Auto merge of #116666 - Urgau:check-cfg-pre-mcp636, r=petrochenkov

Improve check-cfg diagnostics

This PR tries to improve some of the diagnostics of check-cfg.

The main changes is the unexpected name or value being added to the main diagnostic:
```diff
- warning: unexpected `cfg` condition name
+ warning: unexpected `cfg` condition name: `widnows`
```

It also cherry-pick the better sensible logic for when we print the list of expected values when we have a matching value for a very similar name.

Address https://github.com/rust-lang/rust/pull/111072#discussion_r1356818100

r? `@petrochenkov`

3 files changed, 23 insertions, 3 deletions

diff --git a/compiler/rustc_attr/src/builtin.rs b/compiler/rustc_attr/src/builtin.rs
index f013ff45a4f..76659458dad 100644
--- a/compiler/rustc_attr/src/builtin.rs
+++ b/compiler/rustc_attr/src/builtin.rs
@@ -548,7 +548,11 @@ pub fn cfg_matches(
                     UNEXPECTED_CFGS,
                     cfg.span,
                     lint_node_id,
-                    "unexpected `cfg` condition value",
+                    if let Some(value) = cfg.value {
+                        format!("unexpected `cfg` condition value: `{value}`")
+                    } else {
+                        format!("unexpected `cfg` condition value: (none)")
+                    },
                     BuiltinLintDiagnostics::UnexpectedCfgValue(
                         (cfg.name, cfg.name_span),
                         cfg.value.map(|v| (v, cfg.value_span.unwrap())),
@@ -560,7 +564,7 @@ pub fn cfg_matches(
                     UNEXPECTED_CFGS,
                     cfg.span,
                     lint_node_id,
-                    "unexpected `cfg` condition name",
+                    format!("unexpected `cfg` condition name: `{}`", cfg.name),
                     BuiltinLintDiagnostics::UnexpectedCfgName(
                         (cfg.name, cfg.name_span),
                         cfg.value.map(|v| (v, cfg.value_span.unwrap())),
diff --git a/compiler/rustc_errors/src/diagnostic_builder.rs b/compiler/rustc_errors/src/diagnostic_builder.rs
index 5e23ae655fe..85acf8ab5aa 100644
--- a/compiler/rustc_errors/src/diagnostic_builder.rs
+++ b/compiler/rustc_errors/src/diagnostic_builder.rs
@@ -659,6 +659,7 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
         msg: impl Into<SubdiagnosticMessage>,
     ) -> &mut Self);
     forward!(pub fn help(&mut self, msg: impl Into<SubdiagnosticMessage>) -> &mut Self);
+    forward!(pub fn help_once(&mut self, msg: impl Into<SubdiagnosticMessage>) -> &mut Self);
     forward!(pub fn span_help(
         &mut self,
         sp: impl Into<MultiSpan>,
diff --git a/compiler/rustc_lint/src/context.rs b/compiler/rustc_lint/src/context.rs
index 3c5cde4309b..1ed9b86d66c 100644
--- a/compiler/rustc_lint/src/context.rs
+++ b/compiler/rustc_lint/src/context.rs
@@ -727,11 +727,14 @@ pub trait LintContext: Sized {
                                 .collect::<Vec<_>>();
                             possibilities.sort();
 
+                            let mut should_print_possibilities = true;
                             if let Some((value, value_span)) = value {
                                 if best_match_values.contains(&Some(value)) {
                                     db.span_suggestion(name_span, "there is a config with a similar name and value", best_match, Applicability::MaybeIncorrect);
+                                    should_print_possibilities = false;
                                 } else if best_match_values.contains(&None) {
                                     db.span_suggestion(name_span.to(value_span), "there is a config with a similar name and no value", best_match, Applicability::MaybeIncorrect);
+                                    should_print_possibilities = false;
                                 } else if let Some(first_value) = possibilities.first() {
                                     db.span_suggestion(name_span.to(value_span), "there is a config with a similar name and different values", format!("{best_match} = \"{first_value}\""), Applicability::MaybeIncorrect);
                                 } else {
@@ -741,13 +744,25 @@ pub trait LintContext: Sized {
                                 db.span_suggestion(name_span, "there is a config with a similar name", best_match, Applicability::MaybeIncorrect);
                             }
 
-                            if !possibilities.is_empty() {
+                            if !possibilities.is_empty() && should_print_possibilities {
                                 let possibilities = possibilities.join("`, `");
                                 db.help(format!("expected values for `{best_match}` are: `{possibilities}`"));
                             }
                         } else {
                             db.span_suggestion(name_span, "there is a config with a similar name", best_match, Applicability::MaybeIncorrect);
                         }
+                    } else if !possibilities.is_empty() {
+                        let mut possibilities = possibilities.iter()
+                            .map(Symbol::as_str)
+                            .collect::<Vec<_>>();
+                        possibilities.sort();
+                        let possibilities = possibilities.join("`, `");
+
+                        // The list of expected names can be long (even by default) and
+                        // so the diagnostic produced can take a lot of space. To avoid
+                        // cloging the user output we only want to print that diagnostic
+                        // once.
+                        db.help_once(format!("expected names are: `{possibilities}`"));
                     }
                 },
                 BuiltinLintDiagnostics::UnexpectedCfgValue((name, name_span), value) => {