Add Location::file_cstr

This is useful for C/C++ APIs which expect the const char* returned
from __FILE__ or std::source_location::file_name.

Author: cramertj

compiler/rustc_const_eval/src/interpret/place.rs

@@ -1013,30 +1013,50 @@ where
         )
     }
 
-    /// Allocates a string in the interpreter's memory, returning it as a (wide) place.
-    /// This is allocated in immutable global memory and deduplicated.
-    pub fn allocate_str_dedup(
-        &mut self,
-        str: &str,
-    ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::Provenance>> {
-        let bytes = str.as_bytes();
-        let ptr = self.allocate_bytes_dedup(bytes)?;
-
+    /// Combines pointer and metadata into a wide pointer.
+    fn slice_from_ptr_and_len(
+        &self,
+        ptr: Pointer<M::Provenance>,
+        len: usize,
+        slice_ty: Ty<'tcx>,
+    ) -> MPlaceTy<'tcx, M::Provenance> {
         // Create length metadata for the string