Rollup merge of #141003 - clubby789:ternary-improve, r=compiler-errors

Improve ternary operator recovery

This
- Improves the span of the error to not point at the next token
- Where possible, we use the span of the condition to further improve the span of the error to include the cond, and suggest a maybe-incorrect fix

Currently this works on free expressions, not let statements; some more refactoring would be needed to pass the span down, which I'm not sure is worth doing.

### Old
![image](https://github.com/user-attachments/assets/5688cefc-e4ef-4135-a5ba-340ce05ae6f3)

### New
![image](https://github.com/user-attachments/assets/154f5380-e0c8-42c7-9bf8-0adb3d0433fa)

3 files changed, 50 insertions, 11 deletions

diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index 766baf6f80c..31a48b22cfe 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -436,10 +436,28 @@ pub(crate) enum IfExpressionMissingThenBlockSub {
 
 #[derive(Diagnostic)]
 #[diag(parse_ternary_operator)]
-#[help]
 pub(crate) struct TernaryOperator {
     #[primary_span]
     pub span: Span,
+    /// If we have a span for the condition expression, suggest the if/else
+    #[subdiagnostic]
+    pub sugg: Option<TernaryOperatorSuggestion>,
+    /// Otherwise, just print the suggestion message
+    #[help(parse_use_if_else)]
+    pub no_sugg: bool,
+}
+
+#[derive(Subdiagnostic, Copy, Clone)]
+#[multipart_suggestion(parse_use_if_else, applicability = "maybe-incorrect", style = "verbose")]
+pub(crate) struct TernaryOperatorSuggestion {
+    #[suggestion_part(code = "if ")]
+    pub before_cond: Span,
+    #[suggestion_part(code = "{{")]
+    pub question: Span,
+    #[suggestion_part(code = "}} else {{")]
+    pub colon: Span,
+    #[suggestion_part(code = " }}")]
+    pub end: Span,
 }
 
 #[derive(Subdiagnostic)]
diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index 23c8db7bca7..6277dde7c97 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -41,8 +41,9 @@ use crate::errors::{
     IncorrectSemicolon, IncorrectUseOfAwait, IncorrectUseOfUse, PatternMethodParamWithoutBody,
     QuestionMarkInType, QuestionMarkInTypeSugg, SelfParamNotFirst, StructLiteralBodyWithoutPath,
     StructLiteralBodyWithoutPathSugg, SuggAddMissingLetStmt, SuggEscapeIdentifier, SuggRemoveComma,
-    TernaryOperator, UnexpectedConstInGenericParam, UnexpectedConstParamDeclaration,
-    UnexpectedConstParamDeclarationSugg, UnmatchedAngleBrackets, UseEqInstead, WrapType,
+    TernaryOperator, TernaryOperatorSuggestion, UnexpectedConstInGenericParam,
+    UnexpectedConstParamDeclaration, UnexpectedConstParamDeclarationSugg, UnmatchedAngleBrackets,
+    UseEqInstead, WrapType,
 };
 use crate::parser::attr::InnerAttrPolicy;
 use crate::{exp, fluent_generated as fluent};
@@ -497,7 +498,7 @@ impl<'a> Parser<'a> {
             // If the user is trying to write a ternary expression, recover it and
             // return an Err to prevent a cascade of irrelevant diagnostics.
             if self.prev_token == token::Question
-                && let Err(e) = self.maybe_recover_from_ternary_operator()
+                && let Err(e) = self.maybe_recover_from_ternary_operator(None)
             {
                 return Err(e);
             }
@@ -1602,12 +1603,18 @@ impl<'a> Parser<'a> {
     /// Rust has no ternary operator (`cond ? then : else`). Parse it and try
     /// to recover from it if `then` and `else` are valid expressions. Returns
     /// an err if this appears to be a ternary expression.
-    pub(super) fn maybe_recover_from_ternary_operator(&mut self) -> PResult<'a, ()> {
+    /// If we have the span of the condition, we can provide a better error span
+    /// and code suggestion.
+    pub(super) fn maybe_recover_from_ternary_operator(
+        &mut self,
+        cond: Option<Span>,
+    ) -> PResult<'a, ()> {
         if self.prev_token != token::Question {
             return PResult::Ok(());
         }
 
-        let lo = self.prev_token.span.lo();
+        let question = self.prev_token.span;
+        let lo = cond.unwrap_or(question).lo();
         let snapshot = self.create_snapshot_for_diagnostic();
 
         if match self.parse_expr() {
@@ -1620,11 +1627,20 @@ impl<'a> Parser<'a> {
             }
         } {
             if self.eat_noexpect(&token::Colon) {
+                let colon = self.prev_token.span;
                 match self.parse_expr() {
-                    Ok(_) => {
-                        return Err(self
-                            .dcx()
-                            .create_err(TernaryOperator { span: self.token.span.with_lo(lo) }));
+                    Ok(expr) => {
+                        let sugg = cond.map(|cond| TernaryOperatorSuggestion {
+                            before_cond: cond.shrink_to_lo(),
+                            question,
+                            colon,
+                            end: expr.span.shrink_to_hi(),
+                        });
+                        return Err(self.dcx().create_err(TernaryOperator {
+                            span: self.prev_token.span.with_lo(lo),
+                            sugg,
+                            no_sugg: sugg.is_none(),
+                        }));
                     }
                     Err(err) => {
                         err.cancel();
diff --git a/compiler/rustc_parse/src/parser/stmt.rs b/compiler/rustc_parse/src/parser/stmt.rs
index 885a65d4de7..396ded96bde 100644
--- a/compiler/rustc_parse/src/parser/stmt.rs
+++ b/compiler/rustc_parse/src/parser/stmt.rs
@@ -879,7 +879,12 @@ impl<'a> Parser<'a> {
             {
                 // Just check for errors and recover; do not eat semicolon yet.
 
-                let expect_result = self.expect_one_of(&[], &[exp!(Semi), exp!(CloseBrace)]);
+                let expect_result =
+                    if let Err(e) = self.maybe_recover_from_ternary_operator(Some(expr.span)) {
+                        Err(e)
+                    } else {
+                        self.expect_one_of(&[], &[exp!(Semi), exp!(CloseBrace)])
+                    };
 
                 // Try to both emit a better diagnostic, and avoid further errors by replacing
                 // the `expr` with `ExprKind::Err`.