Parse & reject postfix operators after casts

This adds parsing for expressions like 'x as Ty[0]' which will
immediately error out, but still give the rest of the parser a valid
parse tree to continue.

src/librustc_parse/parser/expr.rs

@@ -551,7 +551,7 @@ impl<'a> Parser<'a> {
         // 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() {
+        let type_result = match self.parse_ty_no_plus() {
             Ok(rhs) => Ok(mk_expr(self, rhs)),
             Err(mut type_err) => {
                 // Rewind to before attempting to parse the type with generics, to recover
@@ -616,7 +616,44 @@ impl<'a> Parser<'a> {
                     }
                 }
             }
-        }
+        };
+
+        // Disallow postfix operators such as `.`, `?` or index (`[]`) after casts.
+        // Parses the postfix operator and emits an error.
+        let expr = type_result?;
+        let span = expr.span;
+
+        // The resulting parse tree for `&x as T[0]` has a precedence of `((&x) as T)[0]`.
+        let with_postfix = self.parse_dot_or_call_expr_with_(expr, span)?;
+        if !matches!(with_postfix.kind, ExprKind::Cast(_, _)) {
+            let expr_str = self.span_to_snippet(span);
+
+            let msg = format!(
+                "casts followed by {} are not supported",
+                match with_postfix.kind {
+                    ExprKind::Index(_, _) => "index operators",
+                    ExprKind::Try(_) => "try operators",
+                    ExprKind::Field(_, _) => "field access expressions",
+                    ExprKind::MethodCall(_, _) => "method call expressions",
+                    ExprKind::Await(_) => "awaits",
+                    _ => "expressions",
+                }
+            );
+            let mut err = self.struct_span_err(with_postfix.span, &msg);
+            let suggestion = "try surrounding the expression with parentheses";
+            if let Ok(expr_str) = expr_str {
+                err.span_suggestion(
+                    span,
+                    suggestion,
+                    format!("({})", expr_str),
+                    Applicability::MachineApplicable,
+                )
+            } else {
+                err.span_help(span, suggestion)
+            }
+            .emit();
+        };
+        Ok(with_postfix)
     }
 
     fn parse_assoc_op_ascribe(&mut self, lhs: P<Expr>, lhs_span: Span) -> PResult<'a, P<Expr>> {

src/test/ui/parser/issue-35813-postfix-after-cast.rs

@@ -0,0 +1,63 @@
+// edition:2018
+#![crate_type = "lib"]
+use std::future::Future;
+use std::pin::Pin;
+
+// This tests the parser for "x as Y[z]". It errors, but we want to give useful
+// errors and parse such that further code gives useful errors.
+pub fn index_after_as_cast() {
+    vec![1, 2, 3] as Vec<i32>[0];
+    //~^ ERROR: casts followed by index operators are not supported
+}
+
+pub fn index_after_cast_to_index() {
+    (&[0]) as &[i32][0];
+    //~^ ERROR: casts followed by index operators are not supported
+}
+
+// this tests that the precedence for `!x as Y.Z` is still what we expect
+pub fn precedence() {
+    let x: i32 = &vec![1, 2, 3] as &Vec<i32>[0];
+    //~^ ERROR: casts followed by index operators are not supported
+}
+
+pub fn complex() {
+    let _ = format!(
+        "{}",
+        if true { 33 } else { 44 } as i32.max(0)
+        //~^ ERROR: casts followed by method call expressions are not supported
+    );
+}
+
+pub fn in_condition() {
+    if 5u64 as i32.max(0) == 0 {
+        //~^ ERROR: casts followed by method call expressions are not supported
+    }
+}
+
+pub fn inside_block() {
+    let _ = if true {
+        5u64 as u32.max(0) == 0
+        //~^ ERROR: casts followed by method call expressions are not supported
+    } else { false };
+}
+
+static bar: &[i32] = &(&[1,2,3] as &[i32][0..1]);
+//~^ ERROR: casts followed by index operators are not supported
+
+pub async fn cast_then_await() {
+    Box::pin(noop()) as Pin<Box<dyn Future<Output = ()>>>.await;
+    //~^ ERROR: casts followed by awaits are not supported
+}
+
+pub async fn noop() {}
+
+#[derive(Default)]
+pub struct Foo {
+    pub bar: u32,
+}
+
+pub fn struct_field() {
+    Foo::default() as Foo.bar;
+    //~^ ERROR: casts followed by field access expressions are not supported
+}

