Gracefully handle ternary operator

compiler/rustc_parse/messages.ftl

@@ -722,6 +722,10 @@ parse_sugg_wrap_pattern_in_parens = wrap the pattern in parentheses
 
 parse_switch_mut_let_order =
     switch the order of `mut` and `let`
+
+parse_ternary_operator = Rust has no ternary operator
+    .help = use an `if-else` expression instead
+
 parse_tilde_const_lifetime = `~const` may only modify trait bounds, not lifetime bounds
 
 parse_tilde_is_not_unary_operator = `~` cannot be used as a unary operator

compiler/rustc_parse/src/errors.rs

@@ -365,6 +365,14 @@ pub(crate) enum IfExpressionMissingThenBlockSub {
     AddThenBlock(#[primary_span] Span),
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_ternary_operator)]
+#[help]
+pub struct TernaryOperator {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Subdiagnostic)]
 #[suggestion(parse_extra_if_in_let_else, applicability = "maybe-incorrect", code = "")]
 pub(crate) struct IfExpressionLetSomeSub {

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -14,7 +14,7 @@ use crate::errors::{
     PatternMethodParamWithoutBody, QuestionMarkInType, QuestionMarkInTypeSugg, SelfParamNotFirst,
     StructLiteralBodyWithoutPath, StructLiteralBodyWithoutPathSugg, StructLiteralNeedingParens,
     StructLiteralNeedingParensSugg, SuggAddMissingLetStmt, SuggEscapeIdentifier, SuggRemoveComma,
-    UnexpectedConstInGenericParam, UnexpectedConstParamDeclaration,
+    TernaryOperator, UnexpectedConstInGenericParam, UnexpectedConstParamDeclaration,
     UnexpectedConstParamDeclarationSugg, UnmatchedAngleBrackets, UseEqInstead,
 };
 
@@ -500,6 +500,10 @@ impl<'a> Parser<'a> {
 
         // Special-case "expected `;`" errors
         if expected.contains(&TokenType::Token(token::Semi)) {
+            if self.prev_token == token::Question && self.maybe_recover_from_ternary_operator() {
+                return Ok(true);
+            }
+
             if self.token.span == DUMMY_SP || self.prev_token.span == DUMMY_SP {
                 // Likely inside a macro, can't provide meaningful suggestions.
             } else if !sm.is_multiline(self.prev_token.span.until(self.token.span)) {
@@ -1330,6 +1334,45 @@ 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
+    /// whether it was a ternary operator.
+    pub(super) fn maybe_recover_from_ternary_operator(&mut self) -> bool {
+        if self.prev_token != token::Question {
+            return false;
+        }
+
+        let lo = self.prev_token.span.lo();
+        let snapshot = self.create_snapshot_for_diagnostic();
+
+        if match self.parse_expr() {
+            Ok(_) => true,
+            Err(err) => {
+                err.cancel();
+                // The colon can sometimes be mistaken for type
+                // ascription. Catch when this happens and continue.
+                self.token == token::Colon
+            }
+        } {
+            if self.eat_noexpect(&token::Colon) {
+                match self.parse_expr() {
+                    Ok(_) => {
+                        self.sess.emit_err(TernaryOperator { span: self.token.span.with_lo(lo) });
+                        return true;
+                    }
+                    Err(err) => {
+                        err.cancel();
+                        self.restore_snapshot(snapshot);
+                    }
+                };
+            }
+        } else {
+            self.restore_snapshot(snapshot);
+        };
+
+        false
+    }
+
     pub(super) fn maybe_recover_from_bad_type_plus(&mut self, ty: &Ty) -> PResult<'a, ()> {
         // Do not add `+` to expected tokens.
         if !self.token.is_like_plus() {
@@ -2111,7 +2154,7 @@ impl<'a> Parser<'a> {
             }
             _ => (
                 self.token.span,
-                format!("expected expression, found {}", super::token_descr(&self.token),),
+                format!("expected expression, found {}", super::token_descr(&self.token)),
             ),
         };
         let mut err = self.struct_span_err(span, msg);

