/target/debug/deps -C incremental=/tmp/nm/target/debug/incremental -L dependency=/tmp/nm/target/debug/deps`
    Finished dev [unoptimized + debuginfo] target(s) in 0.22s
$ rustc --edition=2018 --crate-name nm src/lib.rs --color always --crate-type cdylib --emit=dep-info,link -C debuginfo=2 -C metadata=38c165a3ed3ed429 --out-dir /tmp/nm/target/debug/deps -C incremental=/tmp/nm/target/debug/incremental -L dependency=/tmp/nm/target/debug/deps
warning: unused attribute
 --> src/lib.rs:1:1
  |
1 | #[no_mangle]
  | ^^^^^^^^^^^^
  |
  = note: #[warn(unused_attributes)] on by default

As you can see, the first time cargo build in step 3) has no warnings, however, step 4) cause rustc complains #[no_mangle] is unused, which isn't true.

Step 4) can be replaced by:

• modify src/lib.rs, so that it cause rebuild
• run cargo build again

rustc 1.35.0-nightly (70f1309 2019-04-16)

[MikailBag]

I have same error on today's nightly too

[bjorn3]

Same for me

[alexandermerritt]

I see the same warning text during a build for crate type “staticlib” using nightly 1.36.0 commit 5d20ff4 2019-04-18

[nagisa]

I will bisect this and then pass the baton to @oli-obk to fix this.