Parse Ty? as Option<Ty> and provide structured suggestion

Swift has specific syntax that desugars to `Option<T>` similar to our
`?` operator, which means that people might try to use it in Rust. Parse
it and gracefully recover.

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -1,5 +1,5 @@
 use super::pat::Expected;
-use super::ty::AllowPlus;
+use super::ty::{AllowPlus, IsAsCast};
 use super::{
     BlockMode, Parser, PathStyle, RecoverColon, RecoverComma, Restrictions, SemiColonMode, SeqSep,
     TokenExpectType, TokenType,
@@ -1032,6 +1032,34 @@ impl<'a> Parser<'a> {
         }
     }
 
+    /// Swift lets users write `Ty?` to mean `Option<Ty>`. Parse the construct and recover from it.
+    pub(super) fn maybe_recover_from_question_mark(
+        &mut self,
+        ty: P<Ty>,
+        is_as_cast: IsAsCast,
+    ) -> P<Ty> {
+        if let IsAsCast::Yes = is_as_cast {
+            return ty;
+        }
+        if self.token == token::Question {
+            self.bump();
+            self.struct_span_err(self.prev_token.span, "invalid `?` in type")
+                .span_label(self.prev_token.span, "`?` is only allowed on expressions, not types")
+                .multipart_suggestion(
+                    "if you meant to express that the type might not contain a value, use the `Option` wrapper type",
+                    vec![
+                        (ty.span.shrink_to_lo(), "Option<".to_string()),
+                        (self.prev_token.span, ">".to_string()),
+                    ],
+                    Applicability::MachineApplicable,
+                )
+                .emit();
+            self.mk_ty(ty.span.to(self.prev_token.span), TyKind::Err)
+        } else {
+            ty
+        }
+    }
+
     pub(super) fn maybe_recover_from_bad_type_plus(
         &mut self,
         allow_plus: AllowPlus,

compiler/rustc_parse/src/parser/expr.rs

@@ -682,7 +682,7 @@ impl<'a> Parser<'a> {
         // Save the state of the parser before parsing type normally, in case there is a
         // LessThan comparison after this cast.
         let parser_snapshot_before_type = self.clone();
-        let cast_expr = match self.parse_ty_no_plus() {
+        let cast_expr = match self.parse_as_cast_ty() {
             Ok(rhs) => mk_expr(self, lhs, rhs),
             Err(mut type_err) => {
                 // Rewind to before attempting to parse the type with generics, to recover
@@ -808,7 +808,7 @@ impl<'a> Parser<'a> {
                 "casts cannot be followed by {}",
                 match with_postfix.kind {
                     ExprKind::Index(_, _) => "indexing",
-                    ExprKind::Try(_) => "?",
+                    ExprKind::Try(_) => "`?`",
                     ExprKind::Field(_, _) => "a field access",
                     ExprKind::MethodCall(_, _, _) => "a method call",
                     ExprKind::Call(_, _) => "a function call",

compiler/rustc_parse/src/parser/ty.rs

@@ -44,6 +44,11 @@ pub(super) enum RecoverQPath {
     No,
 }
 
+pub(super) enum IsAsCast {
+    Yes,
+    No,
+}
+
 /// Signals whether parsing a type should recover `->`.
 ///
 /// More specifically, when parsing a function like:
@@ -100,6 +105,7 @@ impl<'a> Parser<'a> {
             RecoverQPath::Yes,
             RecoverReturnSign::Yes,
             None,
+            IsAsCast::No,
         )
     }
 
@@ -113,6 +119,7 @@ impl<'a> Parser<'a> {
             RecoverQPath::Yes,
             RecoverReturnSign::Yes,
             Some(ty_params),
+            IsAsCast::No,
         )
     }
 
@@ -126,6 +133,7 @@ impl<'a> Parser<'a> {
             RecoverQPath::Yes,
             RecoverReturnSign::Yes,
             None,
+            IsAsCast::No,
         )
     }
 
@@ -142,9 +150,22 @@ impl<'a> Parser<'a> {
             RecoverQPath::Yes,
             RecoverReturnSign::Yes,
             None,
+            IsAsCast::No,
         )
     }
 
+    /// Parses a type following an `as` cast. Similar to `parse_ty_no_plus`, but signaling origin
+    /// for better diagnostics involving `?`.
+    pub(super) fn parse_as_cast_ty(&mut self) -> PResult<'a, P<Ty>> {
+        self.parse_ty_common(
+            AllowPlus::No,
+            AllowCVariadic::No,
+            RecoverQPath::Yes,
+            RecoverReturnSign::Yes,
+            None,
+            IsAsCast::Yes,
+        )
+    }
     /// Parse a type without recovering `:` as `->` to avoid breaking code such as `where fn() : for<'a>`
     pub(super) fn parse_ty_for_where_clause(&mut self) -> PResult<'a, P<Ty>> {
         self.parse_ty_common(
@@ -153,6 +174,7 @@ impl<'a> Parser<'a> {
             RecoverQPath::Yes,
             RecoverReturnSign::OnlyFatArrow,
             None,
+            IsAsCast::No,
         )
     }
 
@@ -171,6 +193,7 @@ impl<'a> Parser<'a> {
                 recover_qpath,
                 recover_return_sign,
                 None,
+                IsAsCast::No,
             )?;
             FnRetTy::Ty(ty)
         } else if recover_return_sign.can_recover(&self.token.kind) {
@@ -191,6 +214,7 @@ impl<'a> Parser<'a> {
                 recover_qpath,
                 recover_return_sign,
                 None,
+                IsAsCast::No,
             )?;
             FnRetTy::Ty(ty)
         } else {
@@ -205,6 +229,7 @@ impl<'a> Parser<'a> {
         recover_qpath: RecoverQPath,
         recover_return_sign: RecoverReturnSign,
         ty_generics: Option<&Generics>,
+        is_as_cast: IsAsCast,
     ) -> PResult<'a, P<Ty>> {
         let allow_qpath_recovery = recover_qpath == RecoverQPath::Yes;
         maybe_recover_from_interpolated_ty_qpath!(self, allow_qpath_recovery);
@@ -280,6 +305,7 @@ impl<'a> Parser<'a> {
         // Try to recover from use of `+` with incorrect priority.
         self.maybe_report_ambiguous_plus(allow_plus, impl_dyn_multi, &ty);
         self.maybe_recover_from_bad_type_plus(allow_plus, &ty)?;
+        let ty = self.maybe_recover_from_question_mark(ty, is_as_cast);
         self.maybe_recover_from_bad_qpath(ty, allow_qpath_recovery)
     }