Writing an RFC

Generally, an RFC goes through:

1. Pre-RFC discussions on the internals forum
2. RFC
3. Development and stabilization

Please keep in mind our design principles.

For more concrete areas of consideration:

.cargo/config.toml and Cargo.toml

.cargo/config.toml is for environment or transient configuration, being dependent on what directory you are running from and settable on the command-line, independent of other flags like --manifest-path or --package.

On the other hand Cargo.toml is for static, high-level project configuration.

For example,

• RFC 3537 chose configuration for the MSRV-aware resolver because users would likely need to change this setting, like in CI to verify the opposite case of what they run by default.
• The Cargo team rejected a [cfg] table to represent rustc --cfg flags as it was a direct port of low-level rustc behavior that didn’t mesh with the other high level abstractions of manifests.
  • For stabilization, this was worked around through a build script directive and a [lints] field configuration.
• #12738 for exploring how existing config might be representable in Cargo.toml.

Cargo.toml

When adding a table to a manifest,

• Should it be inheritable?
• Ensure the package table and the inheritable table under workspace align
• Care is needed to ensure a workspace = true field doesn’t conflict with other entries
  • e.g. RFC 3389 had to explicitly exclude ever supporting a workspace linter

When adding a field,

• Is it inheritable?
  • Consider whether sharing of the field would be driven by requirements or is a manifestation of the current implementation. For example, in most cases, dependency sources (e.g. version field) should be aligned across a workspace However, frequently dependency features will vary across a workspace.
• When inheriting, can specify it in your package?
• How does specifying a field in both workspace and a package interact?
  • e.g. dependency sources cannot be overridden
  • e.g. dependency features get merged
  • e.g. dependency default-features has been hard to get right (#12162)

When working extending dependencies tables:

• How does this affect cargo add or cargo remove?
• How does this affect [patches] which are just modified dependencies?