Change Token::interpolated_to_tokenstream().

It is currently a method of `Token`, but it only is valid to call if
`self` is a `Token::Interpolated`. This commit eliminates the
possibility of misuse by changing it to an associated function that
takes a `Nonterminal`, which also simplifies the call sites.

This requires splitting out a new function, `nonterminal_to_string`.

src/librustc/hir/lowering.rs

@@ -1131,13 +1131,13 @@ impl<'a> LoweringContext<'a> {
 
     fn lower_token(&mut self, token: Token, span: Span) -> TokenStream {
         match token {
-            Token::Interpolated(_) => {}
-            other => return TokenTree::Token(span, other).into(),
-        }
-
-        let tts = token.interpolated_to_tokenstream(&self.sess.parse_sess, span);
+            Token::Interpolated(nt) => {
+                let tts = Token::interpolated_to_tokenstream(&self.sess.parse_sess, nt, span);
                 self.lower_token_stream(tts)
             }
+            other => TokenTree::Token(span, other).into(),
+        }
+    }
 
     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm {
         hir::Arm {

src/libsyntax/parse/token.rs

@@ -508,14 +508,8 @@ impl Token {
         }
     }
 
-    pub fn interpolated_to_tokenstream(&self, sess: &ParseSess, span: Span)
-        -> TokenStream
-    {
-        let nt = match *self {
-            Token::Interpolated(ref nt) => nt,
-            _ => panic!("only works on interpolated tokens"),
-        };
-
+    pub fn interpolated_to_tokenstream(sess: &ParseSess, nt: Lrc<(Nonterminal, LazyTokenStream)>,
+                                       span: Span) -> TokenStream {
         // An `Interpolated` token means that we have a `Nonterminal`
         // which is often a parsed AST item. At this point we now need
         // to convert the parsed AST to an actual token stream, e.g.
@@ -558,7 +552,7 @@ impl Token {
 
         let tokens_for_real = nt.1.force(|| {
             // FIXME(#43081): Avoid this pretty-print + reparse hack
-            let source = pprust::token_to_string(self);
+            let source = pprust::nonterminal_to_string(&nt.0);
             let filename = FileName::macro_expansion_source_code(&source);
             let (tokens, errors) = parse_stream_from_source_str(
                 filename, source, sess, Some(span));

src/libsyntax/print/pprust.rs

@@ -4,7 +4,7 @@ use crate::ast::{Attribute, MacDelimiter, GenericArg};
 use crate::util::parser::{self, AssocOp, Fixity};
 use crate::attr;
 use crate::source_map::{self, SourceMap, Spanned};
-use crate::parse::token::{self, BinOpToken, Token};
+use crate::parse::token::{self, BinOpToken, Nonterminal, Token};
 use crate::parse::lexer::comments;
 use crate::parse::{self, ParseSess};
 use crate::print::pp::{self, Breaks};
@@ -257,7 +257,12 @@ pub fn token_to_string(tok: &Token) -> String {
         token::Comment              => "/* */".to_string(),
         token::Shebang(s)           => format!("/* shebang: {}*/", s),
 
-        token::Interpolated(ref nt) => match nt.0 {
+        token::Interpolated(ref nt) => nonterminal_to_string(&nt.0),
+    }
+}
+
+pub fn nonterminal_to_string(nt: &Nonterminal) -> String {
+    match *nt {
         token::NtExpr(ref e)        => expr_to_string(e),
         token::NtMeta(ref e)        => meta_item_to_string(e),
         token::NtTy(ref e)          => ty_to_string(e),
@@ -280,7 +285,6 @@ pub fn token_to_string(tok: &Token) -> String {
         token::NtVis(ref e)         => vis_to_string(e),
         token::NtForeignItem(ref e) => foreign_item_to_string(e),
     }
-    }
 }
 
 pub fn ty_to_string(ty: &ast::Ty) -> String {

src/libsyntax_ext/proc_macro_server.rs

@@ -178,8 +178,8 @@ impl FromInternal<(TreeAndJoint, &'_ ParseSess, &'_ mut Vec<Self>)>
                 tt!(Punct::new('#', false))
             }
 
-            Interpolated(_) => {
-                let stream = token.interpolated_to_tokenstream(sess, span);
+            Interpolated(nt) => {
+                let stream = Token::interpolated_to_tokenstream(sess, nt, span);
                 TokenTree::Group(Group {
                     delimiter: Delimiter::None,
                     stream,