Incorrect "can't find crate for `foo`" when really the dependency of `foo` is not found.

[anforowicz]

Repro

1. Create a test crate that depends on a crate (say indexmap) that has some transitive dependencies (say equivalent):

   $ cat Cargo.toml
   [package]
   name = "transitive_dep_missing"
   version = "0.1.0"
   edition = "2024"
   
   [dependencies]
   indexmap = "2.9.0"
   
   $ cat src/main.rs
   #[allow(unused_imports)]
   use indexmap::IndexMap;
   
   fn main() {
       println!("Hello, world!");
   }
   
   $ cargo tree
   transitive_dep_missing v0.1.0 (/home/lukasza/src/cargo/transitive_dep_missing)
   └── indexmap v2.10.0
       ├── equivalent v1.0.2
       └── hashbrown v0.15.4
   

2. Build the crate normally, but use --verbose and note down the rustc command used to build the top-level crate: cargo build --verbose

3. Simulate a bug/error in the build system by trying to build the top-level crate, after deleting one of transitively depended-on crates (this is a simplification of a bug in Chromium builds system - see https://crbug.com/428272362):

   $ rm target/debug/*transitive_dep_missing*
   $ rm target/debug/deps/*equivalent*
   $ rustc --crate-name transitive_dep_missing --edition=2024 src/main.rs --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debuginfo=2 --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values())' -C metadata=f5cd9e5c1e614061 -C extra-filename=-80b808700c2f7ed4 --out-dir /home/lukasza/src/cargo/transitive_dep_missing/target/debug/deps -C incremental=/home/lukasza/src/cargo/transitive_dep_missing/target/debug/incremental -L dependency=/home/lukasza/src/cargo/transitive_dep_missing/target/debug/deps --extern indexmap=/home/lukasza/src/cargo/transitive_dep_missing/target/debug/deps/libindexmap-ed82d862ae804585.rlib
   error[E0463]: can't find crate for `indexmap`
    --> src/main.rs:2:5
     |
   2 | use indexmap::IndexMap;
     |     ^^^^^^^^ can't find crate
   
   error: aborting due to 1 previous error
   
   For more information about this error, try `rustc --explain E0463`.
   

Current output

error[E0463]: can't find crate for `indexmap`
 --> src/main.rs:2:5
  |
2 | use indexmap::IndexMap;
  |     ^^^^^^^^ can't find crate

Desired output

error[E0463]: can't find crate for `equivalent` which `indexmap` depends on
 --> src/main.rs:2:5
  |
2 | use indexmap::IndexMap;
  |     ^^^^^^^^ can't find crate

or maybe

error[E0460]: found possibly newer version of crate `equivalent` which `indexmap` depends on
 --> src/main.rs:1:1
  |
1 | extern crate indexmap;
  | ^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: perhaps that crate needs to be recompiled?
  = note: the following crate versions were found:
          crate `equivalent`: /home/lukasza/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libequivalent-a7e65d9ee6e36542.rmeta
          crate `indexmap`: /home/lukasza/src/cargo/transitive_dep_missing/target/debug/deps/libindexmap-ed82d862ae804585.rlib

Rationale and extra context

In the repro steps above we have explicitly deleted equivalent crate's rlib - this crate is missing. indexmap's rlib file is still there.

In https://crbug.com/428272362 the rlib of the transitive dependency is present, but it is present in another directory and the Chromium's build system seems to have a bug which prevents it from passing -Ldependency=... and --extern foo=path/to.rlib for the transitive dependency crate.

Other cases

Rust Version

$ rustc --version --verbose
rustc 1.90.0-nightly (706f244db 2025-06-23)
binary: rustc
commit-hash: 706f244db581212cabf2e619e0113d70999b2bbe
commit-date: 2025-06-23
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.7

Anything else?

No response

[anforowicz]

Interestingly if I use extern crate indexmap instead of use indexmap::IndexMap then the error message is improved - see below. Note that in the example below equivalent's rlib is found at another searched location, but in https://crbug.com/428272362 we would get almost the error message similar to the one that I suggest above: "error[E0463]: can't find crate for libafl_derive which libafl depends on".

$ cat src/main.rs
extern crate indexmap;

fn main() {
    println!("Hello, world!");
}

lukasza2 in /home/lukasza/src/cargo/transitive_dep_missing
$ rustc --crate-name transitive_dep_missing --edition=2024 src/main.rs --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debuginfo=2 --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values())' -C metadata=f5cd9e5c1e614061 -C extra-filename=-80b808700c2f7ed4 --out-dir /home/lukasza/src/cargo/transitive_dep_missing/target/debug/deps -C incremental=/home/lukasza/src/cargo/transitive_dep_missing/target/debug/incremental -L dependency=/home/lukasza/src/cargo/transitive_dep_missing/target/debug/deps --extern indexmap=/home/lukasza/src/cargo/transitive_dep_missing/target/debug/deps/libindexmap-ed82d862ae804585.rlib
error[E0460]: found possibly newer version of crate `equivalent` which `indexmap` depends on
 --> src/main.rs:1:1
  |
1 | extern crate indexmap;
  | ^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: perhaps that crate needs to be recompiled?
  = note: the following crate versions were found:
          crate `equivalent`: //home/lukasza/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libequivalent-a7e65d9ee6e36542.rmeta
          crate `indexmap`: /home/lukasza/src/cargo/transitive_dep_missing/target/debug/deps/libindexmap-ed82d862ae804585.rlib

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0460`.