Improve the diagnostics in virual workspaces

[VorpalBlade]

Problem

I tried to add the following to my virtual workspace:

[lints.rust]
rust-2018-idioms = "warn"

This results in the following diagnostic:

❯ cargo check
error: failed to parse manifest at `/home/user/src/project/Cargo.toml`

Caused by:
  this virtual manifest specifies a `lints` section, which is not allowed

That diagnostic doesn't give a hint about that the solution is to rename the table to workspace.lints.rust. And googling the error did not turn up very useful results either.

Proposed Solution

❯ cargo check
error: failed to parse manifest at `/home/user/src/project/Cargo.toml`

Caused by:
  this virtual manifest specifies a `lints` section, which is not allowed
Suggestion:
  For workspaces, use `workspace.lints` instead

Note the added suggestion. Rustc is usually very good about giving useful hints. I assume Cargo should be able to do so too.

There is a question about what to do when you have a non-virtual workspace. It is not a feature I use myself, so I have no input on the diagnostic in that case.

Notes

No response

[VorpalBlade]

As an added note, it seems inconsistent which keys need to be prefixed by workspace. in workspaces. There is workspace.dependencies but not workspace.profile.release

[heisen-li]

@rustbot label +A-diagnostics +A-workspaces

[epage]

As an added note, it seems inconsistent which keys need to be prefixed by workspace. in workspaces. There is workspace.dependencies but not workspace.profile.release

The schema for Cargo.toml files is generally flat.

• Most package sub-tables, like dependencies, are top-level
• Most workspace sub-tables, like profile, are top-level

We parented inheritable tables under workspace to disambiguate them from the regular top-level table

• package -> workspace.package
• *dependencies -> workspace.dependencies
• lints -> workspace.lints

These tables (mostly) have no effect on their own but are used to help populate regular tables.