rustc_parse: Move AST -> TokenStream conversion logic to `rustc_ast`

14 files changed, 124 insertions, 284 deletions

diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 0deeb6849a2..07b8a98e47f 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -444,8 +444,7 @@ impl Default for Generics {
 pub struct WhereClause {
     /// `true` if we ate a `where` token: this can happen
     /// if we parsed no predicates (e.g. `struct Foo where {}`).
-    /// This allows us to accurately pretty-print
-    /// in `nt_to_tokenstream`
+    /// This allows us to pretty-print accurately.
     pub has_where_token: bool,
     pub predicates: Vec<WherePredicate>,
     pub span: Span,
diff --git a/compiler/rustc_ast/src/ast_traits.rs b/compiler/rustc_ast/src/ast_traits.rs
index bd401ddbbee..5c30a75a140 100644
--- a/compiler/rustc_ast/src/ast_traits.rs
+++ b/compiler/rustc_ast/src/ast_traits.rs
@@ -108,7 +108,7 @@ macro_rules! impl_has_span {
     };
 }
 
-impl_has_span!(AssocItem, Expr, ForeignItem, Item, Stmt);
+impl_has_span!(AssocItem, Block, Expr, ForeignItem, Item, Pat, Path, Stmt, Ty, Visibility);
 
 impl<T: AstDeref<Target: HasSpan>> HasSpan for T {
     fn span(&self) -> Span {
@@ -116,6 +116,12 @@ impl<T: AstDeref<Target: HasSpan>> HasSpan for T {
     }
 }
 
+impl HasSpan for AttrItem {
+    fn span(&self) -> Span {
+        self.span()
+    }
+}
+
 /// A trait for AST nodes having (or not having) collected tokens.
 pub trait HasTokens {
     fn tokens(&self) -> Option<&LazyTokenStream>;
diff --git a/compiler/rustc_ast/src/tokenstream.rs b/compiler/rustc_ast/src/tokenstream.rs
index 23a039ec868..c58fe7287bf 100644
--- a/compiler/rustc_ast/src/tokenstream.rs
+++ b/compiler/rustc_ast/src/tokenstream.rs
@@ -13,7 +13,9 @@
 //! and a borrowed `TokenStream` is sufficient to build an owned `TokenStream` without taking
 //! ownership of the original.
 
-use crate::token::{self, Delimiter, Token, TokenKind};
+use crate::ast::StmtKind;
+use crate::ast_traits::{HasAttrs, HasSpan, HasTokens};
+use crate::token::{self, Delimiter, Nonterminal, Token, TokenKind};
 use crate::AttrVec;
 
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
@@ -45,12 +47,6 @@ pub enum TokenTree {
     Delimited(DelimSpan, Delimiter, TokenStream),
 }
 
