This is another experiment for #9930.
Comparing preferring over exclusively using MSRV compatible:
Benefits
- Better error messages
- `--ignore-rust-version` is implicitly sticky
Downsides
- Can't backtrack for MSRV compatible version
- Still requires workspace-wide MSRV (compared to our desired end state of declaring MSRV as yet another dependency)
This builds on #12930
Generally, cargo avoids positional arguments. Mostly for the commands
that might forward arguments to another command, like `cargo test`.
It also allows some flexibility in turning flags into options.
For `cargo add` and `cargo remove`, we decided to accept positionals
because the motivations didn't seem to apply as much (similar to `cargo
install`).
This applies the pattern to `cargo update` as well which is in the same
category of commands as `cargo add` and `cargo remove`.
As for `--help` formatting, I'm mixed on whether `[SPEC]...` should be at the top like
other positionals or should be relegated to "Package selection". I went
with the latter mostly to make it easier to visualize the less common
choice.
Switching to a positional for `cargo update` (while keeping `-p` for
backwards compatibility) was referenced in #12425.
Packages in the local workspace can't get updated this way; the user
just needs to point to a different source, or otherwise update the
package themselves.
People encountering a dependency with a newer `rust-version` requirement
may not know about `cargo update --precise`, or may consider alternate
approaches that may be harmful (such as pinning with a `=` dependency).
Provide specific guidance in the error message.
If the user is using `cargo install`, suggest `cargo install --locked` instead.
