Detect bare blocks with type ascription that were meant to be a struct literal

Address part of rust-lang#34255.

Potential improvement: silence the other knock down errors in
`issue-34255-1.rs`.

Author: estebank

compiler/rustc_ast/src/ast.rs

@@ -565,6 +565,14 @@ pub struct Block {
     pub rules: BlockCheckMode,
     pub span: Span,
     pub tokens: Option<LazyTokenStream>,
+    /// The following *isn't* a parse error, but will cause multiple errors in following stages.
+    /// ```
+    /// let x = {
+    ///     foo: var
+    /// };
+    /// ```
+    /// #34255
+    pub could_be_bare_literal: bool,
 }
 
 /// A match pattern.

compiler/rustc_ast/src/mut_visit.rs

@@ -949,7 +949,7 @@ pub fn noop_visit_mt<T: MutVisitor>(MutTy { ty, mutbl: _ }: &mut MutTy, vis: &mu
 }
 
 pub fn noop_visit_block<T: MutVisitor>(block: &mut P<Block>, vis: &mut T) {
-    let Block { id, stmts, rules: _, span, tokens } = block.deref_mut();
+    let Block { id, stmts, rules: _, span, tokens, could_be_bare_literal: _ } = block.deref_mut();
     vis.visit_id(id);
     stmts.flat_map_in_place(|stmt| vis.flat_map_stmt(stmt));
     vis.visit_span(span);

compiler/rustc_builtin_macros/src/deriving/mod.rs

@@ -102,6 +102,7 @@ fn call_unreachable(cx: &ExtCtxt<'_>, span: Span) -> P<ast::Expr> {
         rules: ast::BlockCheckMode::Unsafe(ast::CompilerGenerated),
         span,
         tokens: None,
+        could_be_bare_literal: false,
     }))
 }
 

compiler/rustc_builtin_macros/src/format.rs

@@ -867,6 +867,7 @@ impl<'a, 'b> Context<'a, 'b> {
             rules: BlockCheckMode::Unsafe(UnsafeSource::CompilerGenerated),
             span: self.macsp,
             tokens: None,
+            could_be_bare_literal: false,
         }));
 
         let ident = Ident::from_str_and_span("args", self.macsp);

compiler/rustc_expand/src/build.rs

@@ -197,6 +197,7 @@ impl<'a> ExtCtxt<'a> {
             rules: BlockCheckMode::Default,
             span,
             tokens: None,
+            could_be_bare_literal: false,
         })
     }
 

compiler/rustc_interface/src/util.rs

@@ -810,6 +810,7 @@ impl<'a> MutVisitor for ReplaceBodyWithLoop<'a, '_> {
                 id: resolver.next_node_id(),
                 span: rustc_span::DUMMY_SP,
                 tokens: None,
+                could_be_bare_literal: false,
             }
         }
 

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -446,11 +446,13 @@ impl<'a> Parser<'a> {
                         )
                         .emit();
                     *self = snapshot;
-                    Ok(self.mk_block(
+                    let mut tail = self.mk_block(
                         vec![self.mk_stmt_err(expr.span)],
                         s,
                         lo.to(self.prev_token.span),
-                    ))
+                    );
+                    tail.could_be_bare_literal = true;
+                    Ok(tail)
                 }
                 (Err(mut err), Ok(tail)) => {
                     // We have a block tail that contains a somehow valid type ascription expr.
@@ -463,7 +465,10 @@ impl<'a> Parser<'a> {
                     self.consume_block(token::Brace, ConsumeClosingDelim::Yes);
                     Err(err)
                 }
-                (Ok(_), Ok(tail)) => Ok(tail),
+                (Ok(_), Ok(mut tail)) => {
+                    tail.could_be_bare_literal = true;
+                    Ok(tail)
+                }
             });
         }
         None

compiler/rustc_parse/src/parser/stmt.rs

@@ -574,7 +574,14 @@ impl<'a> Parser<'a> {
     }
 
     pub(super) fn mk_block(&self, stmts: Vec<Stmt>, rules: BlockCheckMode, span: Span) -> P<Block> {
-        P(Block { stmts, id: DUMMY_NODE_ID, rules, span, tokens: None })
+        P(Block {
+            stmts,
+            id: DUMMY_NODE_ID,
+            rules,
+            span,
+            tokens: None,
+            could_be_bare_literal: false,
+        })
     }
 
     pub(super) fn mk_stmt(&self, span: Span, kind: StmtKind) -> Stmt {

compiler/rustc_resolve/src/late.rs

@@ -383,6 +383,11 @@ struct DiagnosticMetadata<'ast> {
     /// Only used for better errors on `fn(): fn()`.
     current_type_ascription: Vec<Span>,
 
+    /// Only used for better errors on `let x = { foo: bar };`.
+    /// In the case of a parse error with `let x = { foo: bar, };`, this isn't needed, it's only
+    /// needed for cases where this parses as a correct type ascription.
+    current_block_could_be_bare_struct_literal: Option<Span>,
+
     /// Only used for better errors on `let <pat>: <expr, not type>;`.
     current_let_binding: Option<(Span, Option<Span>, Option<Span>)>,
 
@@ -1859,6 +1864,7 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                 let instead = res.is_some();
                 let suggestion =
                     if res.is_none() { this.report_missing_type_error(path) } else { None };
+                // get_from_node_id
 
                 this.r.use_injections.push(UseError {
                     err,
@@ -2242,6 +2248,15 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
             self.ribs[ValueNS].push(Rib::new(NormalRibKind));
         }
 
+        let prev = self.diagnostic_metadata.current_block_could_be_bare_struct_literal.take();
+        if let (true, [Stmt { kind: StmtKind::Expr(expr), .. }]) =
+            (block.could_be_bare_literal, &block.stmts[..])
+        {
+            if let ExprKind::Type(..) = expr.kind {
+                self.diagnostic_metadata.current_block_could_be_bare_struct_literal =
+                    Some(block.span);
+            }
+        }
         // Descend into the block.
         for stmt in &block.stmts {
             if let StmtKind::Item(ref item) = stmt.kind {
@@ -2255,6 +2270,7 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
 
             self.visit_stmt(stmt);
         }
+        self.diagnostic_metadata.current_block_could_be_bare_struct_literal = prev;
 
         // Move back up.
         self.parent_scope.module = orig_module;

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -207,6 +207,16 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
         let code = source.error_code(res.is_some());
         let mut err = self.r.session.struct_span_err_with_code(base_span, &base_msg, code);
 
+        if let Some(span) = self.diagnostic_metadata.current_block_could_be_bare_struct_literal {
+            err.multipart_suggestion(
+                "you might have meant to write a `struct` literal",
+                vec![
+                    (span.shrink_to_lo(), "{ SomeStruct ".to_string()),
+                    (span.shrink_to_hi(), "}".to_string()),
+                ],
+                Applicability::HasPlaceholders,
+            );
+        }
         match (source, self.diagnostic_metadata.in_if_condition) {
             (PathSource::Expr(_), Some(Expr { span, kind: ExprKind::Assign(..), .. })) => {
                 err.span_suggestion_verbose(