[rust] [chromium] Minimal rustfmt support for git cl format.

This CL supplements
https://crrev.com/c/chromium/tools/depot_tools/+/3054980 that added
minimal `rustfmt` support to `depot_tools` repo (e.g. to `git cl format`
and `git cl presubmit` commands).

This CL:
- Provides the baseline configuration for `rustfmt`: .rustfmt.toml
- Tells `git` and `rustfmt` that `.rs` files should have Unix line
  endings
- Fixes minor formatting issues that have crept into `.rs` files
  that have already landed in Chromium repo
- Does some other bookkeeping (e.g. providing //styleguide/rust/OWNERS
  for symmetry with how C++ style and the top-level .clang-tidy are
  managed)

Manual tests:

    Setup:
    - Edit .rs files under build/rust/tests
    - Temporarily change the top-level .rustfmt.toml to say:
          comment_width = 40
          wrap_comments = true
    Test:
    - Run `git cl format`
    Verified behavior:
    - New formatting style was applied to the edited .rs file.

Bug: 1231317
Change-Id: I006ed8065dbb0a212162337dc91310d149d38aeb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3187872
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Erik Staab <estaab@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#927003}

.gitattributes

@@ -28,8 +28,10 @@
 *.mojom text eol=lf
 *.pdf -diff
 *.proto text eol=lf
+*.rs text eol=lf
 *.sh text eol=lf
 *.sql   text eol=lf
+*.toml  text eol=lf
 *.txt   text eol=lf
 *.xml   text eol=lf
 *.xslt   text eol=lf

.rustfmt.toml

@@ -0,0 +1,18 @@
+# This file defines the Rust style for automatic reformatting.
+# See also https://rust-lang.github.io/rustfmt
+
+# Rust language edition to be used by the parser.
+edition = "2021"
+
+# Version of the formatting rules to use.
+version = "Two"
+
+# Line endings will be converted to \n.
+newline_style = "Unix"
+
+# The "Default" setting has a heuristic which splits lines too aggresively.
+# We are willing to revisit this setting in future versions of rustfmt.
+# Bugs:
+# * https://github.com/rust-lang/rustfmt/issues/3119
+# * https://github.com/rust-lang/rustfmt/issues/3120
+use_small_heuristics = "Max"

OWNERS

@@ -15,6 +15,7 @@ per-file .git-blame-ignore-revs=thakis@chromium.org
 per-file .gn=estaab@chromium.org
 per-file .gn=file://build/OWNERS
 per-file .mailmap=*
+per-file .rustfmt.toml=file://styleguide/rust/OWNERS
 per-file .vpython*=dpranke@google.com
 per-file .vpython*=iannucci@chromium.org
 per-file .yapfignore=*

build/rust/tests/test_rust_source_set/main.rs

@@ -10,7 +10,8 @@ pub extern "C" fn say_hello_from_cpp() {
 pub fn say_hello() {
     println!(
         "Hello, world - from a Rust library. Calculations suggest that 3+4={}",
-        add_two_ints_via_rust(3, 4));
+        add_two_ints_via_rust(3, 4)
+    );
 }
 
 #[test]

styleguide/rust/OWNERS

@@ -0,0 +1,3 @@
+# TODO(danakj): We should consider having different style OWNERS from
+# code and toolchain OWNERS.  OTOH, for now forwarding seems okay.
+file://build/rust/OWNERS