-#[derive(Copy, Clone)]
-pub enum CanSynthesizeMissingTokens {
-    Yes,
-    No,
-}
-
 // Ensure all fields of `TokenTree` is `Send` and `Sync`.
 #[cfg(parallel_compiler)]
 fn _dummy()
@@ -471,6 +467,89 @@ impl TokenStream {
                 .collect(),
         ))
     }
+
+    fn opt_from_ast(node: &(impl HasAttrs + HasTokens)) -> Option<TokenStream> {
+        let tokens = node.tokens()?;
+        let attrs = node.attrs();
+        let attr_annotated = if attrs.is_empty() {
+            tokens.create_token_stream()
+        } else {
+            let attr_data = AttributesData { attrs: attrs.to_vec().into(), tokens: tokens.clone() };
+            AttrAnnotatedTokenStream::new(vec![(
+                AttrAnnotatedTokenTree::Attributes(attr_data),
+                Spacing::Alone,
+            )])
+        };
+        Some(attr_annotated.to_tokenstream())
+    }
+
+    pub fn from_ast(node: &(impl HasAttrs + HasSpan + HasTokens + fmt::Debug)) -> TokenStream {
+        TokenStream::opt_from_ast(node)
+            .unwrap_or_else(|| panic!("missing tokens for node at {:?}: {:?}", node.span(), node))
+    }
+
+    pub fn from_nonterminal_ast(nt: &Nonterminal) -> TokenStream {
+        match nt {
+            Nonterminal::NtIdent(ident, is_raw) => {
+                TokenTree::token(token::Ident(ident.name, *is_raw), ident.span).into()
+            }
+            Nonterminal::NtLifetime(ident) => {
+                TokenTree::token(token::Lifetime(ident.name), ident.span).into()
+            }
+            Nonterminal::NtItem(item) => TokenStream::from_ast(item),
+            Nonterminal::NtBlock(block) => TokenStream::from_ast(block),
+            Nonterminal::NtStmt(stmt) if let StmtKind::Empty = stmt.kind => {
+                // FIXME: Properly collect tokens for empty statements.
+                TokenTree::token(token::Semi, stmt.span).into()
+            }
+            Nonterminal::NtStmt(stmt) => TokenStream::from_ast(stmt),
+            Nonterminal::NtPat(pat) => TokenStream::from_ast(pat),
+            Nonterminal::NtTy(ty) => TokenStream::from_ast(ty),
+            Nonterminal::NtMeta(attr) => TokenStream::from_ast(attr),
+            Nonterminal::NtPath(path) => TokenStream::from_ast(path),
+            Nonterminal::NtVis(vis) => TokenStream::from_ast(vis),
+            Nonterminal::NtExpr(expr) | Nonterminal::NtLiteral(expr) => TokenStream::from_ast(expr),
+        }
+    }
+
+    fn flatten_token(token: &Token) -> TokenTree {
+        match &token.kind {
+            token::Interpolated(nt) if let token::NtIdent(ident, is_raw) = **nt => {
+                TokenTree::token(token::Ident(ident.name, is_raw), ident.span)
+            }
+            token::Interpolated(nt) => TokenTree::Delimited(
+                DelimSpan::from_single(token.span),
+                Delimiter::Invisible,
+                TokenStream::from_nonterminal_ast(&nt).flattened(),
+            ),
+            _ => TokenTree::Token(token.clone()),
+        }
+    }
+
+    fn flatten_token_tree(tree: &TokenTree) -> TokenTree {
+        match tree {
+            TokenTree::Token(token) => TokenStream::flatten_token(token),
+            TokenTree::Delimited(span, delim, tts) => {
+                TokenTree::Delimited(*span, *delim, tts.flattened())
+            }
+        }
+    }
+
+    #[must_use]
+    pub fn flattened(&self) -> TokenStream {
+        fn can_skip(stream: &TokenStream) -> bool {
+            stream.trees().all(|tree| match tree {
+                TokenTree::Token(token) => !matches!(token.kind, token::Interpolated(_)),
+                TokenTree::Delimited(_, _, inner) => can_skip(inner),
+            })
+        }
+
+        if can_skip(self) {
+            return self.clone();
+        }
+
+        self.trees().map(|tree| TokenStream::flatten_token_tree(tree)).collect()
+    }
 }
 
 // 99.5%+ of the time we have 1 or 2 elements in this vector.
diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
index a01ab32c282..12d1b8404eb 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/compiler/rustc_ast_lowering/src/item.rs
@@ -13,7 +13,6 @@ use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{LocalDefId, CRATE_DEF_ID};
 use rustc_hir::PredicateOrigin;
 use rustc_index::vec::{Idx, IndexVec};
-use rustc_session::utils::NtToTokenstream;
 use rustc_session::Session;
 use rustc_span::source_map::DesugaringKind;
 use rustc_span::symbol::{kw, sym, Ident};
@@ -27,7 +26,6 @@ use std::iter;
 pub(super) struct ItemLowerer<'a, 'hir> {
     pub(super) sess: &'a Session,
     pub(super) resolver: &'a mut dyn ResolverAstLowering,
-    pub(super) nt_to_tokenstream: NtToTokenstream,
     pub(super) arena: &'hir Arena<'hir>,
     pub(super) ast_index: &'a IndexVec<LocalDefId, AstOwner<'a>>,
     pub(super) owners: &'a mut IndexVec<LocalDefId, hir::MaybeOwner<&'hir hir::OwnerInfo<'hir>>>,
@@ -63,7 +61,6 @@ impl<'a, 'hir> ItemLowerer<'a, 'hir> {
             // Pseudo-globals.
             sess: &self.sess,
             resolver: self.resolver,
-            nt_to_tokenstream: self.nt_to_tokenstream,
             arena: self.arena,
 
             // HirId handling.
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 2a7f534bdd9..e59bc9aa6b3 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -37,7 +37,6 @@
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]
 
