relegated use to alternatives, user story, clippy

Author: llogiq

text/0000-deprecation.md

@@ -6,10 +6,9 @@
 # Summary
 
 This RFC proposes to allow library authors to use a `#[deprecated]` attribute,
-with optional `since = "`*version*`"`, `reason = "`*free text*`"` and 
-`use = "`*substitute declaration*`"` fields. The compiler can then
-warn on deprecated items, while `rustdoc` can document their deprecation
-accordingly.
+with optional `since = "`*version*`"` and `reason = "`*free text*`"`fields. The 
+compiler can then warn on deprecated items, while `rustdoc` can document their 
+deprecation accordingly.
 
 # Motivation
 
@@ -30,39 +29,29 @@ fields and enum variants) can be given a `#[deprecated]` attribute. All
 possible fields are optional:
 
 * `since` is defined to contain the version of the crate at the time of
-deprecating the item, following the semver scheme. It makes no sense to put a 
-version number higher than the current newest version here, and this is not 
-checked (but could be by external lints, e.g. 
-[rust-clippy](https://github.com/Manishearth/rust-clippy).
+deprecating the item, following the semver scheme. Rustc does not know about
+versions, thus the content of this field is not checked (but will be by external
+lints, e.g. [rust-clippy](https://github.com/Manishearth/rust-clippy).
 * `reason` should contain a human-readable string outlining the reason for
 deprecating the item. While this field is not required, library authors are
 strongly advised to make use of it to convey the reason for the deprecation to 
 users of their library. The string is interpreted as plain unformatted text 
 (for now) so that rustdoc can include it in the item's documentation without 
 messing up the formatting.
-* `use`, if included, must be the import path (or a semicolon-delimited list of 
-paths) to a set of API items that will replace the functionality of the 
-deprecated item. All crates in scope can be reached by this path. E.g. let's 
-say my `foo()` item was superceded by either the `bar()` or `baz()` functions
-in the `bar` crate, in conjunction with the `bruzz(_)` function in the `baz`
-crate, I can `#[deprecate(use="bar::{bar,baz};baz::bruzz")] foo()`, as long 
-as I have the `bar` and `baz` crates in the library path. Rustc checks if the 
-item is actually available, otherwise returning an error.
 
 On use of a *deprecated* item, `rustc` will `warn` of the deprecation. Note 
-that during Cargo builds, warnings on dependencies get silenced. Note that 
-while this has the upside of keeping things tidy, it has a downside when it 
-comes to deprecation:
+that during Cargo builds, warnings on dependencies get silenced. While this has 
+the upside of keeping things tidy, it has a downside when it comes to 
+deprecation:
 
 Let's say I have my `llogiq` crate that depends on `foobar` which uses a
 deprecated item of `serde`. I will never get the warning about this unless I
 try to build `foobar` directly. We may want to create a service like `crater`
 to warn on use of deprecated items in library crates, however this is outside
 the scope of this RFC.
 
-`rustdoc` will show deprecation on items, with a `[deprecated]`
-box that may optionally show the version, reason and/or link to the replacement 
-if available.
+`rustdoc` will show deprecation on items, with a `[deprecated]` box that may 
+optionally show the version and reason where available.
 
 The language reference will be extended to describe this feature as outlined
 in this RFC. Authors shall be advised to leave their users enough time to react
@@ -71,38 +60,69 @@ before *removing* a deprecated item.
 The internally used feature can either be subsumed by this or possibly renamed
 to avoid a name clash.
 
+# Intended Use
+
+Crate author Anna wants to evolve her crate's API. She has found that one
+type, `Foo`, has a better implementation in the `rust-foo` crate. Also she has
+written a `frob(Foo)` function to replace the earlier `Foo::frobnicate(self)`
+method. 
+
+So Anna first bumps the version of her crate (because deprecation is always
+done on a version change) from `0.1.1` to `0.2.1`. She also adds the following 
+prefix to the `Foo` type:
+
+```
+extern crate rust_foo;
+
+#[deprecated(since = "0.2.1", use="rust_foo::Foo", 
+    reason="The rust_foo version is more advanced, and this crates' will likely be discontinued")]
+struct Foo { .. }
+```
+
+Users of her crate will see the following once they `cargo update` and `build`:
+
+```
+src/foo_use.rs:27:5: 27:8 warning: Foo is marked deprecated as of version 0.2.1
+src/foo_use.rs:27:5: 27:8 note: The rust_foo version is more advanced, and this crates' will likely be discontinued
+```
+
+Rust-clippy will likely gain more sophisticated checks for deprecation:
+
+* `future_deprecation` will warn on items marked as deprecated, but with a
+version lower than their crates', while `current_deprecation` will warn only on
+those items marked as deprecated where the version is equal or lower to the
+crates' one.
+* `deprecation_syntax` will check that the `since` field really contains a
+semver number and not some random string.
+
+Clippy users can then activate the clippy checks and deactivate the standard
+deprecation checks.
+
 # Drawbacks
 
-* The required checks for the `since` and `use` fields are potentially
-quite complex.
 * Once the feature is public, we can no longer change its design
 
 # Alternatives
 
 * Do nothing
 * make the `since` field required and check that it's a single version
-* Optionally the deprecation lint could check the current version as set by
-cargo in the CARGO_CRATE_VERSION environment variable (the rust build process 
-should set this environment variable, too). This would allow future 
-deprecations to be shown in the docs early, but not warned against by the
-stability lint (there could however be a `future-deprecation` lint that should
-be `Allow` by default)
 * require either `reason` or `use` be present
 * `reason` could include markdown formatting
-* The `use` could simply be plain text, which would remove much of the
-complexity here
-* The `use` field could be left out and added later. However, this would
-lead people to describe a replacement in the `reason` field, as is already
-happening in the case of rustc-private deprecation
+* rename the `reason` field to `note` to clarify it's broader usage.
+* add a `note` field and make `reason` a field with specific meaning, perhaps
+even predefine a number of valid reason strings, as JEP277 currently does
+* Add a `use` field containing a plain text of what to use instead
+* Add a `use` field containing a path to some function, type, etc. to replace
+the current feature. Currently with the rustc-private feature, people are 
+describing a replacement in the `reason` field, which is clearly not the 
+original intention of the field
 * Optionally, `cargo` could offer a new dependency category: "doc-dependencies"
 which are used to pull in other crates' documentations to link them (this is
-obviously not only relevant to deprecation).
+obviously not only relevant to deprecation)
 
 # Unresolved questions
 
 * What other restrictions should we introduce now to avoid being bound to a 
 possibly flawed design?
-* How should the multiple values in the `use` field work? Just split by
-comma or some other delimiter?
 * Can / Should the `std` library make use of the `#[deprecated]` extensions?
 * Bikeshedding: Are the names good enough?