src/test/ui/parser/issue-35813-postfix-after-cast.stderr

@@ -0,0 +1,74 @@
+error: casts followed by index operators are not supported
+  --> $DIR/issue-35813-postfix-after-cast.rs:9:5
+   |
+LL |     vec![1, 2, 3] as Vec<i32>[0];
+   |     -------------------------^^^
+   |     |
+   |     help: try surrounding the expression with parentheses: `(vec![1, 2, 3] as Vec<i32>)`
+
+error: casts followed by index operators are not supported
+  --> $DIR/issue-35813-postfix-after-cast.rs:14:5
+   |
+LL |     (&[0]) as &[i32][0];
+   |     ----------------^^^
+   |     |
+   |     help: try surrounding the expression with parentheses: `((&[0]) as &[i32])`
+
+error: casts followed by index operators are not supported
+  --> $DIR/issue-35813-postfix-after-cast.rs:20:18
+   |
+LL |     let x: i32 = &vec![1, 2, 3] as &Vec<i32>[0];
+   |                  ---------------------------^^^
+   |                  |
+   |                  help: try surrounding the expression with parentheses: `(&vec![1, 2, 3] as &Vec<i32>)`
+
+error: casts followed by method call expressions are not supported
+  --> $DIR/issue-35813-postfix-after-cast.rs:33:8
+   |
+LL |     if 5u64 as i32.max(0) == 0 {
+   |        -----------^^^^^^^
+   |        |
+   |        help: try surrounding the expression with parentheses: `(5u64 as i32)`
+
+error: casts followed by method call expressions are not supported
+  --> $DIR/issue-35813-postfix-after-cast.rs:40:9
+   |
+LL |         5u64 as u32.max(0) == 0
+   |         -----------^^^^^^^
+   |         |
+   |         help: try surrounding the expression with parentheses: `(5u64 as u32)`
+
+error: casts followed by index operators are not supported
+  --> $DIR/issue-35813-postfix-after-cast.rs:45:24
+   |
+LL | static bar: &[i32] = &(&[1,2,3] as &[i32][0..1]);
+   |                        ------------------^^^^^^
+   |                        |
+   |                        help: try surrounding the expression with parentheses: `(&[1,2,3] as &[i32])`
+
+error: casts followed by awaits are not supported
+  --> $DIR/issue-35813-postfix-after-cast.rs:49:5
+   |
+LL |     Box::pin(noop()) as Pin<Box<dyn Future<Output = ()>>>.await;
+   |     -----------------------------------------------------^^^^^^
+   |     |
+   |     help: try surrounding the expression with parentheses: `(Box::pin(noop()) as Pin<Box<dyn Future<Output = ()>>>)`
+
+error: casts followed by field access expressions are not supported
+  --> $DIR/issue-35813-postfix-after-cast.rs:61:5
+   |
+LL |     Foo::default() as Foo.bar;
+   |     ---------------------^^^^
+   |     |
+   |     help: try surrounding the expression with parentheses: `(Foo::default() as Foo)`
+
+error: casts followed by method call expressions are not supported
+  --> $DIR/issue-35813-postfix-after-cast.rs:27:9
+   |
+LL |         if true { 33 } else { 44 } as i32.max(0)
+   |         ---------------------------------^^^^^^^
+   |         |
+   |         help: try surrounding the expression with parentheses: `(if true { 33 } else { 44 } as i32)`
+
+error: aborting due to 9 previous errors
+