Allow #[track_caller] in traits.

The codegen implementation already works for this, so we're:

* propagating track_caller attr from trait def to impl
* relaxing errors
* adding tests

Approved in a recent lang team meeting:
https://github.com/rust-lang/lang-team/blob/master/minutes/2020-01-09.md

src/librustc/lib.rs

@@ -38,6 +38,7 @@
 #![feature(extern_types)]
 #![feature(nll)]
 #![feature(option_expect_none)]
+#![feature(or_patterns)]
 #![feature(range_is_empty)]
 #![feature(specialization)]
 #![feature(trusted_len)]

src/librustc/ty/mod.rs

@@ -2875,8 +2875,8 @@ pub fn opt_associated_item(self, def_id: DefId) -> Option<AssocItem> {
                 _ => false,
             }
         } else {
-            match self.def_kind(def_id).expect("no def for `DefId`") {
-                DefKind::AssocConst | DefKind::AssocFn | DefKind::AssocTy => true,
+            match self.def_kind(def_id) {
+                Some(DefKind::AssocConst | DefKind::AssocFn | DefKind::AssocTy) => true,
                 _ => false,
             }
         };

src/librustc_error_codes/error_codes/E0738.md

@@ -1,48 +1,11 @@
-`#[track_caller]` cannot be used in traits yet. This is due to limitations in
-the compiler which are likely to be temporary. See [RFC 2091] for details on
-this and other restrictions.
+`#[track_caller]` cannot be used to annotate foreign functions.
 
-Erroneous example with a trait method implementation:
+Erroneous example:
 
 ```compile_fail,E0738
 #![feature(track_caller)]
-
-trait Foo {
-    fn bar(&self);
-}
-
-impl Foo for u64 {
+extern "Rust" {
     #[track_caller]
-    fn bar(&self) {}
+    fn bar();
 }
 ```
-
-Erroneous example with a blanket trait method implementation:
-
-```compile_fail,E0738
-#![feature(track_caller)]
-
-trait Foo {
-    #[track_caller]
-    fn bar(&self) {}
-    fn baz(&self);
-}
-```
-
-Erroneous example with a trait method declaration:
-
-```compile_fail,E0738
-#![feature(track_caller)]
-
-trait Foo {
-    fn bar(&self) {}
-
-    #[track_caller]
-    fn baz(&self);
-}
-```
-
-Note that while the compiler may be able to support the attribute in traits in
-the future, [RFC 2091] prohibits their implementation without a follow-up RFC.
-
-[RFC 2091]: https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md

src/librustc_passes/check_attr.rs

@@ -151,17 +151,17 @@ fn check_track_caller(
                 .emit();
                 false
             }
-            Target::Fn | Target::Method(MethodKind::Inherent) => true,
-            Target::Method(_) => {
+            Target::ForeignFn => {
                 struct_span_err!(
                     self.tcx.sess,
                     *attr_span,
                     E0738,
-                    "`#[track_caller]` may not be used on trait methods",
+                    "`#[track_caller]` is not supported on foreign functions",
                 )
                 .emit();
                 false
             }
+            Target::Fn | Target::Method(..) => true,
             _ => {
                 struct_span_err!(
                     self.tcx.sess,

src/librustc_typeck/collect.rs

@@ -2339,6 +2339,9 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, id: DefId) -> CodegenFnAttrs {
     let attrs = tcx.get_attrs(id);
 
     let mut codegen_fn_attrs = CodegenFnAttrs::new();
+    if should_inherit_track_caller(tcx, id) {
+        codegen_fn_attrs.flags |= CodegenFnAttrFlags::TRACK_CALLER;
+    }
 
     let whitelist = tcx.target_features_whitelist(LOCAL_CRATE);
 
@@ -2583,6 +2586,32 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, id: DefId) -> CodegenFnAttrs {
     codegen_fn_attrs
 }
 
