Add rustfmt.toml file (#18197)

My editor runs `rustfmt` on save to format Rust code, not `cargo fmt`.

With our recent bump to the Rust 2024 edition, the formatting that
`rustfmt`/`cargo fmt` applies changed. Unfortunately, `rustfmt` and
`cargo fmt` have different behaviors for determining which edition to
use when formatting: `cargo fmt` looks for the Rust edition in
`Cargo.toml`, whereas `rustfmt` looks for it in `rustfmt.toml`. As a
result, whenever I save, I have to remember to manually run `cargo fmt`
before committing/pushing.

There is an open issue asking for `rustfmt` to also look at `Cargo.toml`
when it's present (https://github.com/rust-lang/rust.vim/issues/368),
but it seems like they "closed" that issue just by bumping the default
edition (six years ago, from 2015 to 2018).

In the meantime, this PR adds a `rustfmt.toml` file with our current
Rust edition so that both invocation have the same behavior. I don't
love that this duplicates information in `Cargo.toml`, but I've added a
reminder comment there to hopefully ensure that we bump the edition in
both places three years from now.

Cargo.toml

@@ -3,6 +3,7 @@ members = ["crates/*"]
 resolver = "2"
 
 [workspace.package]
+# Please update rustfmt.toml when bumping the Rust edition
 edition = "2024"
 rust-version = "1.85"
 homepage = "https://docs.astral.sh/ruff"

crates/ruff_text_size/Cargo.toml

@@ -2,8 +2,13 @@
 name = "ruff_text_size"
 version = "0.0.0"
 publish = false
-edition = "2021"
+authors = { workspace = true }
-rust-version = "1.67.1"
+edition = { workspace = true }
+rust-version = { workspace = true }
+homepage = { workspace = true }
+documentation = { workspace = true }
+repository = { workspace = true }
+license = { workspace = true }
 
 [dependencies]
 serde = { workspace = true, optional = true }

crates/ruff_text_size/src/schemars_impls.rs

@@ -6,7 +6,7 @@
 //! bindings to the Workspace API
 
 use crate::{TextRange, TextSize};
-use schemars::{r#gen::SchemaGenerator, schema::Schema, JsonSchema};
+use schemars::{JsonSchema, r#gen::SchemaGenerator, schema::Schema};
 
 impl JsonSchema for TextSize {
     fn schema_name() -> String {

crates/ruff_text_size/src/serde_impls.rs

@@ -1,6 +1,6 @@
 use {
     crate::{TextRange, TextSize},
-    serde::{de, Deserialize, Deserializer, Serialize, Serializer},
+    serde::{Deserialize, Deserializer, Serialize, Serializer, de},
 };
 
 impl Serialize for TextSize {

crates/ruff_text_size/tests/serde.rs

@@ -1,6 +1,6 @@
 use {
     ruff_text_size::{TextRange, TextSize},
-    serde_test::{assert_de_tokens_error, assert_tokens, Token},
+    serde_test::{Token, assert_de_tokens_error, assert_tokens},
     std::ops,
 };
 

rustfmt.toml

@@ -0,0 +1,2 @@
+edition = "2024"
+style_edition = "2024"