syntax: Apply recovery for casts to type ascription

Fix spans, add some comments

Author: petrochenkov

src/libsyntax/parse/parser.rs

@@ -1081,6 +1081,16 @@ impl<'a> Parser<'a> {
             None => token::CloseDelim(self.token_cursor.frame.delim),
         })
     }
+    fn look_ahead_span(&self, dist: usize) -> Span {
+        if dist == 0 {
+            return self.span
+        }
+
+        match self.token_cursor.frame.tree_cursor.look_ahead(dist - 1) {
+            Some(TokenTree::Token(span, _)) | Some(TokenTree::Delimited(span, _)) => span,
+            None => self.look_ahead_span(dist - 1),
+        }
+    }
     pub fn fatal(&self, m: &str) -> DiagnosticBuilder<'a> {
         self.sess.span_diagnostic.struct_span_fatal(self.span, m)
     }
@@ -2805,13 +2815,10 @@ impl<'a> Parser<'a> {
             }
             // Special cases:
             if op == AssocOp::As {
-                // Save the state of the parser before parsing type normally, in case there is a
-                // LessThan comparison after this cast.
-                lhs = self.parse_assoc_op_as(lhs, lhs_span)?;
+                lhs = self.parse_assoc_op_cast(lhs, lhs_span, ExprKind::Cast)?;
                 continue
             } else if op == AssocOp::Colon {
-                let rhs = self.parse_ty_no_plus()?;
-                lhs = self.mk_expr(lhs_span.to(rhs.span), ExprKind::Type(lhs, rhs), ThinVec::new());
+                lhs = self.parse_assoc_op_cast(lhs, lhs_span, ExprKind::Type)?;
                 continue
             } else if op == AssocOp::DotDot || op == AssocOp::DotDotDot {
                 // If we didn’t have to handle `x..`/`x...`, it would be pretty easy to
@@ -2905,61 +2912,61 @@ impl<'a> Parser<'a> {
         Ok(lhs)
     }
 
-    fn parse_assoc_op_as(&mut self, lhs: P<Expr>, lhs_span: Span) -> PResult<'a, P<Expr>> {
-        let rp = self.clone();
+    fn parse_assoc_op_cast(&mut self, lhs: P<Expr>, lhs_span: Span,
+                           expr_kind: fn(P<Expr>, P<Ty>) -> ExprKind)
+                           -> PResult<'a, P<Expr>> {
+        let mk_expr = |this: &mut Self, rhs: P<Ty>| {
+            this.mk_expr(lhs_span.to(rhs.span), expr_kind(lhs, rhs), ThinVec::new())
+        };
+
+        // Save the state of the parser before parsing type normally, in case there is a
+        // LessThan comparison after this cast.
+        let parser_snapshot_before_type = self.clone();
         match self.parse_ty_no_plus() {
             Ok(rhs) => {
-                Ok(self.mk_expr(lhs_span.to(rhs.span),
-                                ExprKind::Cast(lhs, rhs),
-                                ThinVec::new()))
+                Ok(mk_expr(self, rhs))
             }
-            Err(mut err) => {
-                let rp_err = self.clone();
-                let sp = rp_err.span.clone();
+            Err(mut type_err) => {
+                // Rewind to before attempting to parse the type with generics, to recover
+                // from situations like `x as usize < y` in which we first tried to parse
+                // `usize < y` as a type with generic arguments.
+                let parser_snapshot_after_type = self.clone();
+                mem::replace(self, parser_snapshot_before_type);
 