-use rustc_ast::tokenstream::{CanSynthesizeMissingTokens, TokenStream};
 use rustc_ast::visit;
 use rustc_ast::{self as ast, *};
 use rustc_ast_pretty::pprust;
@@ -56,7 +55,6 @@ use rustc_hir::{ConstArg, GenericArg, ItemLocalId, ParamName, TraitCandidate};
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_query_system::ich::StableHashingContext;
 use rustc_session::parse::feature_err;
-use rustc_session::utils::{FlattenNonterminals, NtToTokenstream};
 use rustc_session::Session;
 use rustc_span::hygiene::{ExpnId, MacroKind};
 use rustc_span::source_map::DesugaringKind;
@@ -89,11 +87,6 @@ struct LoweringContext<'a, 'hir: 'a> {
 
     resolver: &'a mut dyn ResolverAstLowering,
 
-    /// HACK(Centril): there is a cyclic dependency between the parser and lowering
-    /// if we don't have this function pointer. To avoid that dependency so that
-    /// `rustc_middle` is independent of the parser, we use dynamic dispatch here.
-    nt_to_tokenstream: NtToTokenstream,
-
     /// Used to allocate HIR nodes.
     arena: &'hir Arena<'hir>,
 
@@ -436,7 +429,6 @@ pub fn lower_crate<'a, 'hir>(
     sess: &'a Session,
     krate: &'a Crate,
     resolver: &'a mut dyn ResolverAstLowering,
-    nt_to_tokenstream: NtToTokenstream,
     arena: &'hir Arena<'hir>,
 ) -> &'hir hir::Crate<'hir> {
     let _prof_timer = sess.prof.verbose_generic_activity("hir_lowering");
@@ -447,15 +439,8 @@ pub fn lower_crate<'a, 'hir>(
         IndexVec::from_fn_n(|_| hir::MaybeOwner::Phantom, resolver.definitions().def_index_count());
 
     for def_id in ast_index.indices() {
-        item::ItemLowerer {
-            sess,
-            resolver,
-            nt_to_tokenstream,
-            arena,
-            ast_index: &ast_index,
-            owners: &mut owners,
-        }
-        .lower_node(def_id);
+        item::ItemLowerer { sess, resolver, arena, ast_index: &ast_index, owners: &mut owners }
+            .lower_node(def_id);
     }
 
     let hir_hash = compute_hir_hash(resolver, &owners);
@@ -875,11 +860,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 // ```
                 //
                 // In both cases, we don't want to synthesize any tokens
-                MacArgs::Delimited(
-                    dspan,
-                    delim,
-                    self.lower_token_stream(tokens.clone(), CanSynthesizeMissingTokens::No),
-                )
+                MacArgs::Delimited(dspan, delim, tokens.flattened())
             }
             // This is an inert key-value attribute - it will never be visible to macros
             // after it gets lowered to HIR. Therefore, we can extract literals to handle
@@ -904,19 +885,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
     }
 
-    fn lower_token_stream(
-        &self,
-        tokens: TokenStream,
-        synthesize_tokens: CanSynthesizeMissingTokens,
-    ) -> TokenStream {
-        FlattenNonterminals {
-            parse_sess: &self.sess.parse_sess,
-            synthesize_tokens,
-            nt_to_tokenstream: self.nt_to_tokenstream,
-        }
-        .process_token_stream(tokens)
-    }
-
     /// Given an associated type constraint like one of these:
     ///
     /// ```ignore (illustrative)
diff --git a/compiler/rustc_ast_pretty/src/pprust/mod.rs b/compiler/rustc_ast_pretty/src/pprust/mod.rs
index d2e2fd520cd..ac9e7d06c4e 100644
--- a/compiler/rustc_ast_pretty/src/pprust/mod.rs
+++ b/compiler/rustc_ast_pretty/src/pprust/mod.rs
@@ -4,42 +4,12 @@ mod tests;
 pub mod state;
 pub use state::{print_crate, AnnNode, Comments, PpAnn, PrintState, State};
 
+use rustc_ast as ast;
 use rustc_ast::token::{Nonterminal, Token, TokenKind};
 use rustc_ast::tokenstream::{TokenStream, TokenTree};
-use rustc_ast::{self as ast, AstDeref};
 
 use std::borrow::Cow;
 
