From fb84edc0a9d30163b75e35b54cc4bf726022e3f2 Mon Sep 17 00:00:00 2001
From: John Chen <johnhuichen@gmail.com>
Date: Sun, 18 Aug 2024 09:11:19 -0400
Subject: [PATCH] rust-lang/style-team#189:
 rhs-should-use-indent-of-last-line-of-lhs

---
 src/expr.rs                              | 12 ++++++++++++
 src/shape.rs                             |  6 ++++++
 tests/source/rust-lang/style-team#189.rs | 24 ++++++++++++++++++++++++
 tests/target/rust-lang/style-team#189.rs | 24 ++++++++++++++++++++++++
 4 files changed, 66 insertions(+)
 create mode 100644 tests/source/rust-lang/style-team#189.rs
 create mode 100644 tests/target/rust-lang/style-team#189.rs

diff --git a/src/expr.rs b/src/expr.rs
index 02372e7be13..f5741b2e72a 100644
--- a/src/expr.rs
+++ b/src/expr.rs
@@ -2060,6 +2060,18 @@ fn rewrite_assignment(
     let lhs_shape = shape.sub_width(operator_str.len() + 1)?;
     let lhs_str = format!("{} {}", lhs.rewrite(context, lhs_shape)?, operator_str);
 
+    let last_line = lhs_str.rsplitn(2, "\n").next().unwrap_or_default();
+    let tab_spaces = context.config.tab_spaces();
+    let new_shape = shape
+        .block_indent(tab_spaces)
+        .saturating_sub_width(tab_spaces);
+    let extra_indent_string = new_shape.to_string(&context.config).to_string();
+    let shape = if last_line.starts_with(&extra_indent_string) {
+        new_shape
+    } else {
+        shape
+    };
+
     rewrite_assign_rhs(
         context,
         lhs_str,
diff --git a/src/shape.rs b/src/shape.rs
index 4376fd12b52..d2115befdb4 100644
--- a/src/shape.rs
+++ b/src/shape.rs
@@ -278,6 +278,12 @@ impl Shape {
         offset_indent.to_string_inner(config, 0)
     }
 
+    pub(crate) fn to_string(&self, config: &Config) -> Cow<'static, str> {
+        let mut offset_indent = self.indent;
+        offset_indent.alignment = self.offset;
+        offset_indent.to_string_inner(config, 1)
+    }
+
     /// Creates a `Shape` with a virtually infinite width.
     pub(crate) fn infinite_width(&self) -> Shape {
         Shape {
diff --git a/tests/source/rust-lang/style-team#189.rs b/tests/source/rust-lang/style-team#189.rs
new file mode 100644
index 00000000000..51c794aba14
--- /dev/null
+++ b/tests/source/rust-lang/style-team#189.rs
@@ -0,0 +1,24 @@
+// rustfmt-style_edition: 2024
+
+impl SomeType {
+    fn method(&mut self) {
+        self.array[array_index as usize]
+            .as_mut()
+            .expect("thing must exist")
+            .extra_info = Some(ExtraInfo {
+            parent,
+            count: count as u16,
+            children: children.into_boxed_slice(),
+        });
+    }
+}
+
+impl SomeType {
+    fn method(&mut self) {
+        self.array[array_index as usize]
+            .as_mut()
+            .expect("thing must exist")
+            .extra_info =
+            long_long_long_long_long_long_long_long_long_long_long_long_long_long_long;
+    }
+}
diff --git a/tests/target/rust-lang/style-team#189.rs b/tests/target/rust-lang/style-team#189.rs
new file mode 100644
index 00000000000..bd09d8b3b62
--- /dev/null
+++ b/tests/target/rust-lang/style-team#189.rs
@@ -0,0 +1,24 @@
+// rustfmt-style_edition: 2024
+
+impl SomeType {
+    fn method(&mut self) {
+        self.array[array_index as usize]
+            .as_mut()
+            .expect("thing must exist")
+            .extra_info = Some(ExtraInfo {
+                parent,
+                count: count as u16,
+                children: children.into_boxed_slice(),
+            });
+    }
+}
+
+impl SomeType {
+    fn method(&mut self) {
+        self.array[array_index as usize]
+            .as_mut()
+            .expect("thing must exist")
+            .extra_info =
+                long_long_long_long_long_long_long_long_long_long_long_long_long_long_long;
+    }
+}