For a project with workspace, Cargo returns issue relative to the workspace root directory. cargo.el instead find the directory with Cargo.toml, which can be different. When these two project root directory differ, Emacs compilation mode fails to find the correct file path. To a user, Emacs is unable to jump to the error location and a window prompt is open.

In this demo project, the file in add-one/src/lib.rs has a typo x1. When one does a cargo build, we see the following compilation log:

-*- mode: cargo-process; default-directory: "~/repos/cargo.el-workspace/add-one/" -*-
Cargo-Process started at Mon Dec 25 15:50:13

cargo build
   Compiling add-one v0.1.0 (file:///Users/benzh/repos/cargo.el-workspace/add-one)
error[E0425]: cannot find value `x1` in this scope
 --> add-one/src/lib.rs:2:5
  |
2 |     x1 + 1
  |     ^^ did you mean `x`?

error: aborting due to previous error

error: Could not compile `add-one`.

To learn more, run the command again with --verbose.

Cargo-Process exited abnormally with code 101 at Mon Dec 25 15:50:15

And Emacs wants to open a non-existant file ~/repos/cargo.el-workspace/add-one/add-one/src/lib.rs. cargo.el finds file relative to ~/repos/cargo.el-workspace/add-one/ while Cargo ~/repos/cargo.el-workspace/.

host:add-one$ rustup default nightly
info: using existing install for 'nightly-x86_64-apple-darwin'
info: default toolchain set to 'nightly-x86_64-apple-darwin'

^P  nightly-x86_64-apple-darwin unchanged - rustc 1.24.0-nightly (c284f8807 2017-12-24)

host:add-one$ cargo build
   Compiling add-one v0.1.0 (file:///Users/benzh/repos/cargo.el-workspace/add-one)
error[E0425]: cannot find value `x1` in this scope
 --> add-one/src/lib.rs:2:5
  |
2 |     x1 + 1
  |     ^^ did you mean `x`?

error: aborting due to previous error

error: Could not compile `add-one`.

To learn more, run the command again with --verbose.

host:add-one$ rustup default stable
info: using existing install for 'stable-x86_64-apple-darwin'
info: default toolchain set to 'stable-x86_64-apple-darwin'

^P  stable-x86_64-apple-darwin unchanged - rustc 1.22.1 (05e2e1c41 2017-11-22)

host:add-one$ cargo build
   Compiling add-one v0.1.0 (file:///Users/benzh/repos/cargo.el-workspace/add-one)
error[E0425]: cannot find value `x1` in this scope
 --> src/lib.rs:2:5
  |
2 |     x1 + 1
  |     ^^ did you mean `x`?

error: aborting due to previous error

error: Could not compile `add-one`.

To learn more, run the command again with --verbose.