2 files changed, 125 insertions, 0 deletions
diff --git a/src/libsyntax_ext/assert.rs b/src/libsyntax_ext/assert.rs
new file mode 100644
index 00000000000..8b29e6adeb9
--- /dev/null
+++ b/src/libsyntax_ext/assert.rs
@@ -0,0 +1,122 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use syntax::ast::*;
+use syntax::codemap::Spanned;
+use syntax::ext::base::*;
+use syntax::ext::build::AstBuilder;
+use syntax::parse::token;
+use syntax::print::pprust;
+use syntax::tokenstream::{TokenStream, TokenTree};
+use syntax_pos::{Span, DUMMY_SP};
+
+pub fn expand_assert<'cx>(
+    cx: &'cx mut ExtCtxt,
+    sp: Span,
+    tts: &[TokenTree],
+) -> Box<MacResult + 'cx> {
+    let mut parser = cx.new_parser_from_tts(tts);
+    let cond_expr = panictry!(parser.parse_expr());
+    let custom_msg_args = if parser.eat(&token::Comma) {
+        let ts = parser.parse_tokens();
+        if !ts.is_empty() {
+            Some(ts)
+        } else {
+            None
+        }
+    } else {
+        None
+    };
+
+    let sp = sp.with_ctxt(sp.ctxt().apply_mark(cx.current_expansion.mark));
+    let panic_call = Mac_ {
+        path: Path::from_ident(sp, Ident::from_str("panic")),
+        tts: if let Some(ts) = custom_msg_args {
+            ts.into()
+        } else {
+            // `expr_to_string` escapes the string literals with `.escape_default()`
+            // which escapes all non-ASCII characters with `\u`.
+            let escaped_expr = escape_format_string(&unescape_printable_unicode(
+                &pprust::expr_to_string(&cond_expr),
+            ));
+
+            TokenStream::from(TokenTree::Token(
+                DUMMY_SP,
+                token::Literal(
+                    token::Lit::Str_(Name::intern(&format!("assertion failed: {}", escaped_expr))),
+                    None,
+                ),
+            )).into()
+        },
+    };
+    let if_expr = cx.expr_if(
+        sp,
+        cx.expr(sp, ExprKind::Unary(UnOp::Not, cond_expr)),
+        cx.expr(
+            sp,
+            ExprKind::Mac(Spanned {
+                span: sp,
+                node: panic_call,
+            }),
+        ),
+        None,
+    );
+    MacEager::expr(if_expr)
+}
+
+/// Escapes a string for use as a formatting string.
+fn escape_format_string(s: &str) -> String {
+    let mut res = String::with_capacity(s.len());
+    for c in s.chars() {
+        res.extend(c.escape_debug());
+        match c {
+            '{' | '}' => res.push(c),
+            _ => {}
+        }
+    }
+    res
+}
+
+#[test]
+fn test_escape_format_string() {
+    assert!(escape_format_string(r"foo{}\") == r"foo{{}}\\");
+}
+
+/// Unescapes the escaped unicodes (`\u{...}`) that are printable.
+fn unescape_printable_unicode(mut s: &str) -> String {
+    use std::{char, u32};
+
+    let mut res = String::with_capacity(s.len());
+
+    loop {
+        if let Some(start) = s.find(r"\u{") {
+            res.push_str(&s[0..start]);
+            s = &s[start..];
+            s.find('}')
+                .and_then(|end| {
+                    let v = u32::from_str_radix(&s[3..end], 16).ok()?;
+                    let c = char::from_u32(v)?;
+                    // Escape unprintable characters.
+                    res.extend(c.escape_debug());
+                    s = &s[end + 1..];
+                    Some(())
+                })
+                .expect("lexer should have rejected invalid escape sequences");
+        } else {
+            res.push_str(s);
+            return res;
+        }
+    }
+}
+
+#[test]
+fn test_unescape_printable_unicode() {
+    assert!(unescape_printable_unicode(r"\u{2603}\n\u{0}") == r"☃\n\u{0}");
+}
diff --git a/src/libsyntax_ext/lib.rs b/src/libsyntax_ext/lib.rs
index 5b078535852..249a64b353f 100644
--- a/src/libsyntax_ext/lib.rs
+++ b/src/libsyntax_ext/lib.rs
@@ -17,6 +17,7 @@
 
 #![feature(proc_macro_internals)]
 #![feature(decl_macro)]
+#![feature(str_escape)]
 
 extern crate fmt_macros;
 #[macro_use]
@@ -26,6 +27,7 @@ extern crate proc_macro;
 extern crate rustc_data_structures;
 extern crate rustc_errors as errors;
 
+mod assert;
 mod asm;
 mod cfg;
 mod compile_error;
@@ -112,6 +114,7 @@ pub fn register_builtins(resolver: &mut syntax::ext::base::Resolver,
         log_syntax: log_syntax::expand_syntax_ext,
         trace_macros: trace_macros::expand_trace_macros,
         compile_error: compile_error::expand_compile_error,
+        assert: assert::expand_assert,
     }
 
     // format_args uses `unstable` things internally.