[Proof of concept] Recover from struct literals with placeholder path

compiler/rustc_errors/src/lib.rs

@@ -507,6 +507,7 @@ struct DiagCtxtInner {
 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
 pub enum StashKey {
     ItemNoType,
+    StructLitNoType,
     UnderscoreForArrayLengths,
     EarlySyntaxWarning,
     CallIntoMethod,

compiler/rustc_hir_typeck/src/expr.rs

@@ -1640,6 +1640,40 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         fields: &'tcx [hir::ExprField<'tcx>],
         base_expr: &'tcx Option<&'tcx hir::Expr<'tcx>>,
     ) -> Ty<'tcx> {
+        // FIXME(fmease): Move this into separate method.
+        // FIXME(fmease): This doesn't get called given `(_ { x: () }).x` (`hir::Field`).
+        //                Figure out why.
+        if let QPath::Resolved(None, hir::Path { res: Res::Err, segments, .. }) = qpath
+            && let [segment] = segments
+            && segment.ident.name == kw::Empty
+            && let Expectation::ExpectHasType(ty) = expected
+            && ty.is_adt()
+            && let Some(guar) = self.dcx().try_steal_modify_and_emit_err(
+                expr.span,
+                StashKey::StructLitNoType,
+                |err| {
+                    // The parser provided a sub-optimal `HasPlaceholders` suggestion for the type.
+                    // We are typeck and have the real type, so remove that and suggest the actual type.
+                    if let Ok(suggestions) = &mut err.suggestions {
+                        suggestions.clear();
+                    }
+
+                    err.span_suggestion(
+                        qpath.span(),
+                        // FIXME(fmease): Make this translatable.
+                        "replace it with the correct type",
+                        // FIXME(fmease): This doesn't qualify paths within the type appropriately.
+                        // FIXME(fmease): This doesn't use turbofish when emitting generic args.
+                        // FIXME(fmease): Make the type suggestable.
+                        ty.to_string(),
+                        Applicability::MaybeIncorrect,
+                    );
+                },
+            )
+        {
+            return Ty::new_error(self.tcx, guar);
+        }
+
         // Find the relevant variant
         let (variant, adt_ty) = match self.check_struct_path(qpath, expr.hir_id) {
             Ok(data) => data,

compiler/rustc_parse/messages.ftl

@@ -708,6 +708,11 @@ parse_struct_literal_needing_parens =
 parse_struct_literal_not_allowed_here = struct literals are not allowed here
     .suggestion = surround the struct literal with parentheses
 
+parse_struct_literal_placeholder_path =
+    the placeholder `_` is not allowed for the path in struct literals
+    .label = not allowed in struct literals
+    .suggestion = replace it with an appropriate type
+
 parse_suffixed_literal_in_attribute = suffixed literals are not allowed in attributes
     .help = instead of using a suffixed literal (`1u8`, `1.0f32`, etc.), use an unsuffixed version (`1`, `1.0`, etc.)
 

compiler/rustc_parse/src/errors.rs

@@ -2974,3 +2974,12 @@ pub(crate) struct AsyncImpl {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(parse_struct_literal_placeholder_path)]
+pub(crate) struct StructLiteralPlaceholderPath {
+    #[primary_span]
+    #[label]
+    #[suggestion(applicability = "has-placeholders", code = "/*Type*/")]
+    pub span: Span,
+}

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -951,6 +951,10 @@ impl<'a> Parser<'a> {
                             return None;
                         }
                     } else {
+                        // FIXME(fmease): Under certain conditions return a struct literal
+                        // here and stash this diagnostic under StructLitNoType.
+                        // FIXME(fmease): Furthermore don't suggest redundant curly braces
+                        // around the potential struct literal if possible.
                         self.dcx().emit_err(StructLiteralBodyWithoutPath {
                             span: expr.span,
                             sugg: StructLiteralBodyWithoutPathSugg {