-pub trait AstPrettyPrint {
-    fn pretty_print(&self) -> String;
-}
-
-impl<T: AstDeref<Target: AstPrettyPrint>> AstPrettyPrint for T {
-    fn pretty_print(&self) -> String {
-        self.ast_deref().pretty_print()
-    }
-}
-
-macro_rules! impl_ast_pretty_print {
-    ($($T:ty => $method:ident),+ $(,)?) => {
-        $(
-            impl AstPrettyPrint for $T {
-                fn pretty_print(&self) -> String {
-                    State::new().$method(self)
-                }
-            }
-        )+
-    };
-}
-
-impl_ast_pretty_print! {
-    ast::Item => item_to_string,
-    ast::AssocItem => assoc_item_to_string,
-    ast::ForeignItem => foreign_item_to_string,
-    ast::Expr => expr_to_string,
-    ast::Stmt => stmt_to_string,
-}
-
 pub fn nonterminal_to_string(nt: &Nonterminal) -> String {
     State::new().nonterminal_to_string(nt)
 }
diff --git a/compiler/rustc_builtin_macros/src/cfg_eval.rs b/compiler/rustc_builtin_macros/src/cfg_eval.rs
index 6485c82a119..89b2c329236 100644
--- a/compiler/rustc_builtin_macros/src/cfg_eval.rs
+++ b/compiler/rustc_builtin_macros/src/cfg_eval.rs
@@ -3,7 +3,6 @@ use crate::util::{check_builtin_macro_attribute, warn_on_duplicate_attribute};
 use rustc_ast as ast;
 use rustc_ast::mut_visit::MutVisitor;
 use rustc_ast::ptr::P;
-use rustc_ast::tokenstream::CanSynthesizeMissingTokens;
 use rustc_ast::visit::Visitor;
 use rustc_ast::NodeId;
 use rustc_ast::{mut_visit, visit};
@@ -13,7 +12,6 @@ use rustc_expand::config::StripUnconfigured;
 use rustc_expand::configure;
 use rustc_feature::Features;
 use rustc_parse::parser::{ForceCollect, Parser};
-use rustc_session::utils::FlattenNonterminals;
 use rustc_session::Session;
 use rustc_span::symbol::sym;
 use rustc_span::Span;
@@ -174,8 +172,6 @@ impl CfgEval<'_, '_> {
             _ => unreachable!(),
         };
 
-        let mut orig_tokens = annotatable.to_tokens(&self.cfg.sess.parse_sess);
-
         // 'Flatten' all nonterminals (i.e. `TokenKind::Interpolated`)
         // to `None`-delimited groups containing the corresponding tokens. This
         // is normally delayed until the proc-macro server actually needs to
@@ -189,12 +185,7 @@ impl CfgEval<'_, '_> {
         // where `$item` is `#[cfg_attr] struct Foo {}`. We want to make
         // sure to evaluate *all* `#[cfg]` and `#[cfg_attr]` attributes - the simplest
         // way to do this is to do a single parse of a stream without any nonterminals.
-        let mut flatten = FlattenNonterminals {
-            nt_to_tokenstream: rustc_parse::nt_to_tokenstream,
-            parse_sess: &self.cfg.sess.parse_sess,
-            synthesize_tokens: CanSynthesizeMissingTokens::No,
-        };
-        orig_tokens = flatten.process_token_stream(orig_tokens);
+        let orig_tokens = annotatable.to_tokens().flattened();
 
         // Re-parse the tokens, setting the `capture_cfg` flag to save extra information
         // to the captured `AttrAnnotatedTokenStream` (specifically, we capture
diff --git a/compiler/rustc_expand/src/base.rs b/compiler/rustc_expand/src/base.rs
index 6ef03d1243d..e48ce602185 100644
--- a/compiler/rustc_expand/src/base.rs
+++ b/compiler/rustc_expand/src/base.rs
@@ -4,7 +4,7 @@ use crate::module::DirOwnership;
 use rustc_ast::attr::MarkedAttrs;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Nonterminal};
-use rustc_ast::tokenstream::{CanSynthesizeMissingTokens, TokenStream};
+use rustc_ast::tokenstream::TokenStream;
 use rustc_ast::visit::{AssocCtxt, Visitor};
 use rustc_ast::{self as ast, Attribute, HasAttrs, Item, NodeId, PatKind};
 use rustc_attr::{self as attr, Deprecation, Stability};
@@ -13,7 +13,7 @@ use rustc_data_structures::sync::{self, Lrc};
 use rustc_errors::{Applicability, DiagnosticBuilder, ErrorGuaranteed, MultiSpan, PResult};
 use rustc_lint_defs::builtin::PROC_MACRO_BACK_COMPAT;
 use rustc_lint_defs::BuiltinLintDiagnostics;
-use rustc_parse::{self, parser, to_token_stream, MACRO_ARGUMENTS};
+use rustc_parse::{self, parser, MACRO_ARGUMENTS};
 use rustc_session::{parse::ParseSess, Limit, Session};
 use rustc_span::def_id::{CrateNum, DefId, LocalDefId};
 use rustc_span::edition::Edition;
@@ -109,20 +109,18 @@ impl Annotatable {
         }
     }
 
