Remove non-keywords from keywords list.

Specifically, `Empty`, `PathRoot`, `DollarCrate`, and `Underscore`. They
are turned into ordinary symbols.

`is_special` is changed: `{{root}}` and the empty symbol are no longer
considered special, but `$crate` and `_` still are. This is required to
avoid changing any behaviour (in tests, etc.)

Author: nnethercote

compiler/rustc_ast/src/ast.rs

@@ -92,7 +92,7 @@ impl fmt::Display for Lifetime {
 pub struct Path {
     pub span: Span,
     /// The segments in the path: the things separated by `::`.
-    /// Global paths begin with `kw::PathRoot`.
+    /// Global paths begin with `sym::path_root`.
     pub segments: ThinVec<PathSegment>,
     pub tokens: Option<LazyAttrTokenStream>,
 }
@@ -121,7 +121,7 @@ impl Path {
     }
 
     pub fn is_global(&self) -> bool {
-        !self.segments.is_empty() && self.segments[0].ident.name == kw::PathRoot
+        !self.segments.is_empty() && self.segments[0].ident.name == sym::path_root
     }
 
     /// If this path is a single identifier with no arguments, does not ensure
@@ -156,7 +156,7 @@ impl PathSegment {
     }
 
     pub fn path_root(span: Span) -> Self {
-        PathSegment::from_ident(Ident::new(kw::PathRoot, span))
+        PathSegment::from_ident(Ident::new(sym::path_root, span))
     }
 
     pub fn span(&self) -> Span {

compiler/rustc_ast/src/token.rs

@@ -326,7 +326,7 @@ fn ident_can_begin_type(name: Symbol, span: Span, is_raw: IdentIsRaw) -> bool {
 
     !ident_token.is_reserved_ident()
         || ident_token.is_path_segment_keyword()
-        || [kw::Underscore, kw::For, kw::Impl, kw::Fn, kw::Unsafe, kw::Extern, kw::Typeof, kw::Dyn]
+        || [sym::underscore, kw::For, kw::Impl, kw::Fn, kw::Unsafe, kw::Extern, kw::Typeof, kw::Dyn]
             .contains(&name)
 }
 
@@ -922,8 +922,7 @@ impl Token {
         self.is_non_raw_ident_where(Ident::is_any_keyword)
     }
 
-    /// Returns true for reserved identifiers used internally for elided lifetimes,
-    /// unnamed method parameters, crate root module, error recovery etc.
+    /// Returns true for reserved identifiers.
     pub fn is_special_ident(&self) -> bool {
         self.is_non_raw_ident_where(Ident::is_special)
     }

compiler/rustc_ast_lowering/src/asm.rs

@@ -7,7 +7,6 @@ use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_session::parse::feature_err;
-use rustc_span::symbol::kw;
 use rustc_span::{Span, sym};
 use rustc_target::asm;
 
@@ -232,7 +231,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             self.create_def(
                                 parent_def_id,
                                 node_id,
-                                kw::Empty,
+                                sym::empty,
                                 DefKind::AnonConst,
                                 *op_sp,
                             );

compiler/rustc_ast_lowering/src/expr.rs

@@ -13,7 +13,7 @@ use rustc_middle::span_bug;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::errors::report_lit_error;
 use rustc_span::source_map::{Spanned, respan};
-use rustc_span::symbol::{Ident, Symbol, kw, sym};
+use rustc_span::symbol::{Ident, Symbol, sym};
 use rustc_span::{DUMMY_SP, DesugaringKind, Span};
 use thin_vec::{ThinVec, thin_vec};
 use visit::{Visitor, walk_expr};
@@ -457,7 +457,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             if legacy_args_idx.contains(&idx) {
                 let parent_def_id = self.current_hir_id_owner.def_id;
                 let node_id = self.next_node_id();
-                self.create_def(parent_def_id, node_id, kw::Empty, DefKind::AnonConst, f.span);
+                self.create_def(parent_def_id, node_id, sym::empty, DefKind::AnonConst, f.span);
                 let mut visitor = WillCreateDefIdsVisitor {};
                 let const_value = if let ControlFlow::Break(span) = visitor.visit_expr(&arg) {
                     AstP(Expr {

compiler/rustc_ast_lowering/src/format.rs

@@ -6,7 +6,7 @@ use rustc_ast::*;
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_hir as hir;
 use rustc_session::config::FmtDebug;
-use rustc_span::symbol::{Ident, kw};
+use rustc_span::symbol::Ident;
 use rustc_span::{Span, Symbol, sym};
 
 use super::LoweringContext;
@@ -416,7 +416,7 @@ fn expand_format_args<'hir>(
                 &FormatArgsPiece::Placeholder(_) => {
                     // Inject empty string before placeholders when not already preceded by a literal piece.
                     if i == 0 || matches!(fmt.template[i - 1], FormatArgsPiece::Placeholder(_)) {
-                        Some(ctx.expr_str(fmt.span, kw::Empty))
+                        Some(ctx.expr_str(fmt.span, sym::empty))
                     } else {
                         None
                     }

compiler/rustc_ast_lowering/src/item.rs

@@ -564,7 +564,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let path = if trees.is_empty()
                     && !(prefix.segments.is_empty()
                         || prefix.segments.len() == 1
-                            && prefix.segments[0].ident.name == kw::PathRoot)
+                            && prefix.segments[0].ident.name == sym::path_root)
                 {
                     // For empty lists we need to lower the prefix so it is checked for things
                     // like stability later.
@@ -1173,7 +1173,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 // This lets rustdoc render it correctly in documentation.
                 hir::PatKind::Binding(_, _, ident, _) => (ident, false),
                 hir::PatKind::Wild => {
-                    (Ident::with_dummy_span(rustc_span::symbol::kw::Underscore), false)
+                    (Ident::with_dummy_span(rustc_span::symbol::sym::underscore), false)
                 }
                 _ => {
                     // Replace the ident for bindings that aren't simple.