compiler/rustc_parse/src/parser/expr.rs

@@ -1505,6 +1505,10 @@ impl<'a> Parser<'a> {
             } else if this.check_keyword(kw::Let) {
                 this.parse_expr_let(restrictions)
             } else if this.eat_keyword(kw::Underscore) {
+                if let Some(expr) = this.maybe_recover_bad_struct_literal_path()? {
+                    return Ok(expr);
+                }
+
                 Ok(this.mk_expr(this.prev_token.span, ExprKind::Underscore))
             } else if this.token.uninterpolated_span().at_least_rust_2018() {
                 // `Span::at_least_rust_2018()` is somewhat expensive; don't get it repeatedly.
@@ -3720,6 +3724,28 @@ impl<'a> Parser<'a> {
         });
     }
 
+    fn maybe_recover_bad_struct_literal_path(&mut self) -> PResult<'a, Option<P<Expr>>> {
+        if self.may_recover()
+            && self.check_noexpect(&token::OpenDelim(Delimiter::Brace))
+            && (!self.restrictions.contains(Restrictions::NO_STRUCT_LITERAL)
+                || self.is_certainly_not_a_block())
+        {
+            let span = self.prev_token.span;
+            self.bump();
+
+            let expr =
+                self.parse_expr_struct(None, Path::from_ident(Ident::new(kw::Empty, span)), false)?;
+
+            self.dcx()
+                .create_err(errors::StructLiteralPlaceholderPath { span: expr.span })
+                .stash(expr.span, StashKey::StructLitNoType);
+
+            Ok(Some(expr))
+        } else {
+            Ok(None)
+        }
+    }
+
     fn err_dotdotdot_syntax(&self, span: Span) {
         self.dcx().emit_err(errors::DotDotDot { span });
     }

compiler/rustc_resolve/src/late.rs

@@ -4304,6 +4304,13 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
             }
 
             ExprKind::Struct(ref se) => {
+                // FIXME(fmease): Add a comment maybe.
+                if se.path == kw::Empty
+                    && self.r.dcx().has_stashed_diagnostic(expr.span, StashKey::StructLitNoType)
+                {
+                    return;
+                }
+
                 self.smart_resolve_path(expr.id, &se.qself, &se.path, PathSource::Struct);
                 // This is the same as `visit::walk_expr(self, expr);`, but we want to pass the
                 // parent in for accurate suggestions when encountering `Foo { bar }` that should

tests/ui/suggestions/struct-lit-placeholder-path.rs

@@ -0,0 +1,21 @@
+// Regression test for issue #98282.
+
+mod blah {
+    pub struct Stuff { x: i32 }
+    pub fn do_stuff(_: Stuff) {}
+}
+
+fn main() {
+    blah::do_stuff(_ { x: 10 });
+    //~^ ERROR the placeholder `_` is not allowed for the path in struct literals
+    //~| NOTE not allowed in struct literals
+    //~| HELP replace it with the correct type
+}
+
+#[cfg(FALSE)]
+fn disabled() {
+    blah::do_stuff(_ { x: 10 });
+    //~^ ERROR the placeholder `_` is not allowed for the path in struct literals
+    //~| NOTE not allowed in struct literals
+    //~| HELP replace it with an appropriate type
+}

tests/ui/suggestions/struct-lit-placeholder-path.stderr

@@ -0,0 +1,20 @@
+error: the placeholder `_` is not allowed for the path in struct literals
+  --> $DIR/struct-lit-placeholder-path.rs:9:20
+   |
+LL |     blah::do_stuff(_ { x: 10 });
+   |                    -^^^^^^^^^^
+   |                    |
+   |                    not allowed in struct literals
+   |                    help: replace it with the correct type: `Stuff`
+
+error: the placeholder `_` is not allowed for the path in struct literals
+  --> $DIR/struct-lit-placeholder-path.rs:17:20
+   |
+LL |     blah::do_stuff(_ { x: 10 });
+   |                    ^^^^^^^^^^^
+   |                    |
+   |                    not allowed in struct literals
+   |                    help: replace it with an appropriate type: `/*Type*/`
+
+error: aborting due to 2 previous errors
+