Recover from struct literals with placeholder or empty path

compiler/rustc_parse/messages.ftl

@@ -822,9 +822,19 @@ parse_struct_literal_body_without_path =
     struct literal body without path
     .suggestion = you might have forgotten to add the struct literal inside the block
 
+parse_struct_literal_body_without_path_late =
+    struct literal body without path
+    .label = struct name missing for struct literal
+    .suggestion = add the correct type
+
 parse_struct_literal_not_allowed_here = struct literals are not allowed here
     .suggestion = surround the struct literal with parentheses
 
+parse_struct_literal_placeholder_path =
+    placeholder `_` is not allowed for the path in struct literals
+    .label = not allowed in struct literals
+    .suggestion = replace it with the correct 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

@@ -3684,3 +3684,22 @@ pub(crate) struct ImplReuseInherentImpl {
     #[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 */", style = "verbose")]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_struct_literal_body_without_path_late)]
+pub(crate) struct StructLiteralWithoutPathLate {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    #[suggestion(applicability = "has-placeholders", code = "/* Type */ ", style = "verbose")]
+    pub suggestion_span: Span,
+}

compiler/rustc_parse/src/parser/expr.rs

@@ -1468,6 +1468,9 @@ impl<'a> Parser<'a> {
             } else if this.check(exp!(OpenParen)) {
                 this.parse_expr_tuple_parens(restrictions)
             } else if this.check(exp!(OpenBrace)) {
+                if let Some(expr) = this.maybe_recover_bad_struct_literal_path(false)? {
+                    return Ok(expr);
+                }
                 this.parse_expr_block(None, lo, BlockCheckMode::Default)
             } else if this.check(exp!(Or)) || this.check(exp!(OrOr)) {
                 this.parse_expr_closure().map_err(|mut err| {
@@ -1542,6 +1545,9 @@ impl<'a> Parser<'a> {
             } else if this.check_keyword(exp!(Let)) {
                 this.parse_expr_let(restrictions)
             } else if this.eat_keyword(exp!(Underscore)) {
+                if let Some(expr) = this.maybe_recover_bad_struct_literal_path(true)? {
+                    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.
@@ -3698,6 +3704,45 @@ impl<'a> Parser<'a> {
         }
     }
 
+    fn maybe_recover_bad_struct_literal_path(
+        &mut self,
+        is_underscore_entry_point: bool,
+    ) -> PResult<'a, Option<Box<Expr>>> {
+        if self.may_recover()
+            && self.check_noexpect(&token::OpenBrace)
+            && (!self.restrictions.contains(Restrictions::NO_STRUCT_LITERAL)
+                && self.is_likely_struct_lit())
+        {
+            let span = if is_underscore_entry_point {
+                self.prev_token.span
+            } else {
+                self.token.span.shrink_to_lo()
+            };
+
+            self.bump(); // {
+            let expr = self.parse_expr_struct(
+                None,
+                Path::from_ident(Ident::new(kw::Underscore, span)),
+                false,
+            )?;
+
+            let guar = if is_underscore_entry_point {
+                self.dcx().create_err(errors::StructLiteralPlaceholderPath { span }).emit()
+            } else {
+                self.dcx()
+                    .create_err(errors::StructLiteralWithoutPathLate {
+                        span: expr.span,
+                        suggestion_span: expr.span.shrink_to_lo(),
+                    })
+                    .emit()
+            };
+
+            Ok(Some(self.mk_expr_err(expr.span, guar)))
+        } else {
+            Ok(None)
+        }
+    }
+
     pub(super) fn parse_struct_fields(
         &mut self,
         pth: ast::Path,

tests/ui/parser/bare-struct-body.stderr

@@ -21,14 +21,12 @@ LL |       let x = {
    |  _____________^
 LL | |         val: (),
 LL | |     };
-   | |_____^
+   | |_____^ struct name missing for struct literal
    |
-help: you might have forgotten to add the struct literal inside the block
-   |
-LL ~     let x = { SomeStruct {
-LL |         val: (),
-LL ~     } };
+help: add the correct type
    |
+LL |     let x = /* Type */ {
+   |             ++++++++++
 
 error[E0308]: mismatched types
   --> $DIR/bare-struct-body.rs:11:14

tests/ui/parser/struct-lit-placeholder-or-empty-path.rs

@@ -0,0 +1,14 @@
+fn main() {
+    let _ = {foo: (), bar: {} }; //~ ERROR struct literal body without path
+    //~| NOTE struct name missing for struct literal
+    //~| HELP add the correct type
+    let _ = _ {foo: (), bar: {} }; //~ ERROR placeholder `_` is not allowed for the path in struct literals
+    //~| NOTE not allowed in struct literals
+    //~| HELP replace it with the correct type
+    let _ = {foo: ()}; //~ ERROR struct literal body without path
+    //~| NOTE struct name missing for struct literal
+    //~| HELP add the correct type
+    let _ = _ {foo: ()}; //~ ERROR placeholder `_` is not allowed for the path in struct literals
+    //~| NOTE not allowed in struct literals
+    //~| HELP replace it with the correct type
+}

tests/ui/parser/struct-lit-placeholder-or-empty-path.stderr

@@ -0,0 +1,48 @@
+error: struct literal body without path
+  --> $DIR/struct-lit-placeholder-or-empty-path.rs:2:13
+   |
+LL |     let _ = {foo: (), bar: {} };
+   |             ^^^^^^^^^^^^^^^^^^^ struct name missing for struct literal
+   |
+help: add the correct type
+   |
+LL |     let _ = /* Type */ {foo: (), bar: {} };
+   |             ++++++++++
+
+error: placeholder `_` is not allowed for the path in struct literals
+  --> $DIR/struct-lit-placeholder-or-empty-path.rs:5:13
+   |
+LL |     let _ = _ {foo: (), bar: {} };
+   |             ^ not allowed in struct literals
+   |
+help: replace it with the correct type
+   |
+LL -     let _ = _ {foo: (), bar: {} };
+LL +     let _ = /* Type */ {foo: (), bar: {} };
+   |
+
+error: struct literal body without path
+  --> $DIR/struct-lit-placeholder-or-empty-path.rs:8:13
+   |
+LL |     let _ = {foo: ()};
+   |             ^^^^^^^^^ struct name missing for struct literal
+   |
+help: add the correct type
+   |
+LL |     let _ = /* Type */ {foo: ()};
+   |             ++++++++++
+
+error: placeholder `_` is not allowed for the path in struct literals
+  --> $DIR/struct-lit-placeholder-or-empty-path.rs:11:13
+   |
+LL |     let _ = _ {foo: ()};
+   |             ^ not allowed in struct literals
+   |
+help: replace it with the correct type
+   |
+LL -     let _ = _ {foo: ()};
+LL +     let _ = /* Type */ {foo: ()};
+   |
+
+error: aborting due to 4 previous errors
+

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 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 placeholder `_` is not allowed for the path in struct literals
+    //~| NOTE not allowed in struct literals
+    //~| HELP replace it with the correct type
+}

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

@@ -0,0 +1,26 @@
+error: 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
+   |
+LL -     blah::do_stuff(_ { x: 10 });
+LL +     blah::do_stuff(/* Type */ { x: 10 });
+   |
+
+error: 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 the correct type
+   |
+LL -     blah::do_stuff(_ { x: 10 });
+LL +     blah::do_stuff(/* Type */ { x: 10 });
+   |
+
+error: aborting due to 2 previous errors
+