RFC: Deprecate the per-build-target edition field in Cargo.toml

text/3772-build-target-edition.md

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+- Feature Name: `build-target-edition`
+- Start Date: 2025-02-13
+- RFC PR: [rust-lang/rfcs#3772](https://github.com/rust-lang/rfcs/pull/3772)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+Deprecate `lib.edition`, etc in favor of only setting `package.edition`, removing the fields in the next Edition.
+
+# Motivation
+[motivation]: #motivation
+
+Cargo supports setting the edition per-build-target:
+```toml
+[package]
+name = "foo"
+edition = "2021"
+
+[lib]
+edition = "2015"
+
+[[bin]]
+name = "foo"
+path = "src/main.rs"
+edition = "2015"
+
+[[example]]
+name = "foo"
+path = "examples/foo.rs"
+edition = "2015"
+
+[[test]]
+name = "foo"
+path = "tests/foo.rs"
+edition = "2015"
+
+[[bench]]
+name = "foo"
+path = "benches/foo.rs"
+edition = "2015"
+```
+
+This was intended for ([cargo#5661](https://github.com/rust-lang/cargo/issues/5661)):
+- Migrating to a new edition per build-target
+- Per edition tests
+
+In practice, this feature does not seem to be in common use.
+Searching the latest `Cargo.toml` files of every package on crates.io,
+we found 13 packages using this feature
+([zulip](https://rust-lang.zulipchat.com/#narrow/channel/246057-t-cargo/topic/Deprecate.20build-target.20.60edition.60.20field.3F/near/499047806)):
+- 4 set `edition` on the sole build-target, rather than on the `package`
+- 3 set `edition` because they enumerated every build-target field but then forgot to update them when updating `package.edition`
+- 3 (+2 forks) have per-edition tests
+- 1 has every version yanked
+
+While this does not account for transient use of this feature during an Edition migration,
+from our experience and observing others, we think this practice is not very common.
+In fact, it seems more likely that migrating a lint at a time may be more beneficial
+([cargo#11125](https://github.com/rust-lang/cargo/issues/11125#issuecomment-2641119791)).
+There is also an open question on the Cargo team on how much to focus on multiple build-targets per package
+vs individual packages
+(see [This Development-cycle in Cargo: 1.77](https://blog.rust-lang.org/inside-rust/2024/02/13/this-development-cycle-in-cargo-1-77.html#when-to-use-packages-or-workspaces)).
+
+Drawbacks of this feature include:
+- Using this has a lot of friction as users have to explicitly
+  enumerate each build target they want to set `edition` on which usually requires
+  also setting the `name` and `path`.
+- This has led to bugs where people thought they migrated editions but did not
+- This is an easily overlooked feature to take into account when extending Cargo
+- Cargo cannot tell a `build.rs` what Edition to generate code for as it may generate code for one of several
+  ([cargo#6408](https://github.com/rust-lang/cargo/issues/6408)).
+  This will become more important once we have [metabuild](https://github.com/rust-lang/cargo/issues/14903) for delegating build scripts to dependencies.
+- Making it more difficult for tools like `cargo fmt`
+  ([rustfmt#5071](https://github.com/rust-lang/rustfmt/pull/5071)) which need to map
+  a file back to its edition which requires heuristics to associate a `.rs`
+  file with a `Cargo.toml` and then to associate it with a specific build-target.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Documentation updates:
+
+## Configuring a target
+
+*From the [Cargo book](https://doc.rust-lang.org/cargo/reference/cargo-targets.html#configuring-a-target)*
+
+...
+
+```toml
+[lib]
+name = "foo"           # The name of the target.
+path = "src/lib.rs"    # The source file of the target.
+test = true            # Is tested by default.
+doctest = true         # Documentation examples are tested by default.
+bench = true           # Is benchmarked by default.
+doc = true             # Is documented by default.
+proc-macro = false     # Set to `true` for a proc-macro library.
+harness = true         # Use libtest harness.
+crate-type = ["lib"]   # The crate types to generate.
+required-features = [] # Features required to build this target (N/A for lib).
+
+edition = "2015"       # Deprecated, N/A for Edition 20XX+
+```
+
+...
+
+### The `edition` field
+
+The `edition` field defines the [Rust edition] the target will use. If not
+specified, it defaults to the [`edition` field][package-edition] for the
+`[package]`.
+
+This field is deprecated and unsupported for Edition 20XX+
+
+## Migration guide
+
+*From [Rust Edition Guide: Advanced migration strategies](https://doc.rust-lang.org/nightly/edition-guide/editions/advanced-migrations.html#migrating-a-large-project-or-workspace)*
+
+### Migrating a large project or workspace
+
+You can migrate a large project incrementally to make the process easier if you run into problems.
+
+In a [Cargo workspace], each package defines its own edition, so the process naturally involves migrating one package at a time.
+
+Within a [Cargo package], you can either migrate the entire package at once, or migrate individual [Cargo targets] one at a time.
+For example, if you have multiple binaries, tests, and examples, you can use specific target selection flags with `cargo fix --edition` to migrate just that one target.
+By default, `cargo fix` uses `--all-targets`.
+
+*(removed talk of the build-target `edition` field)*
+
+### Migrating macros
+
+...
+
+If you have macros, you are encouraged to make sure you have tests that fully
+cover the macro's syntax. You may also want to test the macros by importing and
+using them in crates from multiple editions, just to ensure it works correctly
+everywhere.
+You can do this in doctests by setting the [edition attribute](https://doc.rust-lang.org/stable/rustdoc/write-documentation/documentation-tests.html#attributes)
+or by creating a package for your tests in your workspace for each edition.
+
+If you run into issues, you'll need to read through the chapters of
+this guide to understand how the code can be changed to work across all
+editions.
+
+*(added a testing strategy which was previously left unspoken)*
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+A non-`None` edition will be considered deprecated
+- A deprecation message will eventually be shown by Cargo
+  - Timing depends on if this will be blocked on having `[lints]` control over this or not
+- When `package.edition` is set to Edition 20XX+, an error will be reported when a `<build-target>.edition` field is set.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+- This makes testing macros more difficult as they are limited to either
+  - doctests
+  - creating packages just for the sake of defining tests
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+## One Edition field controlling another
+
+The exact semantics of `package.edition` vs `<build-target>.edition` have not been well defined when it comes to the manifest format itself.
+
+`package.edition`'s [documentation](https://doc.rust-lang.org/cargo/reference/manifest.html#the-edition-field) says:
+
+> [it] affects which Rust Edition your package is compiled with
+
+while `<build-target>.edition` [documentation](https://doc.rust-lang.org/cargo/reference/cargo-targets.html#the-edition-field) says:
+
+> [it] defines the Rust edition the target will use
+
+For Edition 2024, support for `<build-target>.proc_macro` and `<build-target>.crate_type`
+was removed based on `package.edition` and not `<build-target>.edition`.
+
+By having `package.edition` affect `<build-target>.edition`,
+we are effectively saying that `package.edition` affects the manifest format
+while `<build-target>.edition` affects only affects the source code of the build-target.
+
+# Prior art
+[prior-art]: #prior-art
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+- When will Cargo start to report the deprecation message?
+  - Cargo currently lacks lint control for itself ([cargo#12235](https://github.com/rust-lang/cargo/issues/12235)) which we could wait for
+  - We could unconditionally report the warning but the Cargo team avoids adding warnings without a way of suppressing them without changing behavior
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+- Reporting the Edition to `build.rs`