[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/librustc/error_codes.rs

@@ -1640,6 +1640,16 @@ each method; it is not possible to annotate the entire impl with an `#[inline]`
 attribute.
 "##,
 
+E0900: r##"
+TODO: change error number
+TODO: track_caller: invalid syntax
+"##,
+
+E0901: r##"
+TODO: change error number
+TODO: track_caller: no naked functions
+"##,
+
 E0522: r##"
 The lang attribute is intended for marking special items that are built-in to
 Rust itself. This includes special traits (like `Copy` and `Sized`) that affect

src/librustc/hir/check_attr.rs

@@ -94,6 +94,7 @@ impl CheckAttrVisitor<'tcx> {
     /// Checks any attribute.
     fn check_attributes(&self, item: &hir::Item, target: Target) {
         let mut is_valid = true;
+        let mut track_caller_span = None;
         for attr in &item.attrs {
             is_valid &= if attr.check_name(sym::inline) {
                 self.check_inline(attr, &item.span, target)
@@ -103,6 +104,9 @@ impl CheckAttrVisitor<'tcx> {
                 self.check_marker(attr, item, target)
             } else if attr.check_name(sym::target_feature) {
                 self.check_target_feature(attr, item, target)
+            } else if attr.check_name(sym::track_caller) {
+                track_caller_span = Some(attr.span);
+                self.check_track_caller(attr, &item, target)
             } else {
                 true
             };
@@ -118,6 +122,19 @@ impl CheckAttrVisitor<'tcx> {
 
         self.check_repr(item, target);
         self.check_used(item, target);
+
+        // Checks if `#[track_caller]` and `#[naked]` are both used.
+        if let Some(span) = track_caller_span {
+            if item.attrs.iter().any(|attr| attr.check_name(sym::naked)) {
+                struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0901,
+                    "cannot use `#[track_caller]` with `#[naked]`",
+                )
+                .emit();
+            }
+        }
     }
 
     /// Checks if an `#[inline]` is applied to a function or a closure. Returns `true` if valid.
@@ -135,6 +152,23 @@ impl CheckAttrVisitor<'tcx> {
         }
     }
 
+    /// Checks if a `#[target_feature]` can be applied.
+    fn check_track_caller(&self, attr: &hir::Attribute, item: &hir::Item, target: Target) -> bool {
+        if target != Target::Fn {
+            struct_span_err!(
+                self.tcx.sess,
+                attr.span,
+                E0900,
+                "attribute should be applied to function"
+            )
+            .span_label(item.span, "not a function")
+            .emit();
+            false
+        } else {
+            true
+        }
+    }
+
     /// Checks if the `#[non_exhaustive]` attribute on an `item` is valid. Returns `true` if valid.
     fn check_non_exhaustive(
         &self,

src/librustc/hir/mod.rs

@@ -2721,7 +2721,9 @@ bitflags! {
         const USED                      = 1 << 9;
         /// #[ffi_returns_twice], indicates that an extern function can return
         /// multiple times
-        const FFI_RETURNS_TWICE = 1 << 10;
+        const FFI_RETURNS_TWICE         = 1 << 10;
+        /// #[track_caller]: allow access to the caller location
+        const TRACK_CALLER              = 1 << 11;
     }
 }