Auto merge of #56999 - petrochenkov:macrecov2, r=estebank

AST/HIR: Introduce `ExprKind::Err` for better error recovery in the front-end

This way we can avoid aborting compilation if expansion produces errors and generate `ExprKind::Err`s instead.

10 files changed, 61 insertions, 47 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 7e228d8d7cf..e3a8980a975 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -444,7 +444,6 @@ pub struct Block {
     /// Distinguishes between `unsafe { ... }` and `{ ... }`
     pub rules: BlockCheckMode,
     pub span: Span,
-    pub recovered: bool,
 }
 
 #[derive(Clone, RustcEncodable, RustcDecodable)]
@@ -1001,6 +1000,7 @@ impl Expr {
             ExprKind::Paren(..) => ExprPrecedence::Paren,
             ExprKind::Try(..) => ExprPrecedence::Try,
             ExprKind::Yield(..) => ExprPrecedence::Yield,
+            ExprKind::Err => ExprPrecedence::Err,
         }
     }
 }
@@ -1160,6 +1160,9 @@ pub enum ExprKind {
 
     /// A `yield`, with an optional value to be yielded.
     Yield(Option<P<Expr>>),
+
+    /// Placeholder for an expression that wasn't syntactically well formed in some way.
+    Err,
 }
 
 /// The explicit `Self` type in a "qualified path". The actual
diff --git a/src/libsyntax/ext/base.rs b/src/libsyntax/ext/base.rs
index d576397c942..1efe0b3478d 100644
--- a/src/libsyntax/ext/base.rs
+++ b/src/libsyntax/ext/base.rs
@@ -2,7 +2,7 @@ pub use self::SyntaxExtension::*;
 
 use ast::{self, Attribute, Name, PatKind, MetaItem};
 use attr::HasAttrs;
-use source_map::{self, SourceMap, Spanned, respan};
+use source_map::{SourceMap, Spanned, respan};
 use syntax_pos::{Span, MultiSpan, DUMMY_SP};
 use edition::Edition;
 use errors::{DiagnosticBuilder, DiagnosticId};
@@ -456,7 +456,8 @@ impl MacResult for MacEager {
 #[derive(Copy, Clone)]
 pub struct DummyResult {
     expr_only: bool,
-    span: Span
+    is_error: bool,
+    span: Span,
 }
 
 impl DummyResult {
@@ -464,8 +465,13 @@ impl DummyResult {
     ///
     /// Use this as a return value after hitting any errors and
     /// calling `span_err`.
-    pub fn any(sp: Span) -> Box<dyn MacResult+'static> {
-        Box::new(DummyResult { expr_only: false, span: sp })
+    pub fn any(span: Span) -> Box<dyn MacResult+'static> {
+        Box::new(DummyResult { expr_only: false, is_error: true, span })
+    }
+
+    /// Same as `any`, but must be a valid fragment, not error.
+    pub fn any_valid(span: Span) -> Box<dyn MacResult+'static> {
+        Box::new(DummyResult { expr_only: false, is_error: false, span })
     }
 
     /// Create a default MacResult that can only be an expression.
@@ -473,15 +479,15 @@ impl DummyResult {
     /// Use this for macros that must expand to an expression, so even
     /// if an error is encountered internally, the user will receive
     /// an error that they also used it in the wrong place.
-    pub fn expr(sp: Span) -> Box<dyn MacResult+'static> {
-        Box::new(DummyResult { expr_only: true, span: sp })
+    pub fn expr(span: Span) -> Box<dyn MacResult+'static> {
+        Box::new(DummyResult { expr_only: true, is_error: true, span })
     }
 