-    pub fn to_tokens(&self, sess: &ParseSess) -> TokenStream {
+    pub fn to_tokens(&self) -> TokenStream {
         match self {
-            Annotatable::Item(node) => to_token_stream(node, sess, CanSynthesizeMissingTokens::No),
+            Annotatable::Item(node) => TokenStream::from_ast(node),
             Annotatable::TraitItem(node) | Annotatable::ImplItem(node) => {
-                to_token_stream(node, sess, CanSynthesizeMissingTokens::No)
-            }
-            Annotatable::ForeignItem(node) => {
-                to_token_stream(node, sess, CanSynthesizeMissingTokens::No)
+                TokenStream::from_ast(node)
             }
+            Annotatable::ForeignItem(node) => TokenStream::from_ast(node),
             Annotatable::Stmt(node) => {
                 assert!(!matches!(node.kind, ast::StmtKind::Empty));
-                to_token_stream(node, sess, CanSynthesizeMissingTokens::No)
+                TokenStream::from_ast(node)
             }
-            Annotatable::Expr(node) => to_token_stream(node, sess, CanSynthesizeMissingTokens::No),
+            Annotatable::Expr(node) => TokenStream::from_ast(node),
             Annotatable::Arm(..)
             | Annotatable::ExprField(..)
             | Annotatable::PatField(..)
diff --git a/compiler/rustc_expand/src/expand.rs b/compiler/rustc_expand/src/expand.rs
index 676abc92b63..5af6b777abe 100644
--- a/compiler/rustc_expand/src/expand.rs
+++ b/compiler/rustc_expand/src/expand.rs
@@ -679,9 +679,12 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                                     )
                                 ) =>
                         {
-                            rustc_parse::fake_token_stream(&self.cx.sess.parse_sess, item_inner)
+                            rustc_parse::fake_token_stream_for_item(
+                                &self.cx.sess.parse_sess,
+                                item_inner,
+                            )
                         }
-                        _ => item.to_tokens(&self.cx.sess.parse_sess),
+                        _ => item.to_tokens(),
                     };
                     let attr_item = attr.unwrap_normal_item();
                     if let MacArgs::Eq(..) = attr_item.args {
diff --git a/compiler/rustc_expand/src/proc_macro.rs b/compiler/rustc_expand/src/proc_macro.rs
index 6c74d462fb8..b3679b31c6c 100644
--- a/compiler/rustc_expand/src/proc_macro.rs
+++ b/compiler/rustc_expand/src/proc_macro.rs
@@ -96,7 +96,7 @@ impl MultiItemModifier for ProcMacroDerive {
             };
             TokenTree::token(token::Interpolated(Lrc::new(nt)), DUMMY_SP).into()
         } else {
-            item.to_tokens(&ecx.sess.parse_sess)
+            item.to_tokens()
         };
 
         let stream = {
diff --git a/compiler/rustc_expand/src/proc_macro_server.rs b/compiler/rustc_expand/src/proc_macro_server.rs
index 5b4c65a306b..d4407c03d03 100644
--- a/compiler/rustc_expand/src/proc_macro_server.rs
+++ b/compiler/rustc_expand/src/proc_macro_server.rs
@@ -2,14 +2,13 @@ use crate::base::ExtCtxt;
 
