Add basic checks for well-formedness of fn/fn_mut lang items

src/test/ui/lang-items/fn-fn_mut-call-ill-formed.rs

@@ -0,0 +1,27 @@
+// Make sure that an error is reported if the `call` function of the
+// `fn`/`fn_mut` lang item is grossly ill-formed.
+
+#![feature(lang_items)]
+#![feature(no_core)]
+#![no_core]
+
+#[lang = "fn"]
+trait MyFn<T> {
+    const call: i32 = 42;
+    //~^ ERROR: `call` trait item in `fn`/`fn_mut` lang item must be a function
+}
+
+#[lang = "fn_mut"]
+trait MyFnMut<T> {
+    fn call(i: i32, j: i32) -> i32 { i + j }
+    //~^ ERROR: first argument of `call` in `fn`/`fn_mut` lang item must be a reference
+}
+
+fn main() {
+    let a = || 42;
+    a();
+
+    let mut i = 0;
+    let mut b = || { i += 1; };
+    b();
+}

src/test/ui/lang-items/fn-fn_mut-call-ill-formed.stderr

@@ -0,0 +1,14 @@
+error: `call` trait item in `fn`/`fn_mut` lang item must be a function
+  --> $DIR/fn-fn_mut-call-ill-formed.rs:10:5
+   |
+LL |     const call: i32 = 42;
+   |     ^^^^^^^^^^^^^^^^^^^^^
+
+error: first argument of `call` in `fn`/`fn_mut` lang item must be a reference
+  --> $DIR/fn-fn_mut-call-ill-formed.rs:16:16
+   |
+LL |     fn call(i: i32, j: i32) -> i32 { i + j }
+   |                ^^^
+
+error: aborting due to 2 previous errors
+

src/test/ui/lang-items/issue-83471.rs

@@ -0,0 +1,23 @@
+// Regression test for the ICE reported in issue #83471.
+
+#![crate_type="lib"]
+#![feature(no_core)]
+#![no_core]
+
+#[lang = "sized"]
+//~^ ERROR: language items are subject to change [E0658]
+trait Sized {}
+
+#[lang = "fn"]
+//~^ ERROR: language items are subject to change [E0658]
+//~| ERROR: `fn` language item must be applied to a trait with 1 generic argument
+trait Fn {
+    fn call(export_name);
+    //~^ ERROR: expected type
+    //~| WARNING: anonymous parameters are deprecated
+    //~| WARNING: this is accepted in the current edition
+}
+fn call_through_fn_trait() {
+    a()
+    //~^ ERROR: cannot find function
+}

src/test/ui/lang-items/issue-83471.stderr

@@ -0,0 +1,51 @@
+error[E0573]: expected type, found built-in attribute `export_name`
+  --> $DIR/issue-83471.rs:15:13
+   |
+LL |     fn call(export_name);
+   |             ^^^^^^^^^^^ not a type
+
+error[E0425]: cannot find function `a` in this scope
+  --> $DIR/issue-83471.rs:21:5
+   |
+LL |     a()
+   |     ^ not found in this scope
+
+error[E0658]: language items are subject to change
+  --> $DIR/issue-83471.rs:7:1
+   |
+LL | #[lang = "sized"]
+   | ^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(lang_items)]` to the crate attributes to enable
+
+error[E0658]: language items are subject to change
+  --> $DIR/issue-83471.rs:11:1
+   |
+LL | #[lang = "fn"]
+   | ^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(lang_items)]` to the crate attributes to enable
+
+warning: anonymous parameters are deprecated and will be removed in the next edition.
+  --> $DIR/issue-83471.rs:15:13
+   |
+LL |     fn call(export_name);
+   |             ^^^^^^^^^^^ help: try naming the parameter or explicitly ignoring it: `_: export_name`
+   |
+   = note: `#[warn(anonymous_parameters)]` on by default
+   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
+   = note: for more information, see issue #41686 <https://github.com/rust-lang/rust/issues/41686>
+
+error[E0718]: `fn` language item must be applied to a trait with 1 generic argument
+  --> $DIR/issue-83471.rs:11:1
+   |
+LL | #[lang = "fn"]
+   | ^^^^^^^^^^^^^^
+...
+LL | trait Fn {
+   |         - this trait has 0 generic arguments
+
+error: aborting due to 5 previous errors; 1 warning emitted
+
+Some errors have detailed explanations: E0425, E0573, E0658, E0718.
+For more information about an error, try `rustc --explain E0425`.