PEP 723: Correct the comparison with Rust (#3277)

In working on the Rust design for single-file packages, we found that
there were too many decisions to make up-front in an RFC, so we
instead opted to approve an experiment (eRFC) to figure out the design
incrementally and then come back with a final proposal. This
implementation is considered unstable and requires a special flag
(``-Zscript``) to access within the nightly builds of Rust.

This commit updates the description of Rust's approach to align
with these facts.

Co-authored-by: Adam Turner <9087854+aa-turner@users.noreply.github.com>

pep-0723.rst

@@ -54,11 +54,11 @@ This PEP defines a mechanism for embedding metadata *within the script itself*,
 and not in an external file.
 
 We choose to follow the latest developments of other modern packaging
-ecosystems (namely `Rust`__ and `Go`__) by embedding the existing
+ecosystems (namely `Go`__ and provisionally `Rust`__) by embedding the existing
 `metadata standard <pyproject metadata_>`_ that is used to describe
 projects.
 
-__ https://github.com/rust-lang/rfcs/blob/master/text/3424-cargo-script.md
+__ https://rust-lang.github.io/rfcs/3424-cargo-script.html
 __ https://github.com/erning/gorun
 
 The format is intended to bridge the gap between different types of users
@@ -139,8 +139,9 @@ The following is an example of a script with an embedded ``pyproject.toml``:
     data = resp.json()
     pprint([(k, v["title"]) for k, v in data.items()][:10])
 
-The following is an example of a single-file Rust project that embeds their
-version of ``pyproject.toml``, which is called ``Cargo.toml``:
+The following is an example of a proposed syntax for single-file Rust project
+that embeds their equivalent of ``pyproject.toml``,
+which is called ``Cargo.toml``:
 
 .. code:: rust
 
@@ -156,19 +157,25 @@ version of ``pyproject.toml``, which is called ``Cargo.toml``:
         println!("Did our date match? {}", re.is_match("2014-01-01"));
     }
 
-One important thing to note is that the metadata is embedded in a comment
-mostly for introspection since Rust documentation is generated from comments.
-Another is that users rarely edit dependencies manually, but rather use their
-Cargo package manager.
+One important thing to note is that the metadata is embedded in a
+`doc-comment`_ (their equivalent of docstrings).
+`Other syntaxes <cargo embedded manifest_>`_ are under consideration
+within the Rust project,
+including using attributes which are somewhat like a
+syntactically recognized equivalent of dunder variables,
+with the key difference between Rust's choice and this PEP being that
+any valid Rust syntax will be allowed,
+requiring one of the Rust syntax parsers to work with it, like `syn`__.
 
-We argue that our choice, in comparison to the Rust format, is easier to read
-and provides easier edits for humans by virtue of the contents starting at the
-beginning of lines so would precisely match the contents of a
-``pyproject.toml`` file. It is also is easier for tools to parse and modify
-this continuous block of text which was `one of the concerns`__ raised in the
-Rust pre-RFC.
+__ https://crates.io/crates/syn
 
-__ https://github.com/epage/cargo-script-mvs/blob/main/0000-cargo-script.md#embedded-manifest-format
+We argue that our choice, in comparison to the `doc-comment`_ approach,
+is easier to read and provides easier edits for humans by virtue
+of the contents starting at the beginning of lines so would precisely match
+the contents of a ``pyproject.toml`` file.
+It is also is easier for tools to parse and modify this continuous block
+of text which was `one of the concerns <cargo embedded manifest_>`_
+raised in the Rust pre-RFC.
 
 Reference Implementation
 ========================
@@ -561,6 +568,8 @@ References
 ==========
 
 .. _pyproject metadata: https://packaging.python.org/en/latest/specifications/declaring-project-metadata/
+.. _doc-comment: https://doc.rust-lang.org/stable/book/ch14-02-publishing-to-crates-io.html#making-useful-documentation-comments
+.. _cargo embedded manifest: https://github.com/epage/cargo-script-mvs/blob/main/0000-cargo-script.md#embedded-manifest-format
 .. _pip-run issue: https://github.com/jaraco/pip-run/issues/44
 .. _pyproject without wheels: https://discuss.python.org/t/projects-that-arent-meant-to-generate-a-wheel-and-pyproject-toml/29684