 use rustc_ast as ast;
 use rustc_ast::token;
-use rustc_ast::tokenstream::{self, CanSynthesizeMissingTokens};
-use rustc_ast::tokenstream::{DelimSpan, Spacing::*, TokenStream, TreeAndSpacing};
+use rustc_ast::tokenstream::{self, DelimSpan, Spacing::*, TokenStream, TreeAndSpacing};
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::{Diagnostic, MultiSpan, PResult};
 use rustc_parse::lexer::nfc_normalize;
-use rustc_parse::{nt_to_tokenstream, parse_stream_from_source_str};
+use rustc_parse::parse_stream_from_source_str;
 use rustc_session::parse::ParseSess;
 use rustc_span::def_id::CrateNum;
 use rustc_span::symbol::{self, kw, sym, Symbol};
@@ -179,10 +178,9 @@ impl FromInternal<(TreeAndSpacing, &'_ mut Vec<Self>, &mut Rustc<'_, '_>)>
                 TokenTree::Ident(Ident::new(rustc.sess(), ident.name, is_raw, ident.span))
             }
             Interpolated(nt) => {
-                let stream = nt_to_tokenstream(&nt, rustc.sess(), CanSynthesizeMissingTokens::No);
                 TokenTree::Group(Group {
                     delimiter: pm::Delimiter::None,
-                    stream,
+                    stream: TokenStream::from_nonterminal_ast(&nt),
                     span: DelimSpan::from_single(span),
                     flatten: crate::base::nt_pretty_printing_compatibility_hack(&nt, rustc.sess()),
                 })
@@ -454,7 +452,7 @@ impl server::TokenStream for Rustc<'_, '_> {
 
         // NOTE: For now, limit `expand_expr` to exclusively expand to literals.
         // This may be relaxed in the future.
-        // We don't use `nt_to_tokenstream` as the tokenstream currently cannot
+        // We don't use `TokenStream::from_ast` as the tokenstream currently cannot
         // be recovered in the general case.
         match &expr.kind {
             ast::ExprKind::Lit(l) => {
diff --git a/compiler/rustc_interface/src/passes.rs b/compiler/rustc_interface/src/passes.rs
index 00119267e85..afbf48fceb5 100644
--- a/compiler/rustc_interface/src/passes.rs
+++ b/compiler/rustc_interface/src/passes.rs
@@ -494,13 +494,7 @@ pub fn lower_to_hir<'res, 'tcx>(
     arena: &'tcx rustc_ast_lowering::Arena<'tcx>,
 ) -> &'tcx Crate<'tcx> {
     // Lower AST to HIR.
-    let hir_crate = rustc_ast_lowering::lower_crate(
-        sess,
-        &*krate,
-        resolver,
-        rustc_parse::nt_to_tokenstream,
-        arena,
-    );
+    let hir_crate = rustc_ast_lowering::lower_crate(sess, &*krate, resolver, arena);
 
     // Drop AST to free memory
     sess.time("drop_ast", || std::mem::drop(krate));
diff --git a/compiler/rustc_parse/src/lib.rs b/compiler/rustc_parse/src/lib.rs
index c46a8b58c29..df1765952ce 100644
--- a/compiler/rustc_parse/src/lib.rs
+++ b/compiler/rustc_parse/src/lib.rs
@@ -12,20 +12,16 @@
 extern crate tracing;
 
 use rustc_ast as ast;
-use rustc_ast::token::{self, Nonterminal, Token, TokenKind};
-use rustc_ast::tokenstream::{self, AttributesData, CanSynthesizeMissingTokens, LazyTokenStream};
-use rustc_ast::tokenstream::{AttrAnnotatedTokenStream, AttrAnnotatedTokenTree};
-use rustc_ast::tokenstream::{Spacing, TokenStream};
+use rustc_ast::token;
+use rustc_ast::tokenstream::TokenStream;
 use rustc_ast::Attribute;
 use rustc_ast::{AttrItem, MetaItem};
-use rustc_ast::{HasAttrs, HasSpan, HasTokens};
-use rustc_ast_pretty::pprust::{self, AstPrettyPrint};
+use rustc_ast_pretty::pprust;
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::{Applicability, Diagnostic, FatalError, Level, PResult};
 use rustc_session::parse::ParseSess;
 use rustc_span::{FileName, SourceFile, Span};
 
-use std::fmt;
 use std::path::Path;
 
 pub const MACRO_ARGUMENTS: Option<&str> = Some("macro arguments");
@@ -240,111 +236,10 @@ pub fn parse_in<'a, T>(
     Ok(result)
 }
 
