[RFC 2091] Add #[track_caller] attribute.

- The attribute is behind a feature gate.
- Error if both #[naked] and #[track_caller] are applied to the same function.
- Error if #[track_caller] is applied to a non-function item.
- Error if ABI is not "rust"
- Error if #[track_caller] is applied to a trait function.

Error codes and descriptions are pending.

src/test/ui/feature-gates/feature-gate-track_caller.rs

@@ -0,0 +1,6 @@
+
+#[track_caller]
+fn f() {}
+//~^^ ERROR the `#[track_caller]` attribute is an experimental feature
+
+fn main() {}

src/test/ui/feature-gates/feature-gate-track_caller.stderr

@@ -0,0 +1,12 @@
+error[E0658]: the `#[track_caller]` attribute is an experimental feature
+  --> $DIR/feature-gate-track_caller.rs:2:1
+   |
+LL | #[track_caller]
+   | ^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/47809
+   = help: add `#![feature(track_caller)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/rfc-2091-track-caller/error-odd-syntax.rs

@@ -0,0 +1,7 @@
+#![feature(track_caller)]
+
+#[track_caller(1)]
+fn f() {}
+//~^^ ERROR malformed `track_caller` attribute input
+
+fn main() {}

src/test/ui/rfc-2091-track-caller/error-odd-syntax.stderr

@@ -0,0 +1,8 @@
+error: malformed `track_caller` attribute input
+  --> $DIR/error-odd-syntax.rs:3:1
+   |
+LL | #[track_caller(1)]
+   | ^^^^^^^^^^^^^^^^^^ help: must be of the form: `#[track_caller]`
+
+error: aborting due to previous error
+

src/test/ui/rfc-2091-track-caller/error-with-invalid-abi.rs

@@ -0,0 +1,7 @@
+#![feature(track_caller)]
+
+#[track_caller]
+extern "C" fn f() {}
+//~^^ ERROR rust ABI is required to use `#[track_caller]`
+
+fn main() {}

src/test/ui/rfc-2091-track-caller/error-with-invalid-abi.stderr

@@ -0,0 +1,9 @@
+error[E0902]: rust ABI is required to use `#[track_caller]`
+  --> $DIR/error-with-invalid-abi.rs:3:1
+   |
+LL | #[track_caller]
+   | ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0902`.

src/test/ui/rfc-2091-track-caller/error-with-naked.rs

@@ -0,0 +1,8 @@
+#![feature(naked_functions, track_caller)]
+
+#[track_caller]
+#[naked]
+fn f() {}
+//~^^^ ERROR cannot use `#[track_caller]` with `#[naked]`
+
+fn main() {}

src/test/ui/rfc-2091-track-caller/error-with-naked.stderr

@@ -0,0 +1,9 @@
+error[E0901]: cannot use `#[track_caller]` with `#[naked]`
+  --> $DIR/error-with-naked.rs:3:1
+   |
+LL | #[track_caller]
+   | ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0901`.

src/test/ui/rfc-2091-track-caller/error-with-trait-fns.rs

@@ -0,0 +1,13 @@
+#![feature(track_caller)]
+
+trait Trait {
+    #[track_caller]
+    fn unwrap(&self);
+    //~^^ ERROR: `#[track_caller]` is not supported for trait items yet.
+}
+
+impl Trait for u64 {
+    fn unwrap(&self) {}
+}
+
+fn main() {}

src/test/ui/rfc-2091-track-caller/error-with-trait-fns.stderr

@@ -0,0 +1,9 @@
+error[E0903]: `#[track_caller]` is not supported for trait items yet.
+  --> $DIR/error-with-trait-fns.rs:4:5
+   |
+LL |     #[track_caller]
+   |     ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0903`.

src/test/ui/rfc-2091-track-caller/only-for-fns.rs

@@ -0,0 +1,7 @@
+#![feature(track_caller)]
+
+#[track_caller]
+struct S;
+//~^^ ERROR attribute should be applied to function
+
+fn main() {}

src/test/ui/rfc-2091-track-caller/only-for-fns.stderr

@@ -0,0 +1,11 @@
+error[E0900]: attribute should be applied to function
+  --> $DIR/only-for-fns.rs:3:1
+   |
+LL | #[track_caller]
+   | ^^^^^^^^^^^^^^^
+LL | struct S;
+   | --------- not a function
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0900`.