Rename kw::Empty as sym::empty.

Because the empty string is not a keyword.

Author: nnethercote

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::{Ident, Span, Symbol, kw, sym};
+use rustc_span::{Ident, Span, Symbol, sym};
 
 use super::LoweringContext;
 
@@ -418,7 +418,7 @@ 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_codegen_ssa/src/mir/debuginfo.rs

@@ -10,7 +10,7 @@ use rustc_middle::ty::layout::{LayoutOf, TyAndLayout};
 use rustc_middle::ty::{Instance, Ty};
 use rustc_middle::{bug, mir, ty};
 use rustc_session::config::DebugInfo;
-use rustc_span::{BytePos, Span, Symbol, hygiene, kw};
+use rustc_span::{BytePos, Span, Symbol, hygiene, sym};
 
 use super::operand::{OperandRef, OperandValue};
 use super::place::{PlaceRef, PlaceValue};
@@ -283,7 +283,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 // (after #67586 gets fixed).
                 None
             } else {
-                let name = kw::Empty;
+                let name = sym::empty;
                 let decl = &self.mir.local_decls[local];
                 let dbg_var = if full_debug_info {
                     self.adjusted_span_and_dbg_scope(decl.source_info).map(
@@ -318,7 +318,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             None
         } else {
             Some(match whole_local_var.or(fallback_var.clone()) {
-                Some(var) if var.name != kw::Empty => var.name.to_string(),
+                Some(var) if var.name != sym::empty => var.name.to_string(),
                 _ => format!("{local:?}"),
             })
         };

compiler/rustc_hir/src/hir.rs

@@ -85,7 +85,7 @@ impl From<Ident> for LifetimeSyntax {
     fn from(ident: Ident) -> Self {
         let name = ident.name;
 
-        if name == kw::Empty {
+        if name == sym::empty {
             unreachable!("A lifetime name should never be empty");
         } else if name == kw::UnderscoreLifetime {
             LifetimeSyntax::Anonymous

compiler/rustc_passes/src/check_attr.rs

@@ -35,7 +35,7 @@ use rustc_session::lint::builtin::{
     UNKNOWN_OR_MALFORMED_DIAGNOSTIC_ATTRIBUTES, UNUSED_ATTRIBUTES,
 };
 use rustc_session::parse::feature_err;
-use rustc_span::{BytePos, DUMMY_SP, Span, Symbol, edition, kw, sym};
+use rustc_span::{BytePos, DUMMY_SP, Span, Symbol, edition, sym};
 use rustc_trait_selection::error_reporting::InferCtxtErrorExt;
 use rustc_trait_selection::infer::{TyCtxtInferExt, ValuePairs};
 use rustc_trait_selection::traits::ObligationCtxt;
@@ -936,7 +936,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         let span = meta.name_value_literal_span().unwrap_or_else(|| meta.span());
         let attr_str =
             &format!("`#[doc(alias{})]`", if is_list { "(\"...\")" } else { " = \"...\"" });
-        if doc_alias == kw::Empty {
+        if doc_alias == sym::empty {
             tcx.dcx().emit_err(errors::DocAliasEmpty { span, attr_str });
             return;
         }
@@ -1068,7 +1068,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         }
 
         let doc_keyword = match meta.value_str() {
-            Some(value) if value != kw::Empty => value,
+            Some(value) if value != sym::empty => value,
             _ => return self.doc_attr_str_error(meta, "keyword"),
         };
 

compiler/rustc_resolve/src/rustdoc.rs

@@ -12,7 +12,7 @@ use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::unord::UnordSet;
 use rustc_middle::ty::TyCtxt;
 use rustc_span::def_id::DefId;
-use rustc_span::{DUMMY_SP, InnerSpan, Span, Symbol, kw, sym};
+use rustc_span::{DUMMY_SP, InnerSpan, Span, Symbol, sym};
 use thin_vec::ThinVec;
 use tracing::{debug, trace};
 
@@ -157,7 +157,7 @@ pub fn unindent_doc_fragments(docs: &mut [DocFragment]) {
     };
 
     for fragment in docs {
-        if fragment.doc == kw::Empty {
+        if fragment.doc == sym::empty {
             continue;
         }
 
@@ -177,7 +177,7 @@ pub fn unindent_doc_fragments(docs: &mut [DocFragment]) {
 ///
 /// Note: remove the trailing newline where appropriate
 pub fn add_doc_fragment(out: &mut String, frag: &DocFragment) {
-    if frag.doc == kw::Empty {
+    if frag.doc == sym::empty {
         out.push('\n');
         return;
     }

