avoid not to truncate necessary chars (#3640)

src/formatting.rs

@@ -177,7 +177,12 @@ impl<'a, T: FormatHandler + 'a> FormatContext<'a, T> {
             visitor.format_separate_mod(module, &*source_file);
         };
 
-        debug_assert_eq!(visitor.line_number, count_newlines(&visitor.buffer));
+        debug_assert_eq!(
+            visitor.line_number,
+            count_newlines(&visitor.buffer),
+            "failed in format_file visitor.buffer:\n {:?}",
+            &visitor.buffer
+        );
 
         // For some reason, the source_map does not include terminating
         // newlines so we must add one on for each file. This is sad.

src/visitor.rs

@@ -257,7 +257,20 @@ impl<'b, 'a: 'b> FmtVisitor<'a> {
             } else {
                 self.config.tab_spaces()
             };
-            self.buffer.truncate(total_len - chars_too_many);
+
+            // FIXME this is a temporaly fix
+            // should be remove truncate logic in close_block
+            // avoid not to truncate necessary chars
+            let truncate_start = total_len - chars_too_many;
+            let target_str = &self.buffer[truncate_start..total_len];
+            let truncate_length = target_str.len() - target_str.trim().len();
+
+            if let Some(last_char) = target_str.chars().last() {
+                self.buffer.truncate(total_len - truncate_length);
+                if last_char == '\n' {
+                    self.buffer.push_str("\n");
+                }
+            }
         }
         self.push_str("}");
         self.block_indent = self.block_indent.block_unindent(self.config);

tests/source/issue-3636.rs

@@ -0,0 +1,10 @@
+// rustfmt-file_lines: [{"file":"tests/source/issue-3636.rs","range":[4,7]}]
+
+fn foo() {
+    let x =
+        42;
+    let y =
+        42;
+    let z = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
+    let z = "bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb";   
+}

tests/target/issue-3636.rs

@@ -0,0 +1,8 @@
+// rustfmt-file_lines: [{"file":"tests/source/issue-3636.rs","range":[4,7]}]
+
+fn foo() {
+    let x = 42;
+    let y = 42;
+    let z = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
+    let z = "bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb";
+}