Fix wrapping of comments sometimes making too long lines

In the case where `comment_width` is less than `max_width` and we want
to wrap right on `comment_width`: `rustfmt` would consider it's wrapping
length to be exactly `comment_width`, ignoring the length of a trailing
indent. The fix is essentially how similar formatting code behaves, see
e.g. `shape::Shape::comment`.

This involved rewriting some existing tests, without impacting what
they're meant to be asserting on. This emphasises that this patch _is
not_ backwards compatible: while I believe the change to be correct
(some tests had comments longer than `comment_width` that weren't being
formatted). So there's a trade-off to be made between correctness and
stability, since `comment_width` is still not stabilised this may be
acceptable.

Issue: 6801

Author: matthewhughes934

src/missed_spans.rs

@@ -263,7 +263,7 @@ impl<'a> FmtVisitor<'a> {
         };
 
         let comment_width = ::std::cmp::min(
-            self.config.comment_width(),
+            self.config.comment_width() - self.block_indent.width(),
             self.config.max_width() - self.block_indent.width(),
         );
         let comment_shape = Shape::legacy(comment_width, comment_indent);

tests/source/issue_6801.rs

@@ -0,0 +1,7 @@
+// rustfmt-wrap_comments: true
+
+fn foo() {
+    // this is a comment that could be broken right on max_comment and should break before that
+
+    ()
+}

tests/target/issue_6801.rs

@@ -0,0 +1,8 @@
+// rustfmt-wrap_comments: true
+
+fn foo() {
+    // this is a comment that could be broken right on max_comment and should
+    // break before that
+
+    ()
+}

tests/target/trailing_comments/hard_tabs.rs

@@ -18,10 +18,10 @@ fn lorem_ipsum() {
 	// nunc. Mauris consequat, enim vitae venenatis sollicitudin, dolor orci
 	// bibendum enim, a sagittis nulla nunc quis elit. Phasellus augue. Nunc
 	// suscipit, magna tincidunt lacinia faucibus, lacus tellus ornare purus, a
-	// pulvinar lacus orci eget nibh.  Maecenas sed nibh non lacus tempor faucibus.
-	// In hac habitasse platea dictumst. Vivamus a orci at nulla tristique
-	// condimentum. Donec arcu quam, dictum accumsan, convallis accumsan, cursus sit
-	// amet, ipsum.  In pharetra sagittis nunc.
+	// pulvinar lacus orci eget nibh.  Maecenas sed nibh non lacus tempor
+	// faucibus. In hac habitasse platea dictumst. Vivamus a orci at nulla
+	// tristique condimentum. Donec arcu quam, dictum accumsan, convallis
+	// accumsan, cursus sit amet, ipsum.  In pharetra sagittis nunc.
 	let b = baz();
 
 	let normalized = self.ctfont.all_traits().normalized_weight(); // [-1.0, 1.0]

tests/target/trailing_comments/soft_tabs.rs

@@ -18,10 +18,10 @@ fn foo() {
     // nunc. Mauris consequat, enim vitae venenatis sollicitudin, dolor orci
     // bibendum enim, a sagittis nulla nunc quis elit. Phasellus augue. Nunc
     // suscipit, magna tincidunt lacinia faucibus, lacus tellus ornare purus, a
-    // pulvinar lacus orci eget nibh.  Maecenas sed nibh non lacus tempor faucibus.
-    // In hac habitasse platea dictumst. Vivamus a orci at nulla tristique
-    // condimentum. Donec arcu quam, dictum accumsan, convallis accumsan, cursus sit
-    // amet, ipsum.  In pharetra sagittis nunc.
+    // pulvinar lacus orci eget nibh.  Maecenas sed nibh non lacus tempor
+    // faucibus. In hac habitasse platea dictumst. Vivamus a orci at nulla
+    // tristique condimentum. Donec arcu quam, dictum accumsan, convallis
+    // accumsan, cursus sit amet, ipsum.  In pharetra sagittis nunc.
     let b = baz();
 
     let normalized = self.ctfont.all_traits().normalized_weight(); // [-1.0, 1.0]

tests/target/unicode.rs

@@ -4,7 +4,8 @@ fn foo() {
     let s = "this line goes to 100: ͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶͶ";
     let s = 42;
 
-    // a comment of length 80, with the starting sigil: ҘҘҘҘҘҘҘҘҘҘ ҘҘҘҘҘҘҘҘҘҘҘҘҘҘ
+    // a comment of length 80, with the starting sigil: ҘҘҘҘҘҘҘҘҘҘ
+    // ҘҘҘҘҘҘҘҘҘҘҘҘҘҘ
     let s = 42;
 }