-// NOTE(Centril): The following probably shouldn't be here but it acknowledges the
-// fact that architecturally, we are using parsing (read on below to understand why).
-
-pub fn to_token_stream(
-    node: &(impl HasAttrs + HasSpan + HasTokens + AstPrettyPrint + fmt::Debug),
-    sess: &ParseSess,
-    synthesize_tokens: CanSynthesizeMissingTokens,
-) -> TokenStream {
-    if let Some(tokens) = prepend_attrs(&node.attrs(), node.tokens()) {
-        return tokens;
-    } else if matches!(synthesize_tokens, CanSynthesizeMissingTokens::Yes) {
-        return fake_token_stream(sess, node);
-    } else {
-        panic!("Missing tokens for nt {:?} at {:?}: {:?}", node, node.span(), node.pretty_print());
-    }
-}
-
-pub fn nt_to_tokenstream(
-    nt: &Nonterminal,
-    sess: &ParseSess,
-    synthesize_tokens: CanSynthesizeMissingTokens,
-) -> TokenStream {
-    // A `Nonterminal` is often a parsed AST item. At this point we now
-    // need to convert the parsed AST to an actual token stream, e.g.
-    // un-parse it basically.
-    //
-    // Unfortunately there's not really a great way to do that in a
-    // guaranteed lossless fashion right now. The fallback here is to just
-    // stringify the AST node and reparse it, but this loses all span
-    // information.
-    //
-    // As a result, some AST nodes are annotated with the token stream they
-    // came from. Here we attempt to extract these lossless token streams
-    // before we fall back to the stringification.
-
-    let convert_tokens =
-        |tokens: Option<&LazyTokenStream>| Some(tokens?.create_token_stream().to_tokenstream());
-
-    let tokens = match *nt {
-        Nonterminal::NtItem(ref item) => prepend_attrs(&item.attrs, item.tokens.as_ref()),
-        Nonterminal::NtBlock(ref block) => convert_tokens(block.tokens.as_ref()),
-        Nonterminal::NtStmt(ref stmt) if let ast::StmtKind::Empty = stmt.kind => {
-            let tokens = AttrAnnotatedTokenStream::new(vec![(
-                tokenstream::AttrAnnotatedTokenTree::Token(Token::new(
-                    TokenKind::Semi,
-                    stmt.span,
-                )),
-                Spacing::Alone,
-            )]);
-            prepend_attrs(&stmt.attrs(), Some(&LazyTokenStream::new(tokens)))
-        }
-        Nonterminal::NtStmt(ref stmt) => prepend_attrs(&stmt.attrs(), stmt.tokens()),
-        Nonterminal::NtPat(ref pat) => convert_tokens(pat.tokens.as_ref()),
-        Nonterminal::NtTy(ref ty) => convert_tokens(ty.tokens.as_ref()),
-        Nonterminal::NtIdent(ident, is_raw) => {
-            Some(tokenstream::TokenTree::token(token::Ident(ident.name, is_raw), ident.span).into())
-        }
-        Nonterminal::NtLifetime(ident) => {
-            Some(tokenstream::TokenTree::token(token::Lifetime(ident.name), ident.span).into())
-        }
-        Nonterminal::NtMeta(ref attr) => convert_tokens(attr.tokens.as_ref()),
-        Nonterminal::NtPath(ref path) => convert_tokens(path.tokens.as_ref()),
-        Nonterminal::NtVis(ref vis) => convert_tokens(vis.tokens.as_ref()),
-        Nonterminal::NtExpr(ref expr) | Nonterminal::NtLiteral(ref expr) => {
-            prepend_attrs(&expr.attrs, expr.tokens.as_ref())
-        }
-    };
-
-    if let Some(tokens) = tokens {
-        return tokens;
-    } else if matches!(synthesize_tokens, CanSynthesizeMissingTokens::Yes) {
-        return nt_fake_token_stream(sess, nt);
-    } else {
-        panic!(
-            "Missing tokens for nt {:?} at {:?}: {:?}",
-            nt,
-            nt.span(),
-            pprust::nonterminal_to_string(nt)
-        );
-    }
-}
-
-fn prepend_attrs(attrs: &[Attribute], tokens: Option<&LazyTokenStream>) -> Option<TokenStream> {
-    let tokens = tokens?;
-    if attrs.is_empty() {
-        return Some(tokens.create_token_stream().to_tokenstream());
-    }
-    let attr_data = AttributesData { attrs: attrs.to_vec().into(), tokens: tokens.clone() };
-    let wrapped = AttrAnnotatedTokenStream::new(vec![(
-        AttrAnnotatedTokenTree::Attributes(attr_data),
-        Spacing::Alone,
-    )]);
-    Some(wrapped.to_tokenstream())
-}
-
-pub fn fake_token_stream(sess: &ParseSess, node: &(impl AstPrettyPrint + HasSpan)) -> TokenStream {
-    let source = node.pretty_print();
-    let filename = FileName::macro_expansion_source_code(&source);
-    parse_stream_from_source_str(filename, source, sess, Some(node.span()))
-}
-
-fn nt_fake_token_stream(sess: &ParseSess, nt: &Nonterminal) -> TokenStream {
-    let source = pprust::nonterminal_to_string(nt);
+pub fn fake_token_stream_for_item(sess: &ParseSess, item: &ast::Item) -> TokenStream {
+    let source = pprust::item_to_string(item);
     let filename = FileName::macro_expansion_source_code(&source);
-    parse_stream_from_source_str(filename, source, sess, Some(nt.span()))
+    parse_stream_from_source_str(filename, source, sess, Some(item.span))
 }
 