     /// A plain dummy expression.
-    pub fn raw_expr(sp: Span) -> P<ast::Expr> {
+    pub fn raw_expr(sp: Span, is_error: bool) -> P<ast::Expr> {
         P(ast::Expr {
             id: ast::DUMMY_NODE_ID,
-            node: ast::ExprKind::Lit(source_map::respan(sp, ast::LitKind::Bool(false))),
+            node: if is_error { ast::ExprKind::Err } else { ast::ExprKind::Tup(Vec::new()) },
             span: sp,
             attrs: ThinVec::new(),
         })
@@ -496,10 +502,11 @@ impl DummyResult {
         }
     }
 
-    pub fn raw_ty(sp: Span) -> P<ast::Ty> {
+    /// A plain dummy type.
+    pub fn raw_ty(sp: Span, is_error: bool) -> P<ast::Ty> {
         P(ast::Ty {
             id: ast::DUMMY_NODE_ID,
-            node: ast::TyKind::Infer,
+            node: if is_error { ast::TyKind::Err } else { ast::TyKind::Tup(Vec::new()) },
             span: sp
         })
     }
@@ -507,7 +514,7 @@ impl DummyResult {
 
 impl MacResult for DummyResult {
     fn make_expr(self: Box<DummyResult>) -> Option<P<ast::Expr>> {
-        Some(DummyResult::raw_expr(self.span))
+        Some(DummyResult::raw_expr(self.span, self.is_error))
     }
 
     fn make_pat(self: Box<DummyResult>) -> Option<P<ast::Pat>> {
@@ -550,13 +557,13 @@ impl MacResult for DummyResult {
     fn make_stmts(self: Box<DummyResult>) -> Option<SmallVec<[ast::Stmt; 1]>> {
         Some(smallvec![ast::Stmt {
             id: ast::DUMMY_NODE_ID,
-            node: ast::StmtKind::Expr(DummyResult::raw_expr(self.span)),
+            node: ast::StmtKind::Expr(DummyResult::raw_expr(self.span, self.is_error)),
             span: self.span,
         }])
     }
 
     fn make_ty(self: Box<DummyResult>) -> Option<P<ast::Ty>> {
-        Some(DummyResult::raw_ty(self.span))
+        Some(DummyResult::raw_ty(self.span, self.is_error))
     }
 }
 
@@ -796,7 +803,6 @@ pub struct ExtCtxt<'a> {
     pub ecfg: expand::ExpansionConfig<'a>,
     pub root_path: PathBuf,
     pub resolver: &'a mut dyn Resolver,
-    pub resolve_err_count: usize,
     pub current_expansion: ExpansionData,
     pub expansions: FxHashMap<Span, Vec<String>>,
 }
@@ -811,7 +817,6 @@ impl<'a> ExtCtxt<'a> {
             ecfg,
             root_path: PathBuf::new(),
             resolver,
-            resolve_err_count: 0,
             current_expansion: ExpansionData {
                 mark: Mark::root(),
                 depth: 0,
diff --git a/src/libsyntax/ext/build.rs b/src/libsyntax/ext/build.rs
index 15faae53916..a8eec1a74dd 100644
--- a/src/libsyntax/ext/build.rs
+++ b/src/libsyntax/ext/build.rs
@@ -587,7 +587,6 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
            id: ast::DUMMY_NODE_ID,
            rules: BlockCheckMode::Default,
            span,
-           recovered: false,
         })
     }
 
diff --git a/src/libsyntax/ext/expand.rs b/src/libsyntax/ext/expand.rs
index 55012bb7f5a..3863778fe72 100644
--- a/src/libsyntax/ext/expand.rs
+++ b/src/libsyntax/ext/expand.rs
@@ -344,8 +344,10 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
             // FIXME(jseyfried): Refactor out the following logic
             let (expanded_fragment, new_invocations) = if let Some(ext) = ext {
                 if let Some(ext) = ext {
-                    let dummy = invoc.fragment_kind.dummy(invoc.span()).unwrap();
-                    let fragment = self.expand_invoc(invoc, &*ext).unwrap_or(dummy);
+                    let (invoc_fragment_kind, invoc_span) = (invoc.fragment_kind, invoc.span());
+                    let fragment = self.expand_invoc(invoc, &*ext).unwrap_or_else(|| {
+                        invoc_fragment_kind.dummy(invoc_span).unwrap()
+                    });
                     self.collect_invocations(fragment, &[])
                 } else if let InvocationKind::Attr { attr: None, traits, item, .. } = invoc.kind {
                     if !item.derive_allowed() {
@@ -431,9 +433,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
 