+/// Checks if the provided DefId is a method in a trait impl for a trait which has track_caller
+/// applied to the method prototype.
+fn should_inherit_track_caller(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
+    if let Some(impl_item) = tcx.opt_associated_item(def_id) {
+        if let ty::AssocItemContainer::ImplContainer(impl_def_id) = impl_item.container {
+            if let Some(trait_def_id) = tcx.trait_id_of_impl(impl_def_id) {
+                if let Some(trait_item) = tcx
+                    .associated_items(trait_def_id)
+                    .filter_by_name_unhygienic(impl_item.ident.name)
+                    .find(move |trait_item| {
+                        trait_item.kind == ty::AssocKind::Method
+                            && tcx.hygienic_eq(impl_item.ident, trait_item.ident, trait_def_id)
+                    })
+                {
+                    return tcx
+                        .codegen_fn_attrs(trait_item.def_id)
+                        .flags
+                        .intersects(CodegenFnAttrFlags::TRACK_CALLER);
+                }
+            }
+        }
+    }
+
+    false
+}
+
 fn check_link_ordinal(tcx: TyCtxt<'_>, attr: &ast::Attribute) -> Option<usize> {
     use rustc_ast::ast::{Lit, LitIntType, LitKind};
     let meta_item_list = attr.meta_item_list();

src/test/ui/rfc-2091-track-caller/error-extern-fn.rs

@@ -0,0 +1,9 @@
+#![feature(track_caller)]
+#![allow(dead_code)]
+
+extern "Rust" {
+    #[track_caller] //~ ERROR: `#[track_caller]` is not supported on foreign functions
+    fn bar();
+}
+
+fn main() {}

src/test/ui/rfc-2091-track-caller/error-extern-fn.stderr

@@ -1,5 +1,5 @@
-error[E0738]: `#[track_caller]` may not be used on trait methods
-  --> $DIR/error-with-trait-decl.rs:4:5
+error[E0738]: `#[track_caller]` is not supported on foreign functions
+  --> $DIR/error-extern-fn.rs:5:5
    |
 LL |     #[track_caller]
    |     ^^^^^^^^^^^^^^^

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

@@ -1,12 +0,0 @@
-#![feature(track_caller)]
-
-trait Trait {
-    #[track_caller] //~ ERROR: `#[track_caller]` may not be used on trait methods
-    fn unwrap(&self);
-}
-
-impl Trait for u64 {
-    fn unwrap(&self) {}
-}
-
-fn main() {}

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

@@ -1,8 +0,0 @@
-#![feature(track_caller)]
-
-trait Trait {
-    #[track_caller] //~ ERROR: `#[track_caller]` may not be used on trait methods
-    fn unwrap(&self) {}
-}
-
-fn main() {}

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

@@ -1,9 +0,0 @@
-error[E0738]: `#[track_caller]` may not be used on trait methods
-  --> $DIR/error-with-trait-default-impl.rs:4:5
-   |
-LL |     #[track_caller]
-   |     ^^^^^^^^^^^^^^^
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0738`.

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

@@ -1,21 +0,0 @@
-// check-fail
-
-#![feature(track_caller)]
-
-trait Trait {
-    fn unwrap(&self);
-}
-
-impl Trait for u64 {
-    #[track_caller] //~ ERROR: `#[track_caller]` may not be used on trait methods
-    fn unwrap(&self) {}
-}
-
-struct S;
-
-impl S {
-    #[track_caller] // ok
-    fn foo() {}
-}
-
-fn main() {}

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

@@ -1,9 +0,0 @@
-error[E0738]: `#[track_caller]` may not be used on trait methods
-  --> $DIR/error-with-trait-fn-impl.rs:10:5
-   |
-LL |     #[track_caller]
-   |     ^^^^^^^^^^^^^^^
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0738`.

src/test/ui/rfc-2091-track-caller/tracked-fn-ptr-with-arg.rs

@@ -14,6 +14,49 @@ fn tracked_unit(_: ()) {
     assert_eq!(location.line(), expected_line, "call shims report location as fn definition");
 }
 
+trait Trait {
+    fn trait_tracked_unit(_: ());
+}
+
+impl Trait for () {
+    #[track_caller]
+    fn trait_tracked_unit(_: ()) {
+        let expected_line = line!() - 1;
+        let location = std::panic::Location::caller();
+        assert_eq!(location.file(), file!());
+        assert_eq!(location.line(), expected_line, "call shims report location as fn definition");
+    }
+}
+
+trait TrackedTrait {
+    #[track_caller]
+    fn trait_tracked_unit_default(_: ()) {
+        let expected_line = line!() - 1;
+        let location = std::panic::Location::caller();
+        assert_eq!(location.file(), file!());
+        assert_eq!(location.line(), expected_line, "call shims report location as fn definition");
+    }
+}
+
+impl TrackedTrait for () {}
+
+trait BlanketTrackedTrait {
+    #[track_caller]
+    fn tracked_blanket(_: ());
+}
+
+impl BlanketTrackedTrait for () {
+    fn tracked_blanket(_: ()) {
+        let expected_line = line!() - 1;
+        let location = std::panic::Location::caller();
+        assert_eq!(location.file(), file!());
+        assert_eq!(location.line(), expected_line, "call shims report location as fn definition");
+    }
+}
+
 fn main() {
     pass_to_ptr_call(tracked_unit, ());
+    pass_to_ptr_call(<() as Trait>::trait_tracked_unit, ());
+    pass_to_ptr_call(<() as TrackedTrait>::trait_tracked_unit_default, ());
+    pass_to_ptr_call(<() as BlanketTrackedTrait>::tracked_blanket, ());
 }

