diff --git a/Cargo.lock b/Cargo.lock
index ff21d5f8c0861..bd121921b0b96 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4054,7 +4054,6 @@ dependencies = [
  "rustc_hir_analysis",
  "rustc_hir_typeck",
  "rustc_incremental",
- "rustc_index",
  "rustc_lint",
  "rustc_macros",
  "rustc_metadata",
diff --git a/compiler/rustc_driver_impl/src/lib.rs b/compiler/rustc_driver_impl/src/lib.rs
index 331843ab05151..635930742fc8b 100644
--- a/compiler/rustc_driver_impl/src/lib.rs
+++ b/compiler/rustc_driver_impl/src/lib.rs
@@ -318,7 +318,7 @@ fn run_compiler(
         return Ok(());
     }
 
-    let cfg = interface::parse_cfgspecs(&early_error_handler, matches.opt_strs("cfg"));
+    let cfg = interface::parse_cfg(&early_error_handler, matches.opt_strs("cfg"));
     let check_cfg = interface::parse_check_cfg(&early_error_handler, matches.opt_strs("check-cfg"));
     let (odir, ofile) = make_output(&matches);
     let mut config = interface::Config {
diff --git a/compiler/rustc_interface/Cargo.toml b/compiler/rustc_interface/Cargo.toml
index ae008674d0127..b280665057cc3 100644
--- a/compiler/rustc_interface/Cargo.toml
+++ b/compiler/rustc_interface/Cargo.toml
@@ -26,7 +26,6 @@ rustc_middle = { path = "../rustc_middle" }
 rustc_ast_lowering = { path = "../rustc_ast_lowering" }
 rustc_ast_passes = { path = "../rustc_ast_passes" }
 rustc_incremental = { path = "../rustc_incremental" }
-rustc_index = { path = "../rustc_index" }
 rustc_traits = { path = "../rustc_traits" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_codegen_ssa = { path = "../rustc_codegen_ssa" }
diff --git a/compiler/rustc_interface/src/callbacks.rs b/compiler/rustc_interface/src/callbacks.rs
index 45b1aeb4a5c36..6fa989bb96ce0 100644
--- a/compiler/rustc_interface/src/callbacks.rs
+++ b/compiler/rustc_interface/src/callbacks.rs
@@ -26,7 +26,7 @@ fn track_span_parent(def_id: rustc_span::def_id::LocalDefId) {
     })
 }
 
-/// This is a callback from `rustc_ast` as it cannot access the implicit state
+/// This is a callback from `rustc_errors` as it cannot access the implicit state
 /// in `rustc_middle` otherwise. It is used when diagnostic messages are
 /// emitted and stores them in the current query, if there is one.
 fn track_diagnostic(diagnostic: &mut Diagnostic, f: &mut dyn FnMut(&mut Diagnostic)) {
diff --git a/compiler/rustc_interface/src/interface.rs b/compiler/rustc_interface/src/interface.rs
index 87badcbdf15ba..7672523292b4f 100644
--- a/compiler/rustc_interface/src/interface.rs
+++ b/compiler/rustc_interface/src/interface.rs
@@ -15,8 +15,10 @@ use rustc_middle::{bug, ty};
 use rustc_parse::maybe_new_parser_from_source_str;
 use rustc_query_impl::QueryCtxt;
 use rustc_query_system::query::print_query_stack;
-use rustc_session::config::{self, CheckCfg, ExpectedValues, Input, OutFileName, OutputFilenames};
-use rustc_session::parse::{CrateConfig, ParseSess};
+use rustc_session::config::{
+    self, Cfg, CheckCfg, ExpectedValues, Input, OutFileName, OutputFilenames,
+};
+use rustc_session::parse::ParseSess;
 use rustc_session::CompilerIO;
 use rustc_session::Session;
 use rustc_session::{lint, EarlyErrorHandler};
@@ -61,14 +63,13 @@ impl Compiler {
     }
 }
 
