Rollup merge of #94175 - Urgau:check-cfg-improvements, r=petrochenkov

Improve `--check-cfg` implementation

This pull-request is a mix of improvements regarding the `--check-cfg` implementation:

- Simpler internal representation (usage of `Option` instead of separate bool)
- Add --check-cfg to the unstable book (based on the RFC)
- Improved diagnostics:
    * List possible values when the value is unexpected
    * Suggest if possible a name or value that is similar
- Add more tests (well known names, mix of combinations, ...)

r? ```@petrochenkov```

compiler/rustc_lint/src/context.rs

@@ -765,7 +765,40 @@ 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)]