 pub fn fake_token_stream_for_crate(sess: &ParseSess, krate: &ast::Crate) -> TokenStream {
diff --git a/compiler/rustc_session/src/utils.rs b/compiler/rustc_session/src/utils.rs
index 9fda5373fcc..bda7b314308 100644
--- a/compiler/rustc_session/src/utils.rs
+++ b/compiler/rustc_session/src/utils.rs
@@ -1,13 +1,7 @@
-use crate::parse::ParseSess;
 use crate::session::Session;
-use rustc_ast::token::{self, Delimiter, Nonterminal, Token};
-use rustc_ast::tokenstream::CanSynthesizeMissingTokens;
-use rustc_ast::tokenstream::{DelimSpan, TokenStream, TokenTree};
 use rustc_data_structures::profiling::VerboseTimingGuard;
 use std::path::{Path, PathBuf};
 
-pub type NtToTokenstream = fn(&Nonterminal, &ParseSess, CanSynthesizeMissingTokens) -> TokenStream;
-
 impl Session {
     pub fn timer<'a>(&'a self, what: &'static str) -> VerboseTimingGuard<'a> {
         self.prof.verbose_generic_activity(what)
@@ -94,55 +88,3 @@ impl CanonicalizedPath {
         &self.original
     }
 }
-
-// FIXME: Find a better spot for this - it needs to be accessible from `rustc_ast_lowering`,
-// and needs to access `ParseSess
-pub struct FlattenNonterminals<'a> {
-    pub parse_sess: &'a ParseSess,
-    pub synthesize_tokens: CanSynthesizeMissingTokens,
-    pub nt_to_tokenstream: NtToTokenstream,
-}
-
-impl<'a> FlattenNonterminals<'a> {
-    pub fn process_token_stream(&mut self, tokens: TokenStream) -> TokenStream {
-        fn can_skip(stream: &TokenStream) -> bool {
-            stream.trees().all(|tree| match tree {
-                TokenTree::Token(token) => !matches!(token.kind, token::Interpolated(_)),
-                TokenTree::Delimited(_, _, inner) => can_skip(inner),
-            })
-        }
-
-        if can_skip(&tokens) {
-            return tokens;
-        }
-
-        tokens.into_trees().flat_map(|tree| self.process_token_tree(tree).into_trees()).collect()
-    }
-
-    pub fn process_token_tree(&mut self, tree: TokenTree) -> TokenStream {
-        match tree {
-            TokenTree::Token(token) => self.process_token(token),
-            TokenTree::Delimited(span, delim, tts) => {
-                TokenTree::Delimited(span, delim, self.process_token_stream(tts)).into()
-            }
-        }
-    }
-
-    pub fn process_token(&mut self, token: Token) -> TokenStream {
-        match token.kind {
-            token::Interpolated(nt) if let token::NtIdent(ident, is_raw) = *nt => {
-                TokenTree::Token(Token::new(token::Ident(ident.name, is_raw), ident.span)).into()
-            }
-            token::Interpolated(nt) => {
-                let tts = (self.nt_to_tokenstream)(&nt, self.parse_sess, self.synthesize_tokens);
-                TokenTree::Delimited(
-                    DelimSpan::from_single(token.span),
-                    Delimiter::Invisible,
-                    self.process_token_stream(tts),
-                )
-                .into()
-            }
-            _ => TokenTree::Token(token).into(),
-        }
-    }
-}