Rollup merge of #154363 - aerooneqq:nested-delegations, r=petrochenkov

delegation: fix zero-args nested delegation ICE

This PR fixes an ICE when during lowering of nested delegation we need to access information about its parent, who is also inside body of another delegation. As a fix we lower delegation body even if there are no arguments in signature function, in this case we will see an error `this function takes 0 arguments but 1 argument was supplied`. Fixes rust-lang/rust#154332. Part of rust-lang/rust#118212.

r? @petrochenkov

compiler/rustc_ast_lowering/src/delegation.rs

@@ -432,6 +432,17 @@ fn lower_delegation_body(
                 args.push(arg);
             }
 
+            // If we have no params in signature function but user still wrote some code in
+            // delegation body, then add this code as first arg, eventually an error will be shown,
+            // also nested delegations may need to access information about this code (#154332),
+            // so it is better to leave this code as opposed to bodies of extern functions,
+            // which are completely erased from existence.
+            if param_count == 0
+                && let Some(block) = block
+            {
+                args.push(this.lower_target_expr(&block));
+            }
+
             let final_expr = this.finalize_body_lowering(delegation, args, generics, span);
 
             (this.arena.alloc_from_iter(parameters), final_expr)

tests/ui/delegation/explicit-paths-pass.rs

@@ -17,11 +17,8 @@ impl Trait for F {}
 
 mod to_reuse {
     pub fn foo(x: i32) -> i32 { x + 1 }
-    pub fn zero_args() -> i32 { 15 }
 }
 
-reuse to_reuse::zero_args { self }
-
 struct S(F);
 impl Trait for S {
     reuse Trait::bar { self.0 }
@@ -49,5 +46,4 @@ fn main() {
     #[inline]
     reuse to_reuse::foo;
     assert_eq!(43, foo(42));
-    assert_eq!(15, zero_args());
 }

tests/ui/delegation/inner-attr.rs

@@ -4,5 +4,6 @@
 fn a() {}
 
 reuse a as b { #![rustc_dummy] self } //~ ERROR an inner attribute is not permitted in this context
+//~^ ERROR: this function takes 0 arguments but 1 argument was supplied
 
 fn main() {}

tests/ui/delegation/inner-attr.stderr

@@ -3,11 +3,29 @@ error: an inner attribute is not permitted in this context
    |
 LL | reuse a as b { #![rustc_dummy] self }
    |                ^^^^^^^^^^^^^^^
-LL |
+...
 LL | fn main() {}
    | ------------ the inner attribute doesn't annotate this function
    |
    = note: inner attributes, like `#![no_std]`, annotate the item enclosing them, and are usually found at the beginning of source files
 
-error: aborting due to 1 previous error
+error[E0061]: this function takes 0 arguments but 1 argument was supplied
+  --> $DIR/inner-attr.rs:6:7
+   |
+LL | reuse a as b { #![rustc_dummy] self }
+   |       ^                        ---- unexpected argument
+   |
+note: function defined here
+  --> $DIR/inner-attr.rs:4:4
+   |
+LL | fn a() {}
+   |    ^
+help: remove the extra argument
+   |
+LL - reuse a as b { #![rustc_dummy] self }
+LL + reuse self }
+   |
 
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0061`.

tests/ui/delegation/zero-args-delegations-ice-154332.rs

@@ -0,0 +1,30 @@
+#![feature(fn_delegation)]
+#![allow(incomplete_features)]
+
+mod test_ice {
+    fn a() {}
+
+    reuse a as b { //~ ERROR: this function takes 0 arguments but 1 argument was supplied
+        let closure = || {
+            fn foo<'a, 'b, T: Clone, const N: usize, U: Clone>(_t: &'a T, _u: &'b U) {}
+
+            reuse foo::<String, 1, String> as bar;
+            bar(&"".to_string(), &"".to_string());
+        };
+
+        closure();
+    }
+}
+
+mod test_2 {
+    mod to_reuse {
+        pub fn zero_args() -> i32 {
+            15
+        }
+    }
+
+    reuse to_reuse::zero_args { self }
+    //~^ ERROR: this function takes 0 arguments but 1 argument was supplied
+}
+
+fn main() {}

tests/ui/delegation/zero-args-delegations-ice-154332.stderr

@@ -0,0 +1,43 @@
+error[E0061]: this function takes 0 arguments but 1 argument was supplied
+  --> $DIR/zero-args-delegations-ice-154332.rs:7:11
+   |
+LL |       reuse a as b {
+   |  ___________^______-
+LL | |         let closure = || {
+LL | |             fn foo<'a, 'b, T: Clone, const N: usize, U: Clone>(_t: &'a T, _u: &'b U) {}
+...  |
+LL | |         closure();
+LL | |     }
+   | |_____- unexpected argument of type `()`
+   |
+note: function defined here
+  --> $DIR/zero-args-delegations-ice-154332.rs:5:8
+   |
+LL |     fn a() {}
+   |        ^
+help: remove the extra argument
+   |
+LL -     reuse a as b {
+LL +     reuse {
+   |
+
+error[E0061]: this function takes 0 arguments but 1 argument was supplied
+  --> $DIR/zero-args-delegations-ice-154332.rs:26:21
+   |
+LL |     reuse to_reuse::zero_args { self }
+   |                     ^^^^^^^^^   ---- unexpected argument
+   |
+note: function defined here
+  --> $DIR/zero-args-delegations-ice-154332.rs:21:16
+   |
+LL |         pub fn zero_args() -> i32 {
+   |                ^^^^^^^^^
+help: remove the extra argument
+   |
+LL -     reuse to_reuse::zero_args { self }
+LL +     reuse to_reuse::zero_argself }
+   |
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0061`.