-/// Converts strings provided as `--cfg [cfgspec]` into a `crate_cfg`.
-pub fn parse_cfgspecs(
-    handler: &EarlyErrorHandler,
-    cfgspecs: Vec<String>,
-) -> FxHashSet<(String, Option<String>)> {
+/// Converts strings provided as `--cfg [cfgspec]` into a `Cfg`.
+pub fn parse_cfg(handler: &EarlyErrorHandler, cfgs: Vec<String>) -> Cfg<String> {
+    // This creates a short-lived `SessionGlobals`, containing an interner. The
+    // parsed values are converted from symbols to strings before exiting
+    // because the symbols are meaningless once the interner is gone.
     rustc_span::create_default_session_if_not_set_then(move |_| {
-        let cfg = cfgspecs
-            .into_iter()
+        cfgs.into_iter()
             .map(|s| {
                 let sess = ParseSess::with_silent_emitter(Some(format!(
                     "this error occurred on the command line: `--cfg={s}`"
@@ -97,7 +98,10 @@ pub fn parse_cfgspecs(
                                 }
                                 MetaItemKind::NameValue(..) | MetaItemKind::Word => {
                                     let ident = meta_item.ident().expect("multi-segment cfg key");
-                                    return (ident.name, meta_item.value_str());
+                                    return (
+                                        ident.name.to_string(),
+                                        meta_item.value_str().map(|sym| sym.to_string()),
+                                    );
                                 }
                             }
                         }
@@ -118,13 +122,14 @@ pub fn parse_cfgspecs(
                     error!(r#"expected `key` or `key="value"`"#);
                 }
             })
-            .collect::<CrateConfig>();
-        cfg.into_iter().map(|(a, b)| (a.to_string(), b.map(|b| b.to_string()))).collect()
+            .collect::<Cfg<String>>()
     })
 }
 
 /// Converts strings provided as `--check-cfg [specs]` into a `CheckCfg`.
-pub fn parse_check_cfg(handler: &EarlyErrorHandler, specs: Vec<String>) -> CheckCfg {
+pub fn parse_check_cfg(handler: &EarlyErrorHandler, specs: Vec<String>) -> CheckCfg<String> {
+    // The comment about `SessionGlobals` and symbols in `parse_cfg` above
+    // applies here too.
     rustc_span::create_default_session_if_not_set_then(move |_| {
         // If any --check-cfg is passed then exhaustive_values and exhaustive_names
         // are enabled by default.
@@ -372,8 +377,8 @@ pub struct Config {
     pub opts: config::Options,
 
     /// cfg! configuration in addition to the default ones
-    pub crate_cfg: FxHashSet<(String, Option<String>)>,
-    pub crate_check_cfg: CheckCfg,
+    pub crate_cfg: Cfg<String>,
+    pub crate_check_cfg: CheckCfg<String>,
 
     pub input: Input,
     pub output_dir: Option<PathBuf>,
diff --git a/compiler/rustc_interface/src/tests.rs b/compiler/rustc_interface/src/tests.rs
index 3d191669b1adb..26cd83ac9687b 100644
--- a/compiler/rustc_interface/src/tests.rs
+++ b/compiler/rustc_interface/src/tests.rs
@@ -1,17 +1,16 @@
 #![allow(rustc::bad_opt_access)]
-use crate::interface::parse_cfgspecs;
-
-use rustc_data_structures::fx::FxHashSet;
+use crate::interface::parse_cfg;
 use rustc_data_structures::profiling::TimePassesFormat;
 use rustc_errors::{emitter::HumanReadableErrorType, registry, ColorConfig};
 use rustc_session::config::rustc_optgroups;
+use rustc_session::config::Cfg;
 use rustc_session::config::DebugInfo;
 use rustc_session::config::Input;
 use rustc_session::config::InstrumentXRay;
 use rustc_session::config::LinkSelfContained;
 use rustc_session::config::Polonius;
 use rustc_session::config::TraitSolver;
-use rustc_session::config::{build_configuration, build_session_options, to_crate_config};
+use rustc_session::config::{build_configuration, build_session_options};
 use rustc_session::config::{
     BranchProtection, Externs, OomStrategy, OutFileName, OutputType, OutputTypes, PAuthKey, PacRet,
     ProcMacroExecutionStrategy, SymbolManglingVersion, WasiExecModel,
@@ -31,26 +30,18 @@ use rustc_span::FileName;
 use rustc_span::SourceFileHashAlgorithm;
 use rustc_target::spec::{CodeModel, LinkerFlavorCli, MergeFunctions, PanicStrategy, RelocModel};
 use rustc_target::spec::{RelroLevel, SanitizerSet, SplitDebuginfo, StackProtector, TlsModel};
-
 use std::collections::{BTreeMap, BTreeSet};
 use std::num::NonZeroUsize;
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
 
