Remove each comment's leading whitespace in comment::rewrite_comment

src/comment.rs

@@ -38,7 +38,7 @@ fn is_custom_comment(comment: &str) -> bool {
     }
 }
 
-#[derive(Copy, Clone, PartialEq, Eq)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub(crate) enum CommentStyle<'a> {
     DoubleSlash,
     TripleSlash,
@@ -263,7 +263,7 @@ pub(crate) fn rewrite_comment(
     shape: Shape,
     config: &Config,
 ) -> Option<String> {
-    identify_comment(orig, block_style, shape, config, false)
+    identify_comment(orig.trim_start(), block_style, shape, config, false)
 }
 
 fn identify_comment(

tests/target/issue_5391.rs

@@ -0,0 +1,123 @@
+// rustfmt-version: Two
+
+fn custom_comments() {
+    // Custom comment is the last element of the block
+    {
+        let x = 0; // X
+        //.Y
+    }
+
+    // Custom comment is not the last element of the block
+    {
+        let x = 0; // X
+        //.Y
+        println!("hello world!");
+    }
+
+    // Variations on custom comment tests with and without extra text
+    {
+        let x = 0; // X
+        //~Y
+    }
+    {
+        let x = 0; // X
+        //~Y
+        println!("hello world!");
+    }
+    {
+        let x = 0; // X
+        //.Y Some more text
+    }
+    {
+        let x = 0; // X
+        //.Y Some more text
+        println!("hello world!");
+    }
+    {
+        let x = 0; // X
+        //~Y Some more text
+    }
+    {
+        let x = 0; // X
+        //~Y Some more text
+        println!("hello world!");
+    }
+}
+
+fn double_slash_comments() {
+    // DoubleSlash comment is the last element of the block
+    {
+        let x = 0; // X
+        // .Y
+    }
+
+    // DoubleSlash comment is not the the last element of the block
+    {
+        let x = 0; // X
+        // .Y
+        println!("hello world!");
+    }
+    {
+        let x = 0; // X
+        // ~Y
+    }
+    {
+        let x = 0; // X
+        // ~Y
+        println!("hello world!");
+    }
+    {
+        let x = 0; // X
+        //Y
+    }
+    {
+        let x = 0; // X
+        //Y
+        println!("hello world!");
+    }
+    {
+        let x = 0; // X
+        // Y
+    }
+    {
+        let x = 0; // X
+        // Y
+        println!("hello world!");
+    }
+}
+
+// Compiler UI Test Error Annotations that use custom comments
+// https://rustc-dev-guide.rust-lang.org/tests/ui.html#error-annotation-examples
+mod compiler_ui_test_error_annotations {
+    /// Use the //~ ERROR idiom:
+    fn positioned_on_error_line() {
+        let x = (1, 2, 3);
+        match x {
+            (_a, _x @ ..) => {} //~ ERROR `_x @` is not allowed in a tuple
+            _ => {}
+        }
+    }
+
+    /// Use the //~^ idiom with number of carets in the string to indicate the number of lines above
+    fn positioned_below_error_line() {
+        let x = (1, 2, 3);
+        match x {
+            (_a, _x @ ..) => {} // <- the error is on this line
+            _ => {}
+        }
+        //~^^^ ERROR `_x @` is not allowed in a tuple
+    }
+
+    struct Binder(i32, i32, i32);
+
+    /// Use the //~| idiom to define the same error line as the error annotation line above
+    fn use_same_error_line_as_defined_on_error_annotation_line_above() {
+        let x = Binder(1, 2, 3);
+        match x {
+            Binder(_a, _x @ ..) => {} // <- the error is on this line
+            _ => {}
+        }
+        //~^^^ ERROR `_x @` is not allowed in a tuple struct
+        //~| ERROR this pattern has 1 field, but the corresponding tuple struct has 3 fields [E0023]
+    }
+}