Auto merge of #135494 - yotamofek:rustdoc-fmt-from_fn, r=fmease

Refactor `fmt::Display` impls in rustdoc
This PR does a couple of things, with the intention of cleaning up and streamlining some of the `fmt::Display` impls in rustdoc:
1. Use the unstable [`fmt::from_fn`](#117729) instead of open-coding it.
2. ~~Replace bespoke implementations of `Itertools::format` with the method itself.~~
4. Some more minor cleanups - DRY, remove unnecessary calls to `Symbol::as_str()`, replace some `format!()` calls with lazier options
The changes are mostly cosmetic but some of them might have a slight positive effect on performance.

Author: bors

src/librustdoc/clean/cfg.rs

@@ -389,6 +389,49 @@ fn write_with_opt_paren<T: fmt::Display>(
 Ok(())
 }
 
+implDisplay<'_>{
+fndisplay_sub_cfgs(
+&self,
+fmt:&mut fmt::Formatter<'_>,
+sub_cfgs:&[Cfg],
+separator:&str,
+) -> fmt::Result{
+let short_longhand = self.1.is_long() && {
+let all_crate_features =
+ sub_cfgs.iter().all(|sub_cfg| matches!(sub_cfg,Cfg::Cfg(sym::feature,Some(_))));
+let all_target_features = sub_cfgs
+.iter()
+.all(|sub_cfg| matches!(sub_cfg,Cfg::Cfg(sym::target_feature,Some(_))));
+
+if all_crate_features {
+ fmt.write_str("crate features ")?;
+true
+}elseif all_target_features {
+ fmt.write_str("target features ")?;
+true
+}else{
+false
+}
+};
+
+for(i, sub_cfg)in sub_cfgs.iter().enumerate(){
+if i != 0{
+ fmt.write_str(separator)?;
+}
+iflet(true,Cfg::Cfg(_,Some(feat))) = (short_longhand, sub_cfg){
+ifself.1.is_html(){
+write!(fmt,"<code>{feat}</code>")?;
+}else{
+write!(fmt,"`{feat}`")?;
+}
+}else{
+write_with_opt_paren(fmt, !sub_cfg.is_all(),Display(sub_cfg,self.1))?;
+}
+}
+Ok(())
+}
+}
+
 impl fmt::DisplayforDisplay<'_>{
 fnfmt(&self,fmt:&mut fmt::Formatter<'_>) -> fmt::Result{
 match*self.0{
@@ -408,79 +451,9 @@ impl fmt::Display for Display<'_> {
 
 Cfg::Any(ref sub_cfgs) => {
 let separator = if sub_cfgs.iter().all(Cfg::is_simple){" or "}else{", or "};
-
-let short_longhand = self.1.is_long() && {
-let all_crate_features = sub_cfgs
-.iter()
-.all(|sub_cfg| matches!(sub_cfg,Cfg::Cfg(sym::feature,Some(_))));
-let all_target_features = sub_cfgs
-.iter()
-.all(|sub_cfg| matches!(sub_cfg,Cfg::Cfg(sym::target_feature,Some(_))));
-
-if all_crate_features {
- fmt.write_str("crate features ")?;
-true
-}elseif all_target_features {
- fmt.write_str("target features ")?;
-true
-}else{
-false
-}
-};
-
-for(i, sub_cfg)in sub_cfgs.iter().enumerate(){
-if i != 0{
- fmt.write_str(separator)?;
-}
-iflet(true,Cfg::Cfg(_,Some(feat))) = (short_longhand, sub_cfg){
-ifself.1.is_html(){
-write!(fmt,"<code>{feat}</code>")?;
-}else{
-write!(fmt,"`{feat}`")?;
-}
-}else{
-write_with_opt_paren(fmt, !sub_cfg.is_all(),Display(sub_cfg,self.1))?;
-}
-}
-Ok(())
-}
-
-Cfg::All(ref sub_cfgs) => {
-let short_longhand = self.1.is_long() && {
-let all_crate_features = sub_cfgs
-.iter()
-.all(|sub_cfg| matches!(sub_cfg,Cfg::Cfg(sym::feature,Some(_))));
-let all_target_features = sub_cfgs
-.iter()
-.all(|sub_cfg| matches!(sub_cfg,Cfg::Cfg(sym::target_feature,Some(_))));
-
-if all_crate_features {
- fmt.write_str("crate features ")?;
-true
-}elseif all_target_features {
- fmt.write_str("target features ")?;
-true
-}else{
-false
-}
-};
-
-for(i, sub_cfg)in sub_cfgs.iter().enumerate(){
-if i != 0{
- fmt.write_str(" and ")?;
-}
-iflet(true,Cfg::Cfg(_,Some(feat))) = (short_longhand, sub_cfg){
-ifself.1.is_html(){
-write!(fmt,"<code>{feat}</code>")?;
-}else{
-write!(fmt,"`{feat}`")?;
-}
-}else{
-write_with_opt_paren(fmt, !sub_cfg.is_simple(),Display(sub_cfg,self.1))?;
-}
-}
-Ok(())
+self.display_sub_cfgs(fmt, sub_cfgs, separator)
 }
+Cfg::All(ref sub_cfgs) => self.display_sub_cfgs(fmt, sub_cfgs," and "),
 
 Cfg::True => fmt.write_str("everywhere"),
 Cfg::False => fmt.write_str("nowhere"),