tests/ui/parser/ternary_operator.rs

@@ -0,0 +1,69 @@
+// A good chunk of these errors aren't shown to the user, but are still
+// required in the test for it to pass.
+
+fn a() { //~ NOTE this function should return `Result` or `Option` to accept `?`
+    let x = 5 > 2 ? true : false;
+    //~^ ERROR Rust has no ternary operator
+    //~| HELP use an `if-else` expression instead
+    //~| ERROR the `?` operator can only be applied to values that implement `Try` [E0277]
+    //~| HELP the trait `Try` is not implemented for `{integer}`
+    //~| ERROR the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`) [E0277]
+    //~| HELP the trait `FromResidual<_>` is not implemented for `()`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE the `?` operator cannot be applied to type `{integer}`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE cannot use the `?` operator in a function that returns `()`
+    //~| NOTE in this expansion of desugaring of operator `?`
+}
+
+fn b() { //~ NOTE this function should return `Result` or `Option` to accept `?`
+    let x = 5 > 2 ? { true } : { false };
+    //~^ ERROR Rust has no ternary operator
+    //~| HELP use an `if-else` expression instead
+    //~| ERROR the `?` operator can only be applied to values that implement `Try` [E0277]
+    //~| HELP the trait `Try` is not implemented for `{integer}`
+    //~| ERROR the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`) [E0277]
+    //~| HELP the trait `FromResidual<_>` is not implemented for `()`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE the `?` operator cannot be applied to type `{integer}`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE cannot use the `?` operator in a function that returns `()`
+    //~| NOTE in this expansion of desugaring of operator `?`
+}
+
+fn c() { //~ NOTE this function should return `Result` or `Option` to accept `?`
+    let x = 5 > 2 ? f32::MAX : f32::MIN;
+    //~^ ERROR Rust has no ternary operator
+    //~| HELP use an `if-else` expression instead
+    //~| ERROR the `?` operator can only be applied to values that implement `Try` [E0277]
+    //~| HELP the trait `Try` is not implemented for `{integer}`
+    //~| ERROR the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`) [E0277]
+    //~| HELP the trait `FromResidual<_>` is not implemented for `()`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE the `?` operator cannot be applied to type `{integer}`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE cannot use the `?` operator in a function that returns `()`
+    //~| NOTE in this expansion of desugaring of operator `?`
+}
+
+fn main() { //~ NOTE this function should return `Result` or `Option` to accept `?`
+    let x = 5 > 2 ? { let x = vec![]: Vec<u16>; x } : { false };
+    //~^ ERROR Rust has no ternary operator
+    //~| HELP use an `if-else` expression instead
+    //~| ERROR expected one of `.`, `;`, `?`, `else`, or an operator, found `:`
+    //~| NOTE expected one of `.`, `;`, `?`, `else`, or an operator
+    //~| ERROR the `?` operator can only be applied to values that implement `Try` [E0277]
+    //~| HELP the trait `Try` is not implemented for `{integer}`
+    //~| ERROR the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`) [E0277]
+    //~| HELP the trait `FromResidual<_>` is not implemented for `()`
+    //~| NOTE type ascription syntax has been removed, see issue #101728 <https://github.com/rust-lang/rust/issues/101728>
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE the `?` operator cannot be applied to type `{integer}`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE cannot use the `?` operator in a function that returns `()`
+    //~| NOTE in this expansion of desugaring of operator `?`
+}

