Improve ternary operator recovery

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();