     fn resolve_imports(&mut self) {
         if self.monotonic {
-            let err_count = self.cx.parse_sess.span_diagnostic.err_count();
             self.cx.resolver.resolve_imports();
-            self.cx.resolve_err_count += self.cx.parse_sess.span_diagnostic.err_count() - err_count;
         }
     }
 
@@ -457,11 +457,9 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         };
 
         if self.monotonic {
-            let err_count = self.cx.parse_sess.span_diagnostic.err_count();
-            let mark = self.cx.current_expansion.mark;
-            self.cx.resolver.visit_ast_fragment_with_placeholders(mark, &fragment_with_placeholders,
-                                                                  derives);
-            self.cx.resolve_err_count += self.cx.parse_sess.span_diagnostic.err_count() - err_count;
+            self.cx.resolver.visit_ast_fragment_with_placeholders(
+                self.cx.current_expansion.mark, &fragment_with_placeholders, derives
+            );
         }
 
         (fragment_with_placeholders, invocations)
@@ -724,7 +722,6 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                     emit_feature_err(this.cx.parse_sess, &*feature.as_str(), span,
                                      GateIssue::Library(Some(issue)), &explain);
                     this.cx.trace_macros_diag();
-                    return Err(kind.dummy(span));
                 }
             }
 
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 62269a8f163..70df403d0c2 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -1272,16 +1272,15 @@ impl<'a> Context<'a> {
                 return;
             }
         }
-        if name.starts_with("rustc_") {
-            gate_feature!(self, rustc_attrs, attr.span,
-                          "unless otherwise specified, attributes \
-                           with the prefix `rustc_` \
-                           are reserved for internal compiler diagnostics");
-        } else if !attr::is_known(attr) {
-            // Only run the custom attribute lint during regular feature gate
-            // checking. Macro gating runs before the plugin attributes are
-            // registered, so we skip this in that case.
-            if !is_macro {
+        if !attr::is_known(attr) {
+            if name.starts_with("rustc_") {
+                let msg = "unless otherwise specified, attributes with the prefix `rustc_` \
+                           are reserved for internal compiler diagnostics";
+                gate_feature!(self, rustc_attrs, attr.span, msg);
+            } else if !is_macro {
+                // Only run the custom attribute lint during regular feature gate
+                // checking. Macro gating runs before the plugin attributes are
+                // registered, so we skip this in that case.
                 let msg = format!("The attribute `{}` is currently unknown to the compiler and \
                                    may have meaning added to it in the future", attr.path);
                 gate_feature!(self, custom_attribute, attr.span, &msg);
diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index 0abc74c2314..8ac103856dc 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -892,12 +892,11 @@ fn noop_fold_bounds<T: Folder>(bounds: GenericBounds, 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, recovered}| Block {
+    b.map(|Block {id, stmts, rules, span}| 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,
     })
 }
 