tests/ui/parser/ternary_operator.stderr

@@ -0,0 +1,115 @@
+error: Rust has no ternary operator
+  --> $DIR/ternary_operator.rs:5:19
+   |
+LL |     let x = 5 > 2 ? true : false;
+   |                   ^^^^^^^^^^^^^^^
+   |
+   = help: use an `if-else` expression instead
+
+error: Rust has no ternary operator
+  --> $DIR/ternary_operator.rs:21:19
+   |
+LL |     let x = 5 > 2 ? { true } : { false };
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use an `if-else` expression instead
+
+error: Rust has no ternary operator
+  --> $DIR/ternary_operator.rs:37:19
+   |
+LL |     let x = 5 > 2 ? f32::MAX : f32::MIN;
+   |                   ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use an `if-else` expression instead
+
+error: expected one of `.`, `;`, `?`, `else`, or an operator, found `:`
+  --> $DIR/ternary_operator.rs:53:37
+   |
+LL |     let x = 5 > 2 ? { let x = vec![]: Vec<u16>; x } : { false };
+   |                                     ^ expected one of `.`, `;`, `?`, `else`, or an operator
+   |
+   = note: type ascription syntax has been removed, see issue #101728 <https://github.com/rust-lang/rust/issues/101728>
+
+error: Rust has no ternary operator
+  --> $DIR/ternary_operator.rs:53:19
+   |
+LL |     let x = 5 > 2 ? { let x = vec![]: Vec<u16>; x } : { false };
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use an `if-else` expression instead
+
+error[E0277]: the `?` operator can only be applied to values that implement `Try`
+  --> $DIR/ternary_operator.rs:5:17
+   |
+LL |     let x = 5 > 2 ? true : false;
+   |                 ^^^ the `?` operator cannot be applied to type `{integer}`
+   |
+   = help: the trait `Try` is not implemented for `{integer}`
+
+error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
+  --> $DIR/ternary_operator.rs:5:19
+   |
+LL | fn a() {
+   | ------ this function should return `Result` or `Option` to accept `?`
+LL |     let x = 5 > 2 ? true : false;
+   |                   ^ cannot use the `?` operator in a function that returns `()`
+   |
+   = help: the trait `FromResidual<_>` is not implemented for `()`
+
+error[E0277]: the `?` operator can only be applied to values that implement `Try`
+  --> $DIR/ternary_operator.rs:21:17
+   |
+LL |     let x = 5 > 2 ? { true } : { false };
+   |                 ^^^ the `?` operator cannot be applied to type `{integer}`
+   |
+   = help: the trait `Try` is not implemented for `{integer}`
+
+error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
+  --> $DIR/ternary_operator.rs:21:19
+   |
+LL | fn b() {
+   | ------ this function should return `Result` or `Option` to accept `?`
+LL |     let x = 5 > 2 ? { true } : { false };
+   |                   ^ cannot use the `?` operator in a function that returns `()`
+   |
+   = help: the trait `FromResidual<_>` is not implemented for `()`
+
+error[E0277]: the `?` operator can only be applied to values that implement `Try`
+  --> $DIR/ternary_operator.rs:37:17
+   |
+LL |     let x = 5 > 2 ? f32::MAX : f32::MIN;
+   |                 ^^^ the `?` operator cannot be applied to type `{integer}`
+   |
+   = help: the trait `Try` is not implemented for `{integer}`
+
+error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
+  --> $DIR/ternary_operator.rs:37:19
+   |
+LL | fn c() {
+   | ------ this function should return `Result` or `Option` to accept `?`
+LL |     let x = 5 > 2 ? f32::MAX : f32::MIN;
+   |                   ^ cannot use the `?` operator in a function that returns `()`
+   |
+   = help: the trait `FromResidual<_>` is not implemented for `()`
+
+error[E0277]: the `?` operator can only be applied to values that implement `Try`
+  --> $DIR/ternary_operator.rs:53:17
+   |
+LL |     let x = 5 > 2 ? { let x = vec![]: Vec<u16>; x } : { false };
+   |                 ^^^ the `?` operator cannot be applied to type `{integer}`
+   |
+   = help: the trait `Try` is not implemented for `{integer}`
+
+error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
+  --> $DIR/ternary_operator.rs:53:19
+   |
+LL | fn main() {
+   | --------- this function should return `Result` or `Option` to accept `?`
+LL |     let x = 5 > 2 ? { let x = vec![]: Vec<u16>; x } : { false };
+   |                   ^ cannot use the `?` operator in a function that returns `()`
+   |
+   = help: the trait `FromResidual<_>` is not implemented for `()`
+
+error: aborting due to 13 previous errors
+
+For more information about this error, try `rustc --explain E0277`.