Suggested `cargo fix` command fails for workspace packages not in `default-members`

[Alexendoo]

Problem

With the following root Cargo.toml:

[workspace]
members = ["a"]
default-members = []
resolver = "3"

If the workspace member a emits a fixable diagnostic, e.g.

% cargo check -p a
    Checking a v0.1.0 (/home/gh-Alexendoo/temp/a)
warning: unnecessary parentheses around assigned value
 --> a/src/main.rs:2:13
  |
2 |     let _ = (1);
  |             ^ ^
  |
  = note: `#[warn(unused_parens)]` on by default
help: remove these parentheses
  |
2 -     let _ = (1);
2 +     let _ = 1;
  |

warning: `a` (bin "a") generated 1 warning (run `cargo fix --bin "a"` to apply 1 suggestion)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.05s

Running the suggested command results in an error:

% cargo fix --bin "a"
error: manifest path `/home/gh-Alexendoo/temp` contains no package: The manifest is virtual, and the workspace has no members.

Steps

No response

Possible Solution(s)

No response

Notes

No response

Version

cargo 1.90.0 (840b83a10 2025-07-30)
release: 1.90.0
commit-hash: 840b83a10fb0e039a83f4d70ad032892c287570a
commit-date: 2025-07-30
host: aarch64-unknown-linux-gnu
libgit2: 1.9.1 (sys:0.20.2 vendored)
libcurl: 8.14.1-DEV (sys:0.4.82+curl-8.14.1 vendored ssl:OpenSSL/3.5.0)
ssl: OpenSSL 3.5.0 8 Apr 2025
os: Ubuntu 24.4.0 (noble) [64-bit]

[weihanglo]

It is not only about virtual manifest. It is about the suggestions not making its best effort to respect workspace settings.

We should probably always append -p <pkg> to disambiguate:

cargo/src/cargo/core/compiler/job_queue/mod.rs

Lines 1089 to 1095 in 8325770

	let named = unit.target.description_named();
	// if its a lib we need to add the package to fix
	if unit.target.is_lib() {
	format!("{} -p {}", named, unit.pkg.name())
	} else {
	named
	}

Alternatively, we could be a bit smater like checking the current working direcotry and the workspace setting to avoid a no-op -p, but that sound too much works.

@rustbot label +S-accepted -S-triage +E-easy