src/test/ui/rfc-2091-track-caller/tracked-fn-ptr.rs

@@ -14,6 +14,49 @@ fn tracked() {
     assert_eq!(location.line(), expected_line, "call shims report location as fn definition");
 }
 
+trait Trait {
+    fn trait_tracked();
+}
+
+impl Trait for () {
+    #[track_caller]
+    fn trait_tracked() {
+        let expected_line = line!() - 1;
+        let location = std::panic::Location::caller();
+        assert_eq!(location.file(), file!());
+        assert_eq!(location.line(), expected_line, "call shims report location as fn definition");
+    }
+}
+
+trait TrackedTrait {
+    #[track_caller]
+    fn trait_tracked_default() {
+        let expected_line = line!() - 1;
+        let location = std::panic::Location::caller();
+        assert_eq!(location.file(), file!());
+        assert_eq!(location.line(), expected_line, "call shims report location as fn definition");
+    }
+}
+
+impl TrackedTrait for () {}
+
+trait TraitBlanketTracked {
+    #[track_caller]
+    fn tracked_blanket();
+}
+
+impl TraitBlanketTracked for () {
+    fn tracked_blanket() {
+        let expected_line = line!() - 1;
+        let location = std::panic::Location::caller();
+        assert_eq!(location.file(), file!());
+        assert_eq!(location.line(), expected_line, "call shims report location as fn definition");
+    }
+}
+
 fn main() {
     ptr_call(tracked);
+    ptr_call(<() as Trait>::trait_tracked);
+    ptr_call(<() as TrackedTrait>::trait_tracked_default);
+    ptr_call(<() as TraitBlanketTracked>::tracked_blanket);
 }

src/test/ui/rfc-2091-track-caller/tracked-trait-impls.rs

@@ -0,0 +1,79 @@
+// run-pass
+
+#![feature(track_caller)]
+
+macro_rules! assert_expansion_site_is_tracked {
+    () => {{
+        let location = std::panic::Location::caller();
+        assert_eq!(location.file(), file!());
+        assert_ne!(location.line(), line!(), "line should be outside this fn");
+    }}
+}
+
+trait Tracked {
+    fn local_tracked(&self);
+
+    #[track_caller]
+    fn blanket_tracked(&self);
+
+    #[track_caller]
+    fn default_tracked(&self) {
+        assert_expansion_site_is_tracked!();
+    }
+}
+
+impl Tracked for () {
+    #[track_caller]
+    fn local_tracked(&self) {
+        assert_expansion_site_is_tracked!();
+    }
+
+    fn blanket_tracked(&self) {
+        assert_expansion_site_is_tracked!();
+    }
+}
+
+impl Tracked for bool {
+    #[track_caller]
+    fn local_tracked(&self) {
+        assert_expansion_site_is_tracked!();
+    }
+
+    fn blanket_tracked(&self) {
+        assert_expansion_site_is_tracked!();
+    }
+
+    fn default_tracked(&self) {
+        assert_expansion_site_is_tracked!();
+    }
+}
+
+impl Tracked for u8 {
+    #[track_caller]
+    fn local_tracked(&self) {
+        assert_expansion_site_is_tracked!();
+    }
+
+    fn blanket_tracked(&self) {
+        assert_expansion_site_is_tracked!();
+    }
+
+    #[track_caller]
+    fn default_tracked(&self) {
+        assert_expansion_site_is_tracked!();
+    }
+}
+
+fn main() {
+    ().local_tracked();
+    ().default_tracked();
+    ().blanket_tracked();
+
+    true.local_tracked();
+    true.default_tracked();
+    true.blanket_tracked();
+
+    0u8.local_tracked();
+    0u8.default_tracked();
+    0u8.blanket_tracked();
+}