compiler/rustc_span/src/symbol.rs

@@ -34,17 +34,8 @@ symbols! {
         // unnamed method parameters, crate root module, error recovery etc.
         // Matching predicates: `is_special`/`is_reserved`
         //
-        // Notes about `kw::Empty`:
-        // - Its use can blur the lines between "empty symbol" and "no symbol".
-        //   Using `Option<Symbol>` is preferable, where possible, because that
-        //   is unambiguous.
-        // - For dummy symbols that are never used and absolutely must be
-        //   present, it's better to use `sym::dummy` than `kw::Empty`, because
-        //   it's clearer that it's intended as a dummy value, and more likely
-        //   to be detected if it accidentally does get used.
         // tidy-alphabetical-start
         DollarCrate:        "$crate",
-        Empty:              "",
         PathRoot:           "{{root}}",
         Underscore:         "_",
         // tidy-alphabetical-end
@@ -864,7 +855,7 @@ symbols! {
         drop_types_in_const,
         dropck_eyepatch,
         dropck_parametricity,
-        dummy: "<!dummy!>", // use this instead of `kw::Empty` for symbols that won't be used
+        dummy: "<!dummy!>", // use this instead of `sym::empty` for symbols that won't be used
         dummy_cgu_name,
         dylib,
         dyn_compatible_for_dispatch,
@@ -883,6 +874,14 @@ symbols! {
         emit_enum_variant_arg,
         emit_struct,
         emit_struct_field,
+        // Notes about `sym::empty`:
+        // - It should only be used when it genuinely means "empty symbol". Use
+        //   `Option<Symbol>` when "no symbol" is a possibility.
+        // - For dummy symbols that are never used and absolutely must be
+        //   present, it's better to use `sym::dummy` than `sym::empty`, because
+        //   it's clearer that it's intended as a dummy value, and more likely
+        //   to be detected if it accidentally does get used.
+        empty: "",
         emscripten_wasm_eh,
         enable,
         encode,
@@ -2362,7 +2361,7 @@ impl Ident {
     #[inline]
     /// Constructs a new identifier from a symbol and a span.
     pub fn new(name: Symbol, span: Span) -> Ident {
-        debug_assert_ne!(name, kw::Empty);
+        debug_assert_ne!(name, sym::empty);
         Ident { name, span }
     }
 
@@ -2584,7 +2583,7 @@ impl Symbol {
     }
 
     pub fn is_empty(self) -> bool {
-        self == kw::Empty
+        self == sym::empty
     }
 
     /// This method is supposed to be used in error messages, so it's expected to be
@@ -2593,7 +2592,7 @@ impl Symbol {
     /// or edition, so we have to guess the rawness using the global edition.
     pub fn to_ident_string(self) -> String {
         // Avoid creating an empty identifier, because that asserts in debug builds.
-        if self == kw::Empty { String::new() } else { Ident::with_dummy_span(self).to_string() }
+        if self == sym::empty { String::new() } else { Ident::with_dummy_span(self).to_string() }
     }
 }
 
@@ -2773,7 +2772,7 @@ impl Symbol {
 
     /// Returns `true` if this symbol can be a raw identifier.
     pub fn can_be_raw(self) -> bool {
-        self != kw::Empty && self != kw::Underscore && !self.is_path_segment_keyword()
+        self != sym::empty && self != kw::Underscore && !self.is_path_segment_keyword()
     }
 
     /// Was this symbol predefined in the compiler's `symbols!` macro

compiler/rustc_symbol_mangling/src/v0.rs

@@ -20,7 +20,7 @@ use rustc_middle::ty::{
     self, FloatTy, GenericArg, GenericArgKind, Instance, IntTy, ReifyReason, Ty, TyCtxt,
     TypeVisitable, TypeVisitableExt, UintTy,
 };
-use rustc_span::kw;
+use rustc_span::sym;
 
 pub(super) fn mangle<'tcx>(
     tcx: TyCtxt<'tcx>,
@@ -902,7 +902,7 @@ impl<'tcx> Printer<'tcx> for SymbolMangler<'tcx> {
             print_prefix,
             ns,
             disambiguated_data.disambiguator as u64,
-            name.unwrap_or(kw::Empty).as_str(),
+            name.unwrap_or(sym::empty).as_str(),
         )
     }
 

src/tools/clippy/clippy_lints/src/manual_string_new.rs

@@ -5,7 +5,7 @@ use rustc_hir::{Expr, ExprKind, PathSegment, QPath, TyKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty;
 use rustc_session::declare_lint_pass;
-use rustc_span::{Span, sym, symbol};
+use rustc_span::{Span, sym};
 
 declare_clippy_lint! {
     /// ### What it does
@@ -67,7 +67,7 @@ impl LateLintPass<'_> for ManualStringNew {
 fn is_expr_kind_empty_str(expr_kind: &ExprKind<'_>) -> bool {
     if let ExprKind::Lit(lit) = expr_kind
         && let LitKind::Str(value, _) = lit.node
-        && value == symbol::kw::Empty
+        && value == sym::empty
     {
         return true;
     }

src/tools/clippy/clippy_lints/src/methods/or_fun_call.rs

@@ -12,7 +12,7 @@ use rustc_errors::Applicability;
 use rustc_lint::LateContext;
 use rustc_middle::ty;
 use rustc_span::Span;
-use rustc_span::symbol::{self, Symbol};
+use rustc_span::Symbol;
 use {rustc_ast as ast, rustc_hir as hir};
 
 use super::{OR_FUN_CALL, UNWRAP_OR_DEFAULT};
@@ -265,7 +265,7 @@ fn closure_body_returns_empty_to_string(cx: &LateContext<'_>, e: &hir::Expr<'_>)
             && ident.name == sym::to_string
             && let hir::Expr { kind, .. } = self_arg
             && let hir::ExprKind::Lit(lit) = kind
-            && let ast::LitKind::Str(symbol::kw::Empty, _) = lit.node
+            && let ast::LitKind::Str(rustc_span::sym::empty, _) = lit.node
         {
             return true;
         }