-                // Rewind to before attempting to parse the type with generics, to get
-                // arround #22644.
-                mem::replace(self, rp);
-                let lo = self.span;
                 match self.parse_path_without_generics(PathStyle::Type) {
                     Ok(path) => {
-                        // Successfully parsed the type leaving a `<` yet to parse
-                        err.cancel();
-                        let codemap = self.sess.codemap();
-                        let suggestion_span = lhs_span.to(self.prev_span);
-                        let warn_message = match codemap.span_to_snippet(self.prev_span) {
-                            Ok(lstring) => format!("`{}`", lstring),
-                            _ => "a type".to_string(),
-                        };
+                        // Successfully parsed the type path leaving a `<` yet to parse.
+                        type_err.cancel();
+
+                        // Report non-fatal diagnostics, keep `x as usize` as an expression
+                        // in AST and continue parsing.
                         let msg = format!("`<` is interpreted as a start of generic \
-                                           arguments for {}, not a comparison",
-                                          warn_message);
-                        let mut err = self.sess.span_diagnostic.struct_span_err(sp, &msg);
-                        err.span_label(sp, "interpreted as generic argument");
+                                           arguments for `{}`, not a comparison", path);
+                        let mut err = self.sess.span_diagnostic.struct_span_err(self.span, &msg);
+                        err.span_label(self.look_ahead_span(1).to(parser_snapshot_after_type.span),
+                                       "interpreted as generic arguments");
                         err.span_label(self.span, "not interpreted as comparison");
-                        let suggestion = match codemap.span_to_snippet(suggestion_span) {
-                            Ok(lstring) => format!("({})", lstring),
-                            _ => format!("(<expression> as <type>)")
-                        };
-                        err.span_suggestion(suggestion_span,
+
+                        let expr = mk_expr(self, P(Ty {
+                            span: path.span,
+                            node: TyKind::Path(None, path),
+                            id: ast::DUMMY_NODE_ID
+                        }));
+
+                        let expr_str = self.sess.codemap().span_to_snippet(expr.span)
+                                                .unwrap_or(pprust::expr_to_string(&expr));
+                        err.span_suggestion(expr.span,
                                             "if you want to compare the casted value then write:",
-                                            suggestion);
+                                            format!("({})", expr_str));
                         err.emit();
 
-                        let path = TyKind::Path(None, path);
-                        let span = lo.to(self.prev_span);
-                        let rhs = P(Ty { node: path, span: span, id: ast::DUMMY_NODE_ID });
-                        // Letting the parser accept the recovered type to avoid further errors,
-                        // but the code will still not compile due to the error emitted above.
-                        Ok(self.mk_expr(lhs_span.to(rhs.span),
-                                        ExprKind::Cast(lhs, rhs),
-                                        ThinVec::new()))
+                        Ok(expr)
                     }
                     Err(mut path_err) => {
-                        // Still couldn't parse, return original error and parser state
+                        // Couldn't parse as a path, return original error and parser state.
                         path_err.cancel();
-                        mem::replace(self, rp_err);
-                        Err(err)
+                        mem::replace(self, parser_snapshot_after_type);
+                        Err(type_err)
                     }
                 }
             }

src/test/ui/issue-22644.rs

@@ -10,10 +10,16 @@
 
 fn main() {
     let a : u32 = 0;
-    let b : usize = 0;
+    let long_name : usize = 0;
+
+    println!("{}", a as usize > long_name);
+    println!("{}", a as usize < long_name);
+    println!("{}{}", a as usize < long_name, long_name);
+    println!("{}", a as usize < 4);
+    println!("{}", a: usize > long_name);
+    println!("{}{}", a: usize < long_name, long_name);
+    println!("{}", a: usize < 4);
 
-    println!("{}", a as usize > b);
-    println!("{}", a as usize < b);
     println!("{}", a
                    as
                    usize
@@ -28,4 +34,6 @@ fn main() {
                    usize
                    <
                    5);
+
+    println!("{}", a: &mut 4);
 }

src/test/ui/issue-22644.stderr

@@ -1,48 +1,104 @@
 error: `<` is interpreted as a start of generic arguments for `usize`, not a comparison
-  --> $DIR/issue-22644.rs:16:33
+  --> $DIR/issue-22644.rs:16:31
    |
-16 |     println!("{}", a as usize < b);
-   |                               - ^ interpreted as generic argument
+16 |     println!("{}", a as usize < long_name);
+   |                               ^ --------- interpreted as generic arguments
    |                               |
    |                               not interpreted as comparison
    |
 help: if you want to compare the casted value then write:
    |
-16 |     println!("{}", (a as usize) < b);
+16 |     println!("{}", (a as usize) < long_name);
    |                    ^^^^^^^^^^^^
 