-type CfgSpecs = FxHashSet<(String, Option<String>)>;
-
-fn build_session_options_and_crate_config(
+fn mk_session(
     handler: &mut EarlyErrorHandler,
     matches: getopts::Matches,
-) -> (Options, CfgSpecs) {
-    let sessopts = build_session_options(handler, &matches);
-    let cfg = parse_cfgspecs(handler, matches.opt_strs("cfg"));
-    (sessopts, cfg)
-}
-
-fn mk_session(handler: &mut EarlyErrorHandler, matches: getopts::Matches) -> (Session, CfgSpecs) {
+) -> (Session, Cfg<String>) {
     let registry = registry::Registry::new(&[]);
-    let (sessopts, cfg) = build_session_options_and_crate_config(handler, matches);
+    let sessopts = build_session_options(handler, &matches);
+    let cfg = parse_cfg(handler, matches.opt_strs("cfg"));
     let temps_dir = sessopts.unstable_opts.temps_dir.as_deref().map(PathBuf::from);
     let io = CompilerIO {
         input: Input::Str { name: FileName::Custom(String::new()), input: String::new() },
@@ -141,7 +132,7 @@ fn test_switch_implies_cfg_test() {
         let matches = optgroups().parse(&["--test".to_string()]).unwrap();
         let mut handler = EarlyErrorHandler::new(ErrorOutputType::default());
         let (sess, cfg) = mk_session(&mut handler, matches);
-        let cfg = build_configuration(&sess, to_crate_config(cfg));
+        let cfg = build_configuration(&sess, cfg);
         assert!(cfg.contains(&(sym::test, None)));
     });
 }
@@ -153,7 +144,7 @@ fn test_switch_implies_cfg_test_unless_cfg_test() {
         let matches = optgroups().parse(&["--test".to_string(), "--cfg=test".to_string()]).unwrap();
         let mut handler = EarlyErrorHandler::new(ErrorOutputType::default());
         let (sess, cfg) = mk_session(&mut handler, matches);
-        let cfg = build_configuration(&sess, to_crate_config(cfg));
+        let cfg = build_configuration(&sess, cfg);
         let mut test_items = cfg.iter().filter(|&&(name, _)| name == sym::test);
         assert!(test_items.next().is_some());
         assert!(test_items.next().is_none());
@@ -880,6 +871,6 @@ fn test_edition_parsing() {
     let mut handler = EarlyErrorHandler::new(ErrorOutputType::default());
 