compiler/rustc_ast_lowering/src/lib.rs

@@ -913,7 +913,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 } else {
                     let guar = self.dcx().has_errors().unwrap();
                     MetaItemLit {
-                        symbol: kw::Empty,
+                        symbol: sym::empty,
                         suffix: None,
                         kind: LitKind::Err(guar),
                         span: DUMMY_SP,
@@ -1508,7 +1508,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_fn_params_to_names(&mut self, decl: &FnDecl) -> &'hir [Ident] {
         self.arena.alloc_from_iter(decl.inputs.iter().map(|param| match param.pat.kind {
             PatKind::Ident(_, ident, _) => self.lower_ident(ident),
-            _ => Ident::new(kw::Empty, self.lower_span(param.pat.span)),
+            _ => Ident::new(sym::empty, self.lower_span(param.pat.span)),
         }))
     }
 
@@ -2092,7 +2092,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             // so the def collector didn't create the def ahead of time. That's why we have to do
             // it here.
             let def_id =
-                self.create_def(parent_def_id, node_id, kw::Empty, DefKind::AnonConst, span);
+                self.create_def(parent_def_id, node_id, sym::empty, DefKind::AnonConst, span);
             let hir_id = self.lower_node_id(node_id);
 
             let path_expr = Expr {
@@ -2388,7 +2388,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn elided_dyn_bound(&mut self, span: Span) -> &'hir hir::Lifetime {
         let r = hir::Lifetime {
             hir_id: self.next_id(),
-            ident: Ident::new(kw::Empty, self.lower_span(span)),
+            ident: Ident::new(sym::empty, self.lower_span(span)),
             res: hir::LifetimeName::ImplicitObjectLifetimeDefault,
         };
         debug!("elided_dyn_bound: r={:?}", r);

compiler/rustc_ast_lowering/src/path.rs

@@ -6,7 +6,7 @@ use rustc_hir::def::{DefKind, PartialRes, Res};
 use rustc_hir::def_id::DefId;
 use rustc_middle::span_bug;
 use rustc_session::parse::add_feature_diagnostics;
-use rustc_span::symbol::{Ident, kw, sym};
+use rustc_span::symbol::{Ident, sym};
 use rustc_span::{BytePos, DUMMY_SP, DesugaringKind, Span, Symbol};
 use smallvec::{SmallVec, smallvec};
 use tracing::{debug, instrument};
@@ -445,7 +445,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 let id = NodeId::from_u32(i);
                 let l = self.lower_lifetime(&Lifetime {
                     id,
-                    ident: Ident::new(kw::Empty, elided_lifetime_span),
+                    ident: Ident::new(sym::empty, elided_lifetime_span),
                 });
                 GenericArg::Lifetime(l)
             }),

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -35,7 +35,7 @@ use rustc_session::lint::builtin::{
 };
 use rustc_session::lint::{BuiltinLintDiag, LintBuffer};
 use rustc_span::Span;
-use rustc_span::symbol::{Ident, kw, sym};
+use rustc_span::symbol::{Ident, sym};
 use rustc_target::spec::abi;
 use thin_vec::thin_vec;
 
@@ -209,7 +209,7 @@ impl<'a> AstValidator<'a> {
 
     fn visit_struct_field_def(&mut self, field: &'a FieldDef) {
         if let Some(ref ident) = field.ident
-            && ident.name == kw::Underscore
+            && ident.name == sym::underscore
         {
             self.visit_vis(&field.vis);
             self.visit_ident(ident);
@@ -563,7 +563,7 @@ impl<'a> AstValidator<'a> {
     }
 
     fn check_item_named(&self, ident: Ident, kind: &str) {
-        if ident.name != kw::Underscore {
+        if ident.name != sym::underscore {
             return;
         }
         self.dcx().emit_err(errors::ItemUnderscore { span: ident.span, kind });

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -827,7 +827,7 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
     }
 
     fn print_path_segment(&mut self, segment: &ast::PathSegment, colons_before_params: bool) {
-        if segment.ident.name != kw::PathRoot {
+        if segment.ident.name != sym::path_root {
             self.print_ident(segment.ident);
             if let Some(args) = &segment.args {
                 self.print_generic_args(args, colons_before_params);
@@ -1902,7 +1902,7 @@ impl<'a> State<'a> {
                     self.print_explicit_self(&eself);
                 } else {
                     let invalid = if let PatKind::Ident(_, ident, _) = input.pat.kind {
-                        ident.name == kw::Empty
+                        ident.name == sym::empty
                     } else {
                         false
                     };