Disable trivial_cast_lint inside format_args expansion.

Author: m-ou-se

compiler/rustc_builtin_macros/src/format.rs

@@ -610,11 +610,11 @@ impl<'a, 'b> Context<'a, 'b> {
         let string_length = self.ecx.expr_usize(sp, self.literal.len());
         let string = self.ecx.expr_str(sp, Symbol::intern(&self.literal));
         let string_as_ptr = self.ecx.expr_cast(
-            sp,
+            self.macsp,
             string,
             self.ecx.ty_ptr(
-                sp,
-                self.ecx.ty_path(self.ecx.path_ident(sp, Ident::new(sym::str, sp))),
+                self.macsp,
+                self.ecx.ty_path(self.ecx.path_ident(self.macsp, Ident::new(sym::str, self.macsp))),
                 Mutability::Not,
             ),
         );

compiler/rustc_typeck/src/check/cast.rs

@@ -541,6 +541,11 @@ impl<'a, 'tcx> CastCheck<'tcx> {
     }
 
     fn trivial_cast_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
+        if let Some(id) = self.span.ctxt().outer_expn_data().macro_def_id {
+            if fcx.tcx.is_diagnostic_item(sym::format_args, id) {
+                return;
+            }
+        }
         let t_cast = self.cast_ty;
         let t_expr = self.expr_ty;
         let type_asc_or =

library/core/src/macros/mod.rs

@@ -826,6 +826,7 @@ pub(crate) mod builtin {
     #[allow_internal_unstable(fmt_internals)]
     #[rustc_builtin_macro]
     #[macro_export]
+    #[rustc_diagnostic_item = "format_args"]
     macro_rules! format_args {
         ($fmt:expr) => {{ /* compiler built-in */ }};
         ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};