Implement new panic!() behaviour for Rust 2021.

compiler/rustc_builtin_macros/src/panic.rs

@@ -0,0 +1,48 @@
+use rustc_ast::ptr::P;
+use rustc_ast::tokenstream::{DelimSpan, TokenStream};
+use rustc_ast::*;
+use rustc_expand::base::*;
+use rustc_span::symbol::sym;
+use rustc_span::Span;
+
+// This expands to either
+// - `$crate::panic::panic_2015!(...)` or
+// - `$crate::panic::panic_2021!(...)`
+// depending on the edition.
+//
+// This is used for both std::panic!() and core::panic!().
+//
+// `$crate` will refer to either the `std` or `core` crate depending on which
+// one we're expanding from.
+pub fn expand_panic<'cx>(
+    cx: &'cx mut ExtCtxt<'_>,
+    sp: Span,
+    tts: TokenStream,
+) -> Box<dyn MacResult + 'cx> {
+    let panic = if sp.rust_2021() { sym::panic_2021 } else { sym::panic_2015 };
+
+    let sp = cx.with_call_site_ctxt(sp);
+
+    MacEager::expr(
+        cx.expr(
+            sp,
+            ExprKind::MacCall(MacCall {
+                path: Path {
+                    span: sp,
+                    segments: cx
+                        .std_path(&[sym::panic, panic])
+                        .into_iter()
+                        .map(|ident| PathSegment::from_ident(ident))
+                        .collect(),
+                    tokens: None,
+                },
+                args: P(MacArgs::Delimited(
+                    DelimSpan::from_single(sp),
+                    MacDelimiter::Parenthesis,
+                    tts,
+                )),
+                prior_type_ascription: None,
+            }),
+        ),
+    )
+}