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(())
 }
 
+impl Display<'_> {
+    fn display_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
+            } else if 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)?;
+            }
+            if let (true, Cfg::Cfg(_, Some(feat))) = (short_longhand, sub_cfg) {
+                if self.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::Display for Display<'_> {
     fn fmt(&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
-                    } else if 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)?;
-                    }
-                    if let (true, Cfg::Cfg(_, Some(feat))) = (short_longhand, sub_cfg) {
-                        if self.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
-                    } else if 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 ")?;
-                    }
-                    if let (true, Cfg::Cfg(_, Some(feat))) = (short_longhand, sub_cfg) {
-                        if self.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"),