     let matches = optgroups().parse(&["--edition=2018".to_string()]).unwrap();
-    let (sessopts, _) = build_session_options_and_crate_config(&mut handler, matches);
+    let sessopts = build_session_options(&mut handler, &matches);
     assert!(sessopts.edition == Edition::Edition2018)
 }
diff --git a/compiler/rustc_interface/src/util.rs b/compiler/rustc_interface/src/util.rs
index d2455a036ccd5..4d0be65697a42 100644
--- a/compiler/rustc_interface/src/util.rs
+++ b/compiler/rustc_interface/src/util.rs
@@ -3,18 +3,17 @@ use info;
 use libloading::Library;
 use rustc_ast as ast;
 use rustc_codegen_ssa::traits::CodegenBackend;
-use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::fx::FxHashMap;
 #[cfg(parallel_compiler)]
 use rustc_data_structures::sync;
 use rustc_errors::registry::Registry;
 use rustc_parse::validate_attr;
 use rustc_session as session;
-use rustc_session::config::CheckCfg;
-use rustc_session::config::{self, CrateType};
-use rustc_session::config::{OutFileName, OutputFilenames, OutputTypes};
+use rustc_session::config::{
+    self, Cfg, CheckCfg, CrateType, OutFileName, OutputFilenames, OutputTypes,
+};
 use rustc_session::filesearch::sysroot_candidates;
 use rustc_session::lint::{self, BuiltinLintDiagnostics, LintBuffer};
-use rustc_session::parse::CrateConfig;
 use rustc_session::{filesearch, output, Session};
 use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::edition::Edition;
@@ -38,7 +37,7 @@ pub type MakeBackendFn = fn() -> Box<dyn CodegenBackend>;
 /// This is performed by checking whether a set of permitted features
 /// is available on the target machine, by querying the codegen backend.
 pub fn add_configuration(
-    cfg: &mut CrateConfig,
+    cfg: &mut Cfg<Symbol>,
     sess: &mut Session,
     codegen_backend: &dyn CodegenBackend,
 ) {
@@ -60,8 +59,8 @@ pub fn add_configuration(
 pub fn create_session(
     handler: &EarlyErrorHandler,
     sopts: config::Options,
-    cfg: FxHashSet<(String, Option<String>)>,
-    check_cfg: CheckCfg,
+    cfg: Cfg<String>,
+    check_cfg: CheckCfg<String>,
     locale_resources: &'static [&'static str],
     file_loader: Option<Box<dyn FileLoader + Send + Sync + 'static>>,
     io: CompilerIO,
@@ -121,12 +120,13 @@ pub fn create_session(
 
     codegen_backend.init(&sess);
 
-    let mut cfg = config::build_configuration(&sess, config::to_crate_config(cfg));
+    let mut cfg = config::build_configuration(&sess, cfg);
     add_configuration(&mut cfg, &mut sess, &*codegen_backend);
 
-    let mut check_cfg = config::to_crate_check_config(check_cfg);
+    let mut check_cfg = check_cfg.intern();
     check_cfg.fill_well_known(&sess.target);
 
+    // These configs use symbols, rather than strings.
     sess.parse_sess.config = cfg;
     sess.parse_sess.check_config = check_cfg;
 
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index 7aced414ed67a..854e2b779935c 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -9,14 +9,13 @@ use crate::utils::{CanonicalizedPath, NativeLib, NativeLibKind};
 use crate::{lint, HashStableContext};
 use crate::{EarlyErrorHandler, Session};
 
-use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexSet};
 use rustc_data_structures::stable_hasher::{StableOrd, ToStableHashKey};
 use rustc_target::abi::Align;
 use rustc_target::spec::LinkSelfContainedComponents;
 use rustc_target::spec::{PanicStrategy, RelocModel, SanitizerSet, SplitDebuginfo};
 use rustc_target::spec::{Target, TargetTriple, TargetWarnings, TARGETS};
 
-use crate::parse::{CrateCheckConfig, CrateConfig};
 use rustc_feature::UnstableFeatures;
 use rustc_span::edition::{Edition, DEFAULT_EDITION, EDITION_NAME_LIST, LATEST_STABLE_EDITION};
 use rustc_span::source_map::{FileName, FilePathMapping};
@@ -1248,8 +1247,8 @@ pub const fn default_lib_output() -> CrateType {
     CrateType::Rlib
 }
 
-fn default_configuration(sess: &Session) -> CrateConfig {
-    // NOTE: This should be kept in sync with `CrateCheckConfig::fill_well_known` below.
+fn default_configuration(sess: &Session) -> Cfg<Symbol> {
+    // NOTE: This should be kept in sync with `CheckCfg::<Symbol>::fill_well_known` below.
     let end = &sess.target.endian;
     let arch = &sess.target.arch;
     let wordsz = sess.target.pointer_width.to_string();
@@ -1265,7 +1264,7 @@ fn default_configuration(sess: &Session) -> CrateConfig {
         sess.emit_fatal(err);
     });
 
-    let mut ret = CrateConfig::default();
+    let mut ret = Cfg::default();
     ret.reserve(7); // the minimum number of insertions
     // Target bindings.
     ret.insert((sym::target_os, Some(Symbol::intern(os))));
@@ -1358,15 +1357,17 @@ fn default_configuration(sess: &Session) -> CrateConfig {
     ret
 }
 
