Add absolute_comment_width setting

src/config.rs

@@ -618,6 +618,8 @@ create_config! {
     wrap_comments: bool, false, false, "Break comments to fit on the line";
     comment_width: usize, 80, false,
         "Maximum length of comments. No effect unless wrap_comments = true";
+    absolute_comment_width: bool, false, false,
+        "Treat column_width as a fixed column number to wrap at";
     normalize_comments: bool, false, false, "Convert /* */ comments to // comments where possible";
     wrap_match_arms: bool, true, false, "Wrap the body of arms in blocks when it does not fit on \
                                   the same line with the pattern of arms";

src/missed_spans.rs

@@ -174,7 +174,14 @@ impl<'a> FmtVisitor<'a> {
                     }
 
                     let comment_width = ::std::cmp::min(
-                        self.config.comment_width(),
+                        if self.config.absolute_comment_width() {
+                            self.config
+                                .comment_width()
+                                .checked_sub(self.block_indent.width())
+                                .unwrap_or(0)
+                        } else {
+                            self.config.comment_width()
+                        },
                         self.config.max_width() - self.block_indent.width(),
                     );
                     let comment_indent = Indent::from_width(self.config, self.buffer.cur_offset());

tests/source/configs-absolute_comment_width.rs

@@ -0,0 +1,14 @@
+// rustfmt-wrap_comments: true
+// rustfmt-comment_width: 72
+// rustfmt-absolute_comment_width: true
+impl MyStruct {
+    fn my_fun() {
+        if foo {
+            // Please wrap this comment at column number 72 exactly, rather than column 84
+        }
+
+        fn bar() {
+            //! Please wrap this comment at column number 72 exactly, rather than column 84
+        }
+    }
+}

tests/target/configs-absolute_comment_width.rs

@@ -0,0 +1,16 @@
+// rustfmt-wrap_comments: true
+// rustfmt-comment_width: 72
+// rustfmt-absolute_comment_width: true
+impl MyStruct {
+    fn my_fun() {
+        if foo {
+            // Please wrap this comment at column number 72 exactly,
+            // rather than column 84
+        }
+
+        fn bar() {
+            //! Please wrap this comment at column number 72 exactly,
+            //! rather than column 84
+        }
+    }
+}