Do not emit type errors on recovered blocks

When a parse error occurs on a block, the parser will recover and create
a block with the statements collected until that point. Now a flag
stating that a recovery has been performed in this block is propagated
so that the type checker knows that the type of the block (which will be
identified as `()`) shouldn't be checked against the expectation to
reduce the amount of irrelevant diagnostic errors shown to the user.

Author: estebank

src/librustc/diagnostics.rs

@@ -359,18 +359,21 @@ lifetime elision rules (see below).
 Here are some simple examples of where you'll run into this error:
 
 ```compile_fail,E0106
-struct Foo { x: &bool }        // error
-struct Foo<'a> { x: &'a bool } // correct
+struct Foo1 { x: &bool }
+              // ^ expected lifetime parameter
+struct Foo2<'a> { x: &'a bool } // correct
 
-struct Bar { x: Foo }
-               ^^^ expected lifetime parameter
-struct Bar<'a> { x: Foo<'a> } // correct
+struct Bar1 { x: Foo2 }
+              // ^^^^ expected lifetime parameter
+struct Bar2<'a> { x: Foo2<'a> } // correct
 
-enum Bar { A(u8), B(&bool), }        // error
-enum Bar<'a> { A(u8), B(&'a bool), } // correct
+enum Baz1 { A(u8), B(&bool), }
+                  // ^ expected lifetime parameter
+enum Baz2<'a> { A(u8), B(&'a bool), } // correct
 
-type MyStr = &str;        // error
-type MyStr<'a> = &'a str; // correct
+type MyStr1 = &str;
+           // ^ expected lifetime parameter
+type MyStr2<'a> = &'a str; // correct
 ```
 
 Lifetime elision is a special, limited kind of inference for lifetimes in

src/librustc/hir/lowering.rs

@@ -1790,6 +1790,7 @@ impl<'a> LoweringContext<'a> {
             rules: self.lower_block_check_mode(&b.rules),
             span: b.span,
             targeted_by_break,
+            recovered: b.recovered,
         })
     }
 
@@ -2646,6 +2647,7 @@ impl<'a> LoweringContext<'a> {
                                 rules: hir::DefaultBlock,
                                 span,
                                 targeted_by_break: false,
+                                recovered: blk.recovered,
                             });
                             P(self.expr_block(blk, ThinVec::new()))
                         }
@@ -3462,6 +3464,7 @@ impl<'a> LoweringContext<'a> {
             rules: hir::DefaultBlock,
             span,
             targeted_by_break: false,
+            recovered: false,
         }
     }
 
@@ -3565,6 +3568,7 @@ impl<'a> LoweringContext<'a> {
             stmts,
             expr: Some(expr),
             targeted_by_break: false,
+            recovered: false,
         });
         self.expr_block(block, attrs)
     }

src/librustc/hir/mod.rs

@@ -620,6 +620,11 @@ pub struct Block {
     /// currently permitted in Rust itself, but it is generated as
     /// part of `catch` statements.
     pub targeted_by_break: bool,
+    /// If true, don't emit return value type errors as the parser had
+    /// to recover from a parse error so this block will not have an
+    /// appropriate type. A parse error will have been emitted so the
+    /// compilation will never succeed if this is true.
+    pub recovered: bool,
 }
 
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash)]

src/librustc/ich/impls_hir.rs

@@ -379,12 +379,14 @@ impl<'gcx> HashStable<StableHashingContext<'gcx>> for hir::Block {
             rules,
             span,
             targeted_by_break,
+            recovered,
         } = *self;
 
         stmts.hash_stable(hcx, hasher);
         expr.hash_stable(hcx, hasher);
         rules.hash_stable(hcx, hasher);
         span.hash_stable(hcx, hasher);
+        recovered.hash_stable(hcx, hasher);
         targeted_by_break.hash_stable(hcx, hasher);
     }
 }

src/librustc_driver/pretty.rs

@@ -725,6 +725,7 @@ impl<'a> fold::Folder for ReplaceBodyWithLoop<'a> {
 
     fn fold_block(&mut self, b: P<ast::Block>) -> P<ast::Block> {
         fn expr_to_block(rules: ast::BlockCheckMode,
+                         recovered: bool,
                          e: Option<P<ast::Expr>>,
                          sess: &Session) -> P<ast::Block> {
             P(ast::Block {
@@ -740,20 +741,21 @@ impl<'a> fold::Folder for ReplaceBodyWithLoop<'a> {
                 rules,
                 id: sess.next_node_id(),
                 span: syntax_pos::DUMMY_SP,
+                recovered,
             })
         }
 
         if !self.within_static_or_const {
 
-            let empty_block = expr_to_block(BlockCheckMode::Default, None, self.sess);
+            let empty_block = expr_to_block(BlockCheckMode::Default, false, None, self.sess);
             let loop_expr = P(ast::Expr {
                 node: ast::ExprKind::Loop(empty_block, None),
                 id: self.sess.next_node_id(),
                 span: syntax_pos::DUMMY_SP,
                 attrs: ast::ThinVec::new(),
             });
 
-            expr_to_block(b.rules, Some(loop_expr), self.sess)
+            expr_to_block(b.rules, b.recovered, Some(loop_expr), self.sess)
 
         } else {
             fold::noop_fold_block(b, self)

src/librustc_typeck/check/mod.rs

@@ -4337,7 +4337,12 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 //
                 // #41425 -- label the implicit `()` as being the
                 // "found type" here, rather than the "expected type".
-                if !self.diverges.get().always() {
+                //
+                // #44579 -- if the block was recovered during parsing,
+                // the type would be nonsensical and it is not worth it
+                // to perform the type check, so we avoid generating the
+                // diagnostic output.
+                if !self.diverges.get().always() && !blk.recovered {
                     coerce.coerce_forced_unit(self, &self.misc(blk.span), &mut |err| {
                         if let Some(expected_ty) = expected.only_has_type(self) {
                             self.consider_hint_about_removing_semicolon(blk,

src/libsyntax/ast.rs

@@ -468,6 +468,7 @@ pub struct Block {
     /// Distinguishes between `unsafe { ... }` and `{ ... }`
     pub rules: BlockCheckMode,
     pub span: Span,
+    pub recovered: bool,
 }
 
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash)]

src/libsyntax/ext/build.rs

@@ -594,6 +594,7 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
            id: ast::DUMMY_NODE_ID,
            rules: BlockCheckMode::Default,
            span,
+           recovered: false,
         })
     }
 

src/libsyntax/fold.rs

@@ -851,11 +851,12 @@ fn noop_fold_bounds<T: Folder>(bounds: TyParamBounds, folder: &mut T)
 }
 
 pub fn noop_fold_block<T: Folder>(b: P<Block>, folder: &mut T) -> P<Block> {
-    b.map(|Block {id, stmts, rules, span}| Block {
+    b.map(|Block {id, stmts, rules, span, recovered}| Block {
         id: folder.new_id(id),
         stmts: stmts.move_flat_map(|s| folder.fold_stmt(s).into_iter()),
         rules,
         span: folder.new_span(span),
+        recovered,
     })
 }
 

src/libsyntax/parse/mod.rs

@@ -924,6 +924,7 @@ mod tests {
                                         id: ast::DUMMY_NODE_ID,
                                         rules: ast::BlockCheckMode::Default, // no idea
                                         span: sp(15,21),
+                                        recovered: false,
                                     })),
                             vis: ast::Visibility::Inherited,
                             span: sp(0,21)})));