Expand assert!(expr) to panic() function instead of panic!() macro.

The panic message might contain braces which should never be
interpreted as format placeholders, which panic!() will do in a future
edition.

compiler/rustc_builtin_macros/src/assert.rs

@@ -1,8 +1,8 @@
 use rustc_errors::{Applicability, DiagnosticBuilder};
 
 use rustc_ast::ptr::P;
-use rustc_ast::token::{self, TokenKind};
-use rustc_ast::tokenstream::{DelimSpan, TokenStream, TokenTree};
+use rustc_ast::token;
+use rustc_ast::tokenstream::{DelimSpan, TokenStream};
 use rustc_ast::{self as ast, *};
 use rustc_ast_pretty::pprust;
 use rustc_expand::base::*;
@@ -26,31 +26,41 @@ pub fn expand_assert<'cx>(
     // `core::panic` and `std::panic` are different macros, so we use call-site
     // context to pick up whichever is currently in scope.
     let sp = cx.with_call_site_ctxt(sp);
-    let tokens = custom_message.unwrap_or_else(|| {
-        TokenStream::from(TokenTree::token(
-            TokenKind::lit(
-                token::Str,
-                Symbol::intern(&format!(
-                    "assertion failed: {}",
-                    pprust::expr_to_string(&cond_expr).escape_debug()
-                )),
-                None,
-            ),
-            DUMMY_SP,
-        ))
-    });
-    let args = P(MacArgs::Delimited(DelimSpan::from_single(sp), MacDelimiter::Parenthesis, tokens));
-    let panic_call = MacCall {
-        path: Path::from_ident(Ident::new(sym::panic, sp)),
-        args,
-        prior_type_ascription: None,
+
+    let panic_call = {
+        if let Some(tokens) = custom_message {
+            // Pass the custom message to panic!().
+            cx.expr(
+                sp,
+                ExprKind::MacCall(MacCall {
+                    path: Path::from_ident(Ident::new(sym::panic, sp)),
+                    args: P(MacArgs::Delimited(
+                        DelimSpan::from_single(sp),
+                        MacDelimiter::Parenthesis,
+                        tokens,
+                    )),
+                    prior_type_ascription: None,
+                }),
+            )
+        } else {
+            // Pass our own message directly to $crate::panicking::panic(),
+            // because it might contain `{` and `}` that should always be
+            // passed literally.
+            cx.expr_call_global(
+                sp,
+                cx.std_path(&[sym::panicking, sym::panic]),
+                vec![cx.expr_str(
+                    DUMMY_SP,
+                    Symbol::intern(&format!(
+                        "assertion failed: {}",
+                        pprust::expr_to_string(&cond_expr).escape_debug()
+                    )),
+                )],
+            )
+        }
     };
-    let if_expr = cx.expr_if(
-        sp,
-        cx.expr(sp, ExprKind::Unary(UnOp::Not, cond_expr)),
-        cx.expr(sp, ExprKind::MacCall(panic_call)),
-        None,
-    );
+    let if_expr =
+        cx.expr_if(sp, cx.expr(sp, ExprKind::Unary(UnOp::Not, cond_expr)), panic_call, None);
     MacEager::expr(if_expr)
 }
 

compiler/rustc_span/src/symbol.rs

@@ -791,6 +791,7 @@ symbols! {
         panic_runtime,
         panic_str,
         panic_unwind,
+        panicking,
         param_attrs,
         parent_trait,
         partial_cmp,

library/core/src/macros/mod.rs

@@ -1217,6 +1217,7 @@ pub(crate) mod builtin {
     #[rustc_builtin_macro]
     #[macro_export]
     #[rustc_diagnostic_item = "assert_macro"]
+    #[allow_internal_unstable(core_panic)]
     macro_rules! assert {
         ($cond:expr $(,)?) => {{ /* compiler built-in */ }};
         ($cond:expr, $($arg:tt)+) => {{ /* compiler built-in */ }};