Improve --check-cfg implementation

compiler/rustc_attr/src/builtin.rs

@@ -8,6 +8,7 @@ use rustc_errors::{struct_span_err, Applicability};
 use rustc_feature::{find_gated_cfg, is_builtin_attr_name, Features, GatedCfg};
 use rustc_macros::HashStable_Generic;
 use rustc_session::lint::builtin::UNEXPECTED_CFGS;
+use rustc_session::lint::BuiltinLintDiagnostics;
 use rustc_session::parse::{feature_err, ParseSess};
 use rustc_session::Session;
 use rustc_span::hygiene::Transparency;
@@ -461,29 +462,37 @@ pub fn cfg_matches(cfg: &ast::MetaItem, sess: &ParseSess, features: Option<&Feat
                 true
             }
             MetaItemKind::NameValue(..) | MetaItemKind::Word => {
-                let name = cfg.ident().expect("multi-segment cfg predicate").name;
+                let ident = cfg.ident().expect("multi-segment cfg predicate");
+                let name = ident.name;
                 let value = cfg.value_str();
-                if sess.check_config.names_checked && !sess.check_config.names_valid.contains(&name)
-                {
-                    sess.buffer_lint(
-                        UNEXPECTED_CFGS,
-                        cfg.span,
-                        CRATE_NODE_ID,
-                        "unexpected `cfg` condition name",
-                    );
-                }
-                if let Some(val) = value {
-                    if sess.check_config.values_checked.contains(&name)
-                        && !sess.check_config.values_valid.contains(&(name, val))
-                    {
-                        sess.buffer_lint(
+                if let Some(names_valid) = &sess.check_config.names_valid {
+                    if !names_valid.contains(&name) {
+                        sess.buffer_lint_with_diagnostic(
                             UNEXPECTED_CFGS,
                             cfg.span,
                             CRATE_NODE_ID,
-                            "unexpected `cfg` condition value",
+                            "unexpected `cfg` condition name",
+                            BuiltinLintDiagnostics::UnexpectedCfg(ident.span, name, None),
                         );
                     }
                 }
+                if let Some(value) = value {
+                    if let Some(values) = &sess.check_config.values_valid.get(&name) {
+                        if !values.contains(&value) {
+                            sess.buffer_lint_with_diagnostic(
+                                UNEXPECTED_CFGS,
+                                cfg.span,
+                                CRATE_NODE_ID,
+                                "unexpected `cfg` condition value",
+                                BuiltinLintDiagnostics::UnexpectedCfg(
+                                    cfg.name_value_literal_span().unwrap(),
+                                    name,
+                                    Some(value),
+                                ),
+                            );
+                        }
+                    }
+                }
                 sess.config.contains(&(name, value))
             }
         }

compiler/rustc_interface/src/interface.rs

@@ -169,11 +169,12 @@ pub fn parse_check_cfg(specs: Vec<String>) -> CheckCfg {
                     Ok(meta_item) if parser.token == token::Eof => {
                         if let Some(args) = meta_item.meta_item_list() {
                             if meta_item.has_name(sym::names) {
-                                cfg.names_checked = true;
+                                let names_valid =
+                                    cfg.names_valid.get_or_insert_with(|| FxHashSet::default());
                                 for arg in args {
                                     if arg.is_word() && arg.ident().is_some() {
                                         let ident = arg.ident().expect("multi-segment cfg key");
-                                        cfg.names_valid.insert(ident.name.to_string());
+                                        names_valid.insert(ident.name.to_string());
                                     } else {
                                         error!("`names()` arguments must be simple identifers");
                                     }
@@ -183,13 +184,16 @@ pub fn parse_check_cfg(specs: Vec<String>) -> CheckCfg {
                                 if let Some((name, values)) = args.split_first() {
                                     if name.is_word() && name.ident().is_some() {
                                         let ident = name.ident().expect("multi-segment cfg key");
-                                        cfg.values_checked.insert(ident.to_string());
+                                        let ident_values = cfg
+                                            .values_valid
+                                            .entry(ident.name.to_string())
+                                            .or_insert_with(|| FxHashSet::default());
+
                                         for val in values {
                                             if let Some(LitKind::Str(s, _)) =
                                                 val.literal().map(|lit| &lit.kind)
                                             {
-                                                cfg.values_valid
-                                                    .insert((ident.to_string(), s.to_string()));
+                                                ident_values.insert(s.to_string());
                                             } else {
                                                 error!(
                                                     "`values()` arguments must be string literals"
@@ -219,7 +223,9 @@ pub fn parse_check_cfg(specs: Vec<String>) -> CheckCfg {
             );
         }
 
-        cfg.names_valid.extend(cfg.values_checked.iter().cloned());
+        if let Some(names_valid) = &mut cfg.names_valid {
+            names_valid.extend(cfg.values_valid.keys().cloned());
+        }
         cfg
     })
 }

