Parser: Suggest Placing the Return Type After Function Parameters

Author: veera-sivarajan

compiler/rustc_parse/messages.ftl

@@ -526,6 +526,8 @@ parse_mismatched_closing_delimiter = mismatched closing delimiter: `{$delimiter}
     .label_opening_candidate = closing delimiter possibly meant for this
     .label_unclosed = unclosed delimiter
 
+parse_misplaced_return_type = place the return type after the function parameters
+
 parse_missing_comma_after_match_arm = expected `,` following `match` arm
     .suggestion = missing a comma here to end this `match` arm
 

compiler/rustc_parse/src/errors.rs

@@ -1434,6 +1434,20 @@ pub(crate) struct FnPtrWithGenerics {
     pub sugg: Option<FnPtrWithGenericsSugg>,
 }
 
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(
+    parse_misplaced_return_type,
+    style = "verbose",
+    applicability = "maybe-incorrect"
+)]
+pub(crate) struct MisplacedReturnType {
+    #[suggestion_part(code = " {snippet}")]
+    pub fn_params_end: Span,
+    pub snippet: String,
+    #[suggestion_part(code = "")]
+    pub ret_ty_span: Span,
+}
+
 #[derive(Subdiagnostic)]
 #[multipart_suggestion(parse_suggestion, applicability = "maybe-incorrect")]
 pub(crate) struct FnPtrWithGenericsSugg {
@@ -1448,7 +1462,6 @@ pub(crate) struct FnPtrWithGenericsSugg {
 
 pub(crate) struct FnTraitMissingParen {
     pub span: Span,
-    pub machine_applicable: bool,
 }
 
 impl Subdiagnostic for FnTraitMissingParen {
@@ -1458,16 +1471,11 @@ impl Subdiagnostic for FnTraitMissingParen {
         _: &F,
     ) {
         diag.span_label(self.span, crate::fluent_generated::parse_fn_trait_missing_paren);
-        let applicability = if self.machine_applicable {
-            Applicability::MachineApplicable
-        } else {
-            Applicability::MaybeIncorrect
-        };
         diag.span_suggestion_short(
             self.span.shrink_to_hi(),
             crate::fluent_generated::parse_add_paren,
             "()",
-            applicability,
+            Applicability::MachineApplicable,
         );
     }
 }

compiler/rustc_parse/src/parser/item.rs

@@ -2332,10 +2332,21 @@ impl<'a> Parser<'a> {
                 }
             }
         };
+
+        // Store the end of function parameters to give better diagnostics
+        // inside `parse_fn_body()`.
+        let fn_params_end = self.prev_token.span.shrink_to_hi();
+
         generics.where_clause = self.parse_where_clause()?; // `where T: Ord`
 
+        // `fn_params_end` is needed only when it's followed by a where clause.
+        let fn_params_end =
+            if generics.where_clause.has_where_token { Some(fn_params_end) } else { None };
+
         let mut sig_hi = self.prev_token.span;
-        let body = self.parse_fn_body(attrs, &ident, &mut sig_hi, fn_parse_mode.req_body)?; // `;` or `{ ... }`.
+        // Either `;` or `{ ... }`.
+        let body =
+            self.parse_fn_body(attrs, &ident, &mut sig_hi, fn_parse_mode.req_body, fn_params_end)?;
         let fn_sig_span = sig_lo.to(sig_hi);
         Ok((ident, FnSig { header, decl, span: fn_sig_span }, generics, body))
     }
@@ -2349,6 +2360,7 @@ impl<'a> Parser<'a> {
         ident: &Ident,
         sig_hi: &mut Span,
         req_body: bool,
+        fn_params_end: Option<Span>,
     ) -> PResult<'a, Option<P<Block>>> {
         let has_semi = if req_body {
             self.token.kind == TokenKind::Semi
@@ -2388,20 +2400,58 @@ impl<'a> Parser<'a> {
                     // the AST for typechecking.
                     err.span_label(ident.span, "while parsing this `fn`");
                     err.emit();
-                } else {
-                    // check for typo'd Fn* trait bounds such as
-                    // fn foo<F>() where F: FnOnce -> () {}
-                    if self.token.kind == token::RArrow {
-                        let machine_applicable = [sym::FnOnce, sym::FnMut, sym::Fn]
-                            .into_iter()
-                            .any(|s| self.prev_token.is_ident_named(s));
-
-                        err.subdiagnostic(errors::FnTraitMissingParen {
-                            span: self.prev_token.span,
-                            machine_applicable,
+                } else if self.token.kind == token::RArrow
+                    && let Some(fn_params_end) = fn_params_end
+                {
+                    // Instead of a function body, the parser has encountered a right arrow
+                    // preceded by a where clause.
+
+                    // Find whether token behind the right arrow is a function trait and
+                    // store its span.
+                    let prev_token_is_fn_trait = [sym::FnOnce, sym::FnMut, sym::Fn]
+                        .into_iter()
+                        .any(|s| self.prev_token.is_ident_named(s));
+                    let fn_trait_span = self.prev_token.span;
+
+                    // Parse the return type (along with the right arrow) and store its span.
+                    // If there's a parse error, cancel it and return the existing error
+                    // as we are primarily concerned with the
+                    // expected-function-body-but-found-something-else error here.
+                    let arrow_span = self.token.span;
+                    let ty_span = match self.parse_ret_ty(
+                        AllowPlus::Yes,
+                        RecoverQPath::Yes,
+                        RecoverReturnSign::Yes,
+                    ) {
+                        Ok(ty_span) => ty_span.span().shrink_to_hi(),
+                        Err(parse_error) => {
+                            parse_error.cancel();
+                            return Err(err);
+                        }
+                    };
+                    let ret_ty_span = arrow_span.to(ty_span);
+
+                    if prev_token_is_fn_trait {
+                        // Typo'd Fn* trait bounds such as
+                        // fn foo<F>() where F: FnOnce -> () {}
+                        err.subdiagnostic(errors::FnTraitMissingParen { span: fn_trait_span });
+                    } else if let Ok(snippet) = self.psess.source_map().span_to_snippet(ret_ty_span)
+                    {
+                        // If token behind right arrow is not a Fn* trait, the programmer
+                        // probably misplaced the return type after the where clause like
+                        // `fn foo<T>() where T: Default -> u8 {}`
+                        err.primary_message(
+                            "return type should be specified after the function parameters",
+                        );
+                        err.subdiagnostic(errors::MisplacedReturnType {
+                            fn_params_end,
+                            snippet,
+                            ret_ty_span,
                         });
                     }
                     return Err(err);
+                } else {
+                    return Err(err);
                 }
             }
             (AttrVec::new(), None)

compiler/rustc_parse/src/parser/ty.rs

@@ -21,6 +21,11 @@ use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::{ErrorGuaranteed, Span, Symbol};
 use thin_vec::{thin_vec, ThinVec};
 
+/// Signals whether parsing a type should allow `+`.
+///
+/// For example, let T be the type `impl Default + 'static`
+/// With `AllowPlus::Yes`, T will be parsed successfully
+/// With `AllowPlus::No`, parsing T will return a parse error
 #[derive(Copy, Clone, PartialEq)]
 pub(super) enum AllowPlus {
     Yes,