Rollup merge of #38497 - QuietMisdreavus:rustdoc-where-again, r=steveklabnik

rustdoc: properly calculate line length for where clauses

Apparently, while I was cleaning up #37190, I regressed the formatting for long where clauses, where it wouldn't take the "prefix" length into account when deciding whether to break the line up. This patch fixes that.

src/librustdoc/html/format.rs

@@ -207,7 +207,7 @@ fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         if !f.alternate() {
             clause.push_str("</span>");
             let plain = format!("{:#}", self);
-            if plain.len() > 80 {
+            if plain.len() + pad > 80 {
                 //break it onto its own line regardless, but make sure method impls and trait
                 //blocks keep their fixed padding (2 and 9, respectively)
                 let padding = if pad > 10 {