 error: `<` is interpreted as a start of generic arguments for `usize`, not a comparison
-  --> $DIR/issue-22644.rs:21:20
+  --> $DIR/issue-22644.rs:17:33
    |
-20 |                    <
-   |                    - not interpreted as comparison
-21 |                    4);
-   |                    ^ interpreted as generic argument
+17 |     println!("{}{}", a as usize < long_name, long_name);
+   |                                 ^ -------------------- interpreted as generic arguments
+   |                                 |
+   |                                 not interpreted as comparison
    |
 help: if you want to compare the casted value then write:
    |
-17 |     println!("{}", (a
-18 |                    as
-19 |                    usize)
+17 |     println!("{}{}", (a as usize) < long_name, long_name);
+   |                      ^^^^^^^^^^^^
+
+error: `<` is interpreted as a start of generic arguments for `usize`, not a comparison
+  --> $DIR/issue-22644.rs:18:31
+   |
+18 |     println!("{}", a as usize < 4);
+   |                               ^ - interpreted as generic arguments
+   |                               |
+   |                               not interpreted as comparison
+   |
+help: if you want to compare the casted value then write:
+   |
+18 |     println!("{}", (a as usize) < 4);
+   |                    ^^^^^^^^^^^^
+
+error: `<` is interpreted as a start of generic arguments for `usize`, not a comparison
+  --> $DIR/issue-22644.rs:20:31
+   |
+20 |     println!("{}{}", a: usize < long_name, long_name);
+   |                               ^ -------------------- interpreted as generic arguments
+   |                               |
+   |                               not interpreted as comparison
+   |
+help: if you want to compare the casted value then write:
+   |
+20 |     println!("{}{}", (a: usize) < long_name, long_name);
+   |                      ^^^^^^^^^^
+
+error: `<` is interpreted as a start of generic arguments for `usize`, not a comparison
+  --> $DIR/issue-22644.rs:21:29
    |
+21 |     println!("{}", a: usize < 4);
+   |                             ^ - interpreted as generic arguments
+   |                             |
+   |                             not interpreted as comparison
+   |
+help: if you want to compare the casted value then write:
+   |
+21 |     println!("{}", (a: usize) < 4);
+   |                    ^^^^^^^^^^
 
 error: `<` is interpreted as a start of generic arguments for `usize`, not a comparison
-  --> $DIR/issue-22644.rs:30:20
+  --> $DIR/issue-22644.rs:26:20
    |
-29 |                    <
-   |                    - not interpreted as comparison
-30 |                    5);
-   |                    ^ interpreted as generic argument
+26 |                    <
+   |                    ^ not interpreted as comparison
+27 |                    4);
+   |                    - interpreted as generic arguments
    |
 help: if you want to compare the casted value then write:
    |
-22 |     println!("{}", (a
-23 | 
-24 | 
-25 |                    as
-26 | 
-27 | 
+23 |     println!("{}", (a
+24 |                    as
+25 |                    usize)
+   |
+
+error: `<` is interpreted as a start of generic arguments for `usize`, not a comparison
+  --> $DIR/issue-22644.rs:35:20
+   |
+35 |                    <
+   |                    ^ not interpreted as comparison
+36 |                    5);
+   |                    - interpreted as generic arguments
+   |
+help: if you want to compare the casted value then write:
+   |
+28 |     println!("{}", (a
+29 | 
+30 | 
+31 |                    as
+32 | 
+33 | 
  ...
 
-error: aborting due to 3 previous errors
+error: expected type, found `4`
+  --> $DIR/issue-22644.rs:38:28
+   |
+38 |     println!("{}", a: &mut 4);
+   |                            ^
 

src/test/ui/issue-42954.rs

@@ -10,7 +10,7 @@
 
 macro_rules! is_plainly_printable {
     ($i: ident) => {
-      $i as u32 < 0
+        $i as u32 < 0
     };
 }
 

src/test/ui/issue-42954.stderr

@@ -1,13 +1,18 @@
 error: `<` is interpreted as a start of generic arguments for `u32`, not a comparison
   --> $DIR/issue-42954.rs:13:19
    |
-13 |       $i as u32 < 0
-   |                 - ^ interpreted as generic argument
-   |                 |
-   |                 not interpreted as comparison
+13 |         $i as u32 < 0
+   |                   ^ - interpreted as generic arguments
+   |                   |
+   |                   not interpreted as comparison
+...
+19 |     is_plainly_printable!(c);
+   |     ------------------------- in this macro invocation
    |
 help: if you want to compare the casted value then write:
-   |       ($i as u32) < 0
+   |
+13 |         ($i as u32) < 0
+   |         ^^^^^^^^^^^
 
 error: aborting due to previous error