Remove support for specializing ToString outside the standard library

This is the only trait specializable outside of the standard library.
Before stabilizing specialization we will probably want to remove
support for this. It was originally made specializable to allow a more
efficient ToString in libproc_macro back when this way the only way to
get any data out of a TokenStream. We now support getting individual
tokens, so proc macros no longer need to call it as often.

library/proc_macro/src/lib.rs

@@ -19,9 +19,6 @@
 )]
 #![doc(rust_logo)]
 #![feature(rustdoc_internals)]
-// This library is copied into rust-analyzer to allow loading rustc compiled proc macros.
-// Please avoid unstable features where possible to minimize the amount of changes necessary
-// to make it compile with rust-analyzer on stable.
 #![feature(staged_api)]
 #![feature(allow_internal_unstable)]
 #![feature(decl_macro)]
@@ -30,7 +27,6 @@
 #![feature(panic_can_unwind)]
 #![feature(restricted_std)]
 #![feature(rustc_attrs)]
-#![feature(min_specialization)]
 #![feature(extend_one)]
 #![recursion_limit = "256"]
 #![allow(internal_features)]
@@ -185,16 +181,6 @@ impl FromStr for TokenStream {
     }
 }
 
-// N.B., the bridge only provides `to_string`, implement `fmt::Display`
-// based on it (the reverse of the usual relationship between the two).
-#[doc(hidden)]
-#[stable(feature = "proc_macro_lib", since = "1.15.0")]
-impl ToString for TokenStream {
-    fn to_string(&self) -> String {
-        self.0.as_ref().map(|t| t.to_string()).unwrap_or_default()
-    }
-}
-
 /// Prints the token stream as a string that is supposed to be losslessly convertible back
 /// into the same token stream (modulo spans), except for possibly `TokenTree::Group`s
 /// with `Delimiter::None` delimiters and negative numeric literals.
@@ -210,7 +196,10 @@ impl ToString for TokenStream {
 impl fmt::Display for TokenStream {
     #[allow(clippy::recursive_format_impl)] // clippy doesn't see the specialization
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.write_str(&self.to_string())
+        match &self.0 {
+            Some(ts) => write!(f, "{}", ts.to_string()),
+            None => Ok(()),
+        }
     }
 }
 
@@ -756,21 +745,6 @@ impl From<Literal> for TokenTree {
     }
 }
 
-// N.B., the bridge only provides `to_string`, implement `fmt::Display`
-// based on it (the reverse of the usual relationship between the two).
-#[doc(hidden)]
-#[stable(feature = "proc_macro_lib", since = "1.15.0")]
-impl ToString for TokenTree {
-    fn to_string(&self) -> String {
-        match *self {
-            TokenTree::Group(ref t) => t.to_string(),
-            TokenTree::Ident(ref t) => t.to_string(),
-            TokenTree::Punct(ref t) => t.to_string(),
-            TokenTree::Literal(ref t) => t.to_string(),
-        }
-    }
-}
-
 /// Prints the token tree as a string that is supposed to be losslessly convertible back
 /// into the same token tree (modulo spans), except for possibly `TokenTree::Group`s
 /// with `Delimiter::None` delimiters and negative numeric literals.
@@ -786,7 +760,12 @@ impl ToString for TokenTree {
 impl fmt::Display for TokenTree {
     #[allow(clippy::recursive_format_impl)] // clippy doesn't see the specialization
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.write_str(&self.to_string())
+        match self {
+            TokenTree::Group(t) => write!(f, "{t}"),
+            TokenTree::Ident(t) => write!(f, "{t}"),
+            TokenTree::Punct(t) => write!(f, "{t}"),
+            TokenTree::Literal(t) => write!(f, "{t}"),
+        }
     }
 }
 
@@ -912,16 +891,6 @@ impl Group {
     }
 }
 
-// N.B., the bridge only provides `to_string`, implement `fmt::Display`
-// based on it (the reverse of the usual relationship between the two).
-#[doc(hidden)]
-#[stable(feature = "proc_macro_lib", since = "1.15.0")]
-impl ToString for Group {
-    fn to_string(&self) -> String {
-        TokenStream::from(TokenTree::from(self.clone())).to_string()
-    }
-}
-
 /// Prints the group as a string that should be losslessly convertible back
 /// into the same group (modulo spans), except for possibly `TokenTree::Group`s
 /// with `Delimiter::None` delimiters.
@@ -929,7 +898,7 @@ impl ToString for Group {
 impl fmt::Display for Group {
     #[allow(clippy::recursive_format_impl)] // clippy doesn't see the specialization
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.write_str(&self.to_string())
+        write!(f, "{}", TokenStream::from(TokenTree::from(self.clone())))
     }
 }
 
@@ -1035,14 +1004,6 @@ impl Punct {
     }
 }
 
-#[doc(hidden)]
-#[stable(feature = "proc_macro_lib2", since = "1.29.0")]
-impl ToString for Punct {
-    fn to_string(&self) -> String {
-        self.as_char().to_string()
-    }
-}
-
 /// Prints the punctuation character as a string that should be losslessly convertible
 /// back into the same character.
 #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
@@ -1138,14 +1099,6 @@ impl Ident {
     }
 }
 
-#[doc(hidden)]
-#[stable(feature = "proc_macro_lib2", since = "1.29.0")]
-impl ToString for Ident {
-    fn to_string(&self) -> String {
-        self.0.sym.with(|sym| if self.0.is_raw { ["r#", sym].concat() } else { sym.to_owned() })
-    }
-}
-
 /// Prints the identifier as a string that should be losslessly convertible back
 /// into the same identifier.
 #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
@@ -1520,14 +1473,6 @@ impl FromStr for Literal {
     }
 }
 
-#[doc(hidden)]
-#[stable(feature = "proc_macro_lib2", since = "1.29.0")]
-impl ToString for Literal {
-    fn to_string(&self) -> String {
-        self.with_stringify_parts(|parts| parts.concat())
-    }
-}
-
 /// Prints the literal as a string that should be losslessly convertible
 /// back into the same literal (except for possible rounding for floating point literals).
 #[stable(feature = "proc_macro_lib2", since = "1.29.0")]