@@ -1367,6 +1366,7 @@ pub fn noop_fold_expr<T: Folder>(Expr {id, node, span, attrs}: Expr, folder: &mu
             ExprKind::Yield(ex) => ExprKind::Yield(ex.map(|x| folder.fold_expr(x))),
             ExprKind::Try(ex) => ExprKind::Try(folder.fold_expr(ex)),
             ExprKind::TryBlock(body) => ExprKind::TryBlock(folder.fold_block(body)),
+            ExprKind::Err => ExprKind::Err,
         },
         id: folder.new_id(id),
         span: folder.new_span(span),
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index e6f0d871222..52da8a072c7 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -32,6 +32,7 @@ use ast::{UseTree, UseTreeKind};
 use ast::{BinOpKind, UnOp};
 use ast::{RangeEnd, RangeSyntax};
 use {ast, attr};
+use ext::base::DummyResult;
 use source_map::{self, SourceMap, Spanned, respan};
 use syntax_pos::{self, Span, MultiSpan, BytePos, FileName};
 use errors::{self, Applicability, DiagnosticBuilder, DiagnosticId};
@@ -2870,6 +2871,7 @@ impl<'a> Parser<'a> {
                 let mut err = self.fatal(&format!("unknown macro variable `{}`", name));
                 err.span_label(self.span, "unknown macro variable");
                 err.emit();
+                self.bump();
                 return
             }
             token::Interpolated(ref nt) => {
@@ -4966,16 +4968,16 @@ impl<'a> Parser<'a> {
     /// Precondition: already parsed the '{'.
     fn parse_block_tail(&mut self, lo: Span, s: BlockCheckMode) -> PResult<'a, P<Block>> {
         let mut stmts = vec![];
-        let mut recovered = false;
-
         while !self.eat(&token::CloseDelim(token::Brace)) {
             let stmt = match self.parse_full_stmt(false) {
                 Err(mut err) => {
                     err.emit();
                     self.recover_stmt_(SemiColonMode::Ignore, BlockMode::Ignore);
-                    self.eat(&token::CloseDelim(token::Brace));
-                    recovered = true;
-                    break;
+                    Some(Stmt {
+                        id: ast::DUMMY_NODE_ID,
+                        node: StmtKind::Expr(DummyResult::raw_expr(self.span, true)),
+                        span: self.span,
+                    })
                 }
                 Ok(stmt) => stmt,
             };
@@ -4993,7 +4995,6 @@ impl<'a> Parser<'a> {
             id: ast::DUMMY_NODE_ID,
             rules: s,
             span: lo.to(self.prev_span),
-            recovered,
         }))
     }
 
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 931e91e9c79..2ad3d3a6d64 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -1093,7 +1093,9 @@ impl<'a> State<'a> {
                 self.s.word("_")?;
             }
             ast::TyKind::Err => {
-                self.s.word("?")?;
+                self.popen()?;
+                self.s.word("/*ERROR*/")?;
+                self.pclose()?;
             }
             ast::TyKind::ImplicitSelf => {
                 self.s.word("Self")?;
@@ -2391,6 +2393,11 @@ impl<'a> State<'a> {
                 self.s.space()?;
                 self.print_block_with_attrs(blk, attrs)?
             }
+            ast::ExprKind::Err => {
+                self.popen()?;
+                self.s.word("/*ERROR*/")?;
+                self.pclose()?
+            }
         }
         self.ann.post(self, AnnNode::Expr(expr))?;
         self.end()
diff --git a/src/libsyntax/util/parser.rs b/src/libsyntax/util/parser.rs
index 52390fa5b1d..89d4e53b8d1 100644
--- a/src/libsyntax/util/parser.rs
+++ b/src/libsyntax/util/parser.rs
@@ -267,6 +267,7 @@ pub enum ExprPrecedence {
     TryBlock,
     Struct,
     Async,
+    Err,
 }
 
 impl ExprPrecedence {
@@ -325,7 +326,8 @@ impl ExprPrecedence {
             ExprPrecedence::Block |
             ExprPrecedence::TryBlock |
             ExprPrecedence::Async |
-            ExprPrecedence::Struct => PREC_PAREN,
+            ExprPrecedence::Struct |
+            ExprPrecedence::Err => PREC_PAREN,
         }
     }
 }
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index 43f8a13609e..156546bbba9 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -802,6 +802,7 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
         ExprKind::TryBlock(ref body) => {
             visitor.visit_block(body)
         }
+        ExprKind::Err => {}
     }
 
     visitor.visit_expr_post(expression)