WIP

Author: estebank

compiler/rustc_builtin_macros/src/assert.rs

@@ -103,13 +103,18 @@ struct Assert {
     custom_message: Option<TokenStream>,
 }
 
-/// `if !{ ... } { ... }`
+/// `match <cond> { true => {} _ => <then> }`
 fn expr_if_not(cx: &ExtCtxt<'_>, span: Span, cond: P<Expr>, then: P<Expr>) -> P<Expr> {
-    // Instead of expanding to `if !<cond> { <then> }`, we expand to `if <cond> {} else { <then> }`.
+    // Instead of expanding to `if !<cond> { <then> }`, we expand to
+    // `match <cond> { true => {} _ <then> }`.
     // This allows us to always complain about mismatched types instead of "cannot apply unary
-    // operator `!` to type `X`" when passing an invalid `<cond>`.
+    // operator `!` to type `X`" when passing an invalid `<cond>`, while also allowing `<cond>` to
+    // be `&true`.
     let els = cx.expr_block(cx.block(span, thin_vec![]));
-    cx.expr_if(span, cond, els, Some(then))
+    let mut arms = thin_vec![];
+    arms.push(cx.arm(span, cx.pat_lit(span, cx.expr_bool(span, true)), els));
+    arms.push(cx.arm(span, cx.pat_wild(span), then));
+    cx.expr_match(span, cond, arms)
 }
 
 fn parse_assert<'a>(cx: &ExtCtxt<'a>, sp: Span, stream: TokenStream) -> PResult<'a, Assert> {