Rollup merge of #45784 - harpocrates:fix/print-parens-cast-lt, r=kennytm

Pretty print parens around casts on the LHS of `<`/`<<`

When pretty printing a cast expression occuring on the LHS of a `<` or `<<` expression, we should add parens around the cast. Otherwise, the `<`/`<<` gets interpreted as the beginning of the generics for the type on the RHS of the cast.

Consider:

    $ cat parens_cast.rs
    macro_rules! negative {
        ($e:expr) => { $e < 0 }
    }

    fn main() {
        negative!(1 as i32);
    }

Before this PR, the output of the following is not valid Rust:

    $ rustc -Z unstable-options --pretty=expanded parens_cast.rs
    #![feature(prelude_import)]
    #![no_std]
    #[prelude_import]
    use std::prelude::v1::*;
    #[macro_use]
    extern crate std as std;
    macro_rules! negative(( $ e : expr ) => { $ e < 0 });

    fn main() { 1 as i32 < 0; }

After this PR, the output of the following is valid Rust:

    $ rustc -Z unstable-options --pretty=expanded parens_cast.rs
    #![feature(prelude_import)]
    #![no_std]
    #[prelude_import]
    use std::prelude::v1::*;
    #[macro_use]
    extern crate std as std;
    macro_rules! negative(( $ e : expr ) => { $ e < 0 });

    fn main() { (1 as i32) < 0; }

I've gone through several README/wiki style documents but I'm still not sure where to test this though. I'm not even sure if this sort of thing is tested...

1 files changed, 9 insertions, 0 deletions

diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 227db93cf65..5cb8e8694cf 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -1987,6 +1987,15 @@ impl<'a> State<'a> {
             Fixity::None => (prec + 1, prec + 1),
         };
 
+        let left_prec = match (&lhs.node, op.node) {
+            // These cases need parens: `x as i32 < y` has the parser thinking that `i32 < y` is
+            // the beginning of a path type. It starts trying to parse `x as (i32 < y ...` instead
+            // of `(x as i32) < ...`. We need to convince it _not_ to do that.
+            (&ast::ExprKind::Cast { .. }, ast::BinOpKind::Lt) |
+            (&ast::ExprKind::Cast { .. }, ast::BinOpKind::Shl) => parser::PREC_FORCE_PAREN,
+            _ => left_prec,
+        };
+
         self.print_expr_maybe_paren(lhs, left_prec)?;
         self.s.space()?;
         self.word_space(op.node.to_string())?;