-/// Converts the crate `cfg!` configuration from `String` to `Symbol`.
-/// `rustc_interface::interface::Config` accepts this in the compiler configuration,
-/// but the symbol interner is not yet set up then, so we must convert it later.
-pub fn to_crate_config(cfg: FxHashSet<(String, Option<String>)>) -> CrateConfig {
-    cfg.into_iter().map(|(a, b)| (Symbol::intern(&a), b.map(|b| Symbol::intern(&b)))).collect()
-}
+/// The parsed `--cfg` options that define the compilation environment of the
+/// crate, used to drive conditional compilation. `T` is always `String` or
+/// `Symbol`. Strings are used temporarily very early on. Once the the main
+/// symbol interner is running, they are converted to symbols.
+///
+/// An `FxIndexSet` is used to ensure deterministic ordering of error messages
+/// relating to `--cfg`.
+pub type Cfg<T> = FxIndexSet<(T, Option<T>)>;
 
-/// The parsed `--check-cfg` options
-pub struct CheckCfg<T = String> {
+/// The parsed `--check-cfg` options. The `<T>` structure is similar to `Cfg`.
+pub struct CheckCfg<T> {
     /// Is well known names activated
     pub exhaustive_names: bool,
     /// Is well known values activated
@@ -1385,8 +1386,8 @@ impl<T> Default for CheckCfg<T> {
     }
 }
 
-impl<T> CheckCfg<T> {
-    fn map_data<O: Eq + Hash>(self, f: impl Fn(T) -> O) -> CheckCfg<O> {
+impl CheckCfg<String> {
+    pub fn intern(self) -> CheckCfg<Symbol> {
         CheckCfg {
             exhaustive_names: self.exhaustive_names,
             exhaustive_values: self.exhaustive_values,
@@ -1395,10 +1396,10 @@ impl<T> CheckCfg<T> {
                 .into_iter()
                 .map(|(name, values)| {
                     (
-                        f(name),
+                        Symbol::intern(&name),
                         match values {
                             ExpectedValues::Some(values) => ExpectedValues::Some(
-                                values.into_iter().map(|b| b.map(|b| f(b))).collect(),
+                                values.into_iter().map(|b| b.map(|b| Symbol::intern(&b))).collect(),
                             ),
                             ExpectedValues::Any => ExpectedValues::Any,
                         },
@@ -1441,14 +1442,7 @@ impl<'a, T: Eq + Hash + Copy + 'a> Extend<&'a T> for ExpectedValues<T> {
     }
 }
 
-/// Converts the crate `--check-cfg` options from `String` to `Symbol`.
-/// `rustc_interface::interface::Config` accepts this in the compiler configuration,
-/// but the symbol interner is not yet set up then, so we must convert it later.
-pub fn to_crate_check_config(cfg: CheckCfg) -> CrateCheckConfig {
-    cfg.map_data(|s| Symbol::intern(&s))
-}
-
-impl CrateCheckConfig {
+impl CheckCfg<Symbol> {
     pub fn fill_well_known(&mut self, current_target: &Target) {
         if !self.exhaustive_values && !self.exhaustive_names {
             return;
@@ -1588,7 +1582,13 @@ impl CrateCheckConfig {
     }
 }
 
-pub fn build_configuration(sess: &Session, mut user_cfg: CrateConfig) -> CrateConfig {
+pub fn build_configuration(sess: &Session, user_cfg: Cfg<String>) -> Cfg<Symbol> {
+    // We can now intern these strings.
+    let mut user_cfg: Cfg<Symbol> = user_cfg
+        .into_iter()
+        .map(|(a, b)| (Symbol::intern(&a), b.map(|b| Symbol::intern(&b))))
+        .collect();
+
     // Combine the configuration requested by the session (command line) with
     // some default and generated configuration items.
     let default_cfg = default_configuration(sess);
diff --git a/compiler/rustc_session/src/parse.rs b/compiler/rustc_session/src/parse.rs
index abb0ab5630c13..5b98ee5d992df 100644
--- a/compiler/rustc_session/src/parse.rs
+++ b/compiler/rustc_session/src/parse.rs
@@ -1,7 +1,7 @@
 //! Contains `ParseSess` which holds state living beyond what one `Parser` might.
 //! It also serves as an input to the parser itself.
 