compiler/rustc_lint/src/context.rs

@@ -766,7 +766,40 @@ pub trait LintContext: Sized {
                 BuiltinLintDiagnostics::NamedAsmLabel(help) => {
                     db.help(&help);
                     db.note("see the asm section of Rust By Example <https://doc.rust-lang.org/nightly/rust-by-example/unsafe/asm.html#labels> for more information");
-                }
+                },
+                BuiltinLintDiagnostics::UnexpectedCfg(span, name, value) => {
+                    let possibilities: Vec<Symbol> = if value.is_some() {
+                        let Some(values) = &sess.parse_sess.check_config.values_valid.get(&name) else {
+                            bug!("it shouldn't be possible to have a diagnostic on a value whose name is not in values");
+                        };
+                        values.iter().map(|&s| s).collect()
+                    } else {
+                        let Some(names_valid) = &sess.parse_sess.check_config.names_valid else {
+                            bug!("it shouldn't be possible to have a diagnostic on a name if name checking is not enabled");
+                        };
+                        names_valid.iter().map(|s| *s).collect()
+                    };
+
+                    // Show the full list if all possible values for a given name, but don't do it
+                    // for names as the possibilities could be very long
+                    if value.is_some() {
+                        if !possibilities.is_empty() {
+                            let mut possibilities = possibilities.iter().map(Symbol::as_str).collect::<Vec<_>>();
+                            possibilities.sort();
+
+                            let possibilities = possibilities.join(", ");
+                            db.note(&format!("expected values for `{name}` are: {possibilities}"));
+                        } else {
+                            db.note(&format!("no expected value for `{name}`"));
+                        }
+                    }
+
+                    // Suggest the most probable if we found one
+                    if let Some(best_match) = find_best_match_for_name(&possibilities, value.unwrap_or(name), None) {
+                        let punctuation = if value.is_some() { "\"" } else { "" };
+                        db.span_suggestion(span, "did you mean", format!("{punctuation}{best_match}{punctuation}"), Applicability::MaybeIncorrect);
+                    }
+                },
             }
             // Rewrap `db`, and pass control to the user.
             decorate(LintDiagnosticBuilder::new(db));

compiler/rustc_lint/src/lib.rs

@@ -31,6 +31,7 @@
 #![feature(box_patterns)]
 #![feature(crate_visibility_modifier)]
 #![feature(if_let_guard)]
+#![feature(iter_intersperse)]
 #![feature(iter_order_by)]
 #![feature(let_else)]
 #![feature(never_type)]

compiler/rustc_lint_defs/src/lib.rs

@@ -310,6 +310,7 @@ pub enum BuiltinLintDiagnostics {
     BreakWithLabelAndLoop(Span),
     NamedAsmLabel(String),
     UnicodeTextFlow(Span, String),
+    UnexpectedCfg(Span, Symbol, Option<Symbol>),
 }
 
 /// Lints that are buffered up early on in the `Session` before the

compiler/rustc_session/src/config.rs

@@ -8,7 +8,7 @@ use crate::search_paths::SearchPath;
 use crate::utils::{CanonicalizedPath, NativeLib, NativeLibKind};
 use crate::{early_error, early_warn, Session};
 
-use rustc_data_structures::fx::FxHashSet;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::impl_stable_hash_via_hash;
 
 use rustc_target::abi::{Align, TargetDataLayout};
@@ -1023,34 +1023,30 @@ pub fn to_crate_config(cfg: FxHashSet<(String, Option<String>)>) -> CrateConfig
 
 /// The parsed `--check-cfg` options
 pub struct CheckCfg<T = String> {
-    /// Set if `names()` checking is enabled
-    pub names_checked: bool,
-    /// The union of all `names()`
-    pub names_valid: FxHashSet<T>,
-    /// The set of names for which `values()` was used
-    pub values_checked: FxHashSet<T>,
-    /// The set of all (name, value) pairs passed in `values()`
-    pub values_valid: FxHashSet<(T, T)>,
+    /// The set of all `names()`, if None no name checking is performed
+    pub names_valid: Option<FxHashSet<T>>,
+    /// The set of all `values()`
+    pub values_valid: FxHashMap<T, FxHashSet<T>>,
 }
 
 impl<T> Default for CheckCfg<T> {
     fn default() -> Self {
-        CheckCfg {
-            names_checked: false,
-            names_valid: FxHashSet::default(),
-            values_checked: FxHashSet::default(),
-            values_valid: FxHashSet::default(),
-        }
+        CheckCfg { names_valid: Default::default(), values_valid: Default::default() }
     }
 }
 
 impl<T> CheckCfg<T> {
     fn map_data<O: Eq + Hash>(&self, f: impl Fn(&T) -> O) -> CheckCfg<O> {
         CheckCfg {
-            names_checked: self.names_checked,
-            names_valid: self.names_valid.iter().map(|a| f(a)).collect(),
-            values_checked: self.values_checked.iter().map(|a| f(a)).collect(),
-            values_valid: self.values_valid.iter().map(|(a, b)| (f(a), f(b))).collect(),
+            names_valid: self
+                .names_valid
+                .as_ref()
+                .map(|names_valid| names_valid.iter().map(|a| f(a)).collect()),
+            values_valid: self
+                .values_valid
+                .iter()
+                .map(|(a, b)| (f(a), b.iter().map(|b| f(b)).collect()))
+                .collect(),
         }
     }
 }
@@ -1090,17 +1086,23 @@ impl CrateCheckConfig {
             sym::doctest,
             sym::feature,
         ];
-        for &name in WELL_KNOWN_NAMES {
-            self.names_valid.insert(name);
+        if let Some(names_valid) = &mut self.names_valid {
+            for &name in WELL_KNOWN_NAMES {
+                names_valid.insert(name);
+            }
         }
     }
 
     /// Fills a `CrateCheckConfig` with configuration names and values that are actually active.
     pub fn fill_actual(&mut self, cfg: &CrateConfig) {
         for &(k, v) in cfg {
-            self.names_valid.insert(k);
+            if let Some(names_valid) = &mut self.names_valid {
+                names_valid.insert(k);
+            }
             if let Some(v) = v {
-                self.values_valid.insert((k, v));
+                self.values_valid.entry(k).and_modify(|values| {
+                    values.insert(v);
+                });
             }
         }
     }