Fix incorrect string indentation in macro defs with format_strings

CHANGELOG.md

@@ -40,6 +40,7 @@
 - Handles cases where certain control flow type expressions have comments between patterns/keywords and the pattern ident contains the keyword [#5009](https://github.com/rust-lang/rustfmt/issues/5009)
 - Handles tuple structs that have explicit visibilities and start with a block style comment [#5011](https://github.com/rust-lang/rustfmt/issues/5011)
 - Handles leading line-style comments in certain types of macro calls [#4615](https://github.com/rust-lang/rustfmt/issues/4615)
+- bizarre string indentation in macro defs with `format_strings` [#4036](https://github.com/rust-lang/rustfmt/issues/4036)
 
 
 ### Added

src/utils.rs

@@ -647,8 +647,14 @@ pub(crate) fn trim_left_preserve_layout(
 
 /// Based on the given line, determine if the next line can be indented or not.
 /// This allows to preserve the indentation of multi-line literals.
-pub(crate) fn indent_next_line(kind: FullCodeCharKind, _line: &str, config: &Config) -> bool {
-    !(kind.is_string() || (config.version() == Version::Two && kind.is_commented_string()))
+///
+/// We indicate that multi-line literals need indentation when `format_strings =
+/// true` as the string will only be indented with respect to the formatted code
+/// block after formatting. To end with correct indentation, additional
+/// indentation must be added to the wrapped portion of the multi-line string.
+pub(crate) fn indent_next_line(kind: FullCodeCharKind, line: &str, config: &Config) -> bool {
+    !((kind.is_string() && !(config.format_strings() && line.ends_with('\\')))
+        || (config.version() == Version::Two && kind.is_commented_string()))
 }
 
 pub(crate) fn is_empty_line(s: &str) -> bool {

tests/source/issue-4036.rs

@@ -0,0 +1,11 @@
+// rustfmt-format_strings: true
+
+macro_rules! test {
+    () => {
+        fn from() {
+            None.expect(
+                "We asserted that `buffer.len()` is exactly `$n` so we can expect `ApInt::from_iter` to be successful.",
+            )
+        }
+    };
+}

tests/target/issue-4036.rs

@@ -0,0 +1,12 @@
+// rustfmt-format_strings: true
+
+macro_rules! test {
+    () => {
+        fn from() {
+            None.expect(
+                "We asserted that `buffer.len()` is exactly `$n` so we can expect \
+                 `ApInt::from_iter` to be successful.",
+            )
+        }
+    };
+}