Better error message when registry is added to Cargo.toml instead of Cargo config

[BatmanAoD]

Problem

Alternative registries are added in Cargo-config (e.g. ~/.cargo/config.toml), not in a project's Cargo.toml file.

If a registry is added in Cargo.toml but not used, a helpful warning is printed:

warning: unused manifest key: registries

If, however, a dependency is added that uses the alternate registry, only an error message is shown:

error: failed to parse manifest at `<path>\Cargo.toml`

Caused by:
  no index found for registry: `<name>`

This is actually somewhat confusing: I initially didn't notice that the dependencies section belongs in a different file, so when I got this error, I thought it was a problem with the registry URL or syntax.

Proposed Solution

Multiple possible solutions, with different levels of effort:

• Change the wording of the no index found message to explicitly mention config.toml. For instance, it could be reworded as registry index was not found in any config.toml file
• Print the unused manifest keys warning even if the manifest fails to parse (I'm not sure how easy this would be)
• When the no index found message occurs, explicitly check to see if Cargo.toml has a registries section, and if so, print a separate error message, something like "registries must be defined in config.toml; see https://doc.rust-lang.org/cargo/reference/config.html"
• Just permit registries to be in either config.toml or Cargo.toml (I'm assuming there's a good reason for not doing this, though).

Notes

No response

[BatmanAoD]

It looks like #12531 might indirectly make it easier to control whether warnings are still printed when there's a hard error, making the 2nd suggestion ("Print the unused manifest keys warning...") more viable.

[weihanglo]

In short term, I feel like the option one is the way to go:

registry index was not found in any configuration: {reg_name}

@rustbot label -S-triage +S-accepted +E-easy +A-error +A-registries

[torhovland]

Looks like this issue should be closed as fixed.