Move track caller from pitfall to disallowed

rust-lang · phi-go · Apr 1, 2023 · Apr 2, 2023 · Apr 2, 2023 · Apr 3, 2023
commit 33c76cdc353a1af8f29d72696f771305862d2581
diff --git a/text/0000-explicit-tail-calls.md b/text/0000-explicit-tail-calls.md
@@ -342,25 +342,6 @@ fn bar(n: i32) {
 }
 ```
 
-### Pitfall 4: Caller location aka [`panic::Location::caller`](https://doc.rust-lang.org/std/panic/struct.Location.html#method.caller) and `#[track_caller]`
-
-Since `#[track_caller]` adds an argument, `#[track_caller]` functions can only tail call other `#[track_caller]` functions. So the following example will not compile.
-
-```rust
-use std::panic::Location;
-
-/// Returns the [`Location`] at which it is called.
-#[track_caller]
-fn get_caller_location() -> &'static Location<'static> {
-    Location::caller()
-}
-
-/// Returns a [`Location`] from within this function's definition.
-fn get_just_one_location() -> &'static Location<'static> {
-    become get_caller_location();
-}
-```
-
 # Reference-level explanation
 [reference-level-explanation]: #reference-level-explanation
 <!-- This is the technical portion of the RFC. Explain the design in sufficient detail that:
@@ -453,6 +434,27 @@ In this case, a naive author might assume that this is going to be a stack space
 
 Further confusion could result from the same-signature restriction where the Rust compiler raises an error since fibonacci and `<u64 as Add>::add` do not share a common signature.
 
+### Caller location ([`panic::Location::caller`](https://doc.rust-lang.org/std/panic/struct.Location.html#method.caller) and `#[track_caller]`) are not supported
+
+Since `#[track_caller]` adds an argument, `#[track_caller]` functions can only tail call other `#[track_caller]` functions. So the following example would not compile.
+However, removing `#[track_caller]` should not be a breaking change, as [it is not part of the stable API surface](https://github.com/rust-lang/rust/issues/87401#issuecomment-910628405) for Rust.
+For tail calls, however, removing an argument is a breaking change, so functions with `#[track_caller]` are **not** allowed to use `become`. ([Though, loosening this restriction might be possible.](https://github.com/rust-lang/rfcs/pull/3407/files/3304fee7542a56e3c671b8ce8a85070904cbf2ba#r2202286180)). Additionally, tail calling into a `#[track_caller]` function from a non-tracking function [might require growing the stack](https://github.com/rust-lang/rfcs/pull/3407/files/3304fee7542a56e3c671b8ce8a85070904cbf2ba#r2202286180), as this would defeat the purpose of tail calling, this is also **not** allowed.
+
+```rust
+use std::panic::Location;
+
+/// Returns the [`Location`] at which it is called.
+#[track_caller]
+fn get_caller_location() -> &'static Location<'static> {
+    Location::caller()
+}
+
+/// Returns a [`Location`] from within this function's definition.
+fn get_just_one_location() -> &'static Location<'static> {
+    become get_caller_location();
+}
+```
+
 # Drawbacks
 [drawbacks]: #drawbacks
 <!-- Why should we *not* do this? -->