-use crate::config::CheckCfg;
+use crate::config::{Cfg, CheckCfg};
 use crate::errors::{
     CliFeatureDiagnosticHelp, FeatureDiagnosticForIssue, FeatureDiagnosticHelp, FeatureGateError,
 };
@@ -9,7 +9,7 @@ use crate::lint::{
     builtin::UNSTABLE_SYNTAX_PRE_EXPANSION, BufferedEarlyLint, BuiltinLintDiagnostics, Lint, LintId,
 };
 use rustc_ast::node_id::NodeId;
-use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexSet};
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::sync::{AppendOnlyVec, Lock, Lrc};
 use rustc_errors::{emitter::SilentEmitter, Handler};
 use rustc_errors::{
@@ -25,11 +25,6 @@ use rustc_span::{Span, Symbol};
 use rustc_ast::attr::AttrIdGenerator;
 use std::str;
 
-/// The set of keys (and, optionally, values) that define the compilation
-/// environment of the crate, used to drive conditional compilation.
-pub type CrateConfig = FxIndexSet<(Symbol, Option<Symbol>)>;
-pub type CrateCheckConfig = CheckCfg<Symbol>;
-
 /// Collected spans during parsing for places where a certain feature was
 /// used and should be feature gated accordingly in `check_crate`.
 #[derive(Default)]
@@ -193,8 +188,8 @@ pub fn add_feature_diagnostics_for_issue(
 pub struct ParseSess {
     pub span_diagnostic: Handler,
     pub unstable_features: UnstableFeatures,
-    pub config: CrateConfig,
-    pub check_config: CrateCheckConfig,
+    pub config: Cfg<Symbol>,
+    pub check_config: CheckCfg<Symbol>,
     pub edition: Edition,
     /// Places where raw identifiers were used. This is used to avoid complaining about idents
     /// clashing with keywords in new editions.
@@ -237,8 +232,8 @@ impl ParseSess {
         Self {
             span_diagnostic: handler,
             unstable_features: UnstableFeatures::from_environment(None),
-            config: FxIndexSet::default(),
-            check_config: CrateCheckConfig::default(),
+            config: Cfg::default(),
+            check_config: CheckCfg::default(),
             edition: ExpnId::root().expn_data().edition,
             raw_identifier_spans: Default::default(),
             bad_unicode_identifiers: Lock::new(Default::default()),
diff --git a/src/librustdoc/core.rs b/src/librustdoc/core.rs
index 94e557dcfdb54..bcc61df7c4d84 100644
--- a/src/librustdoc/core.rs
+++ b/src/librustdoc/core.rs
@@ -255,7 +255,7 @@ pub(crate) fn create_config(
 
     interface::Config {
         opts: sessopts,
-        crate_cfg: interface::parse_cfgspecs(handler, cfgs),
+        crate_cfg: interface::parse_cfg(handler, cfgs),
         crate_check_cfg: interface::parse_check_cfg(handler, check_cfgs),
         input,
         output_file: None,
diff --git a/src/librustdoc/doctest.rs b/src/librustdoc/doctest.rs
index 97913345e8fb2..c61c98e4de9c6 100644
--- a/src/librustdoc/doctest.rs
+++ b/src/librustdoc/doctest.rs
@@ -92,7 +92,7 @@ pub(crate) fn run(options: RustdocOptions) -> Result<(), ErrorGuaranteed> {
     cfgs.push("doctest".to_owned());
     let config = interface::Config {
         opts: sessopts,
-        crate_cfg: interface::parse_cfgspecs(&early_error_handler, cfgs),
+        crate_cfg: interface::parse_cfg(&early_error_handler, cfgs),
         crate_check_cfg: interface::parse_check_cfg(
             &early_error_handler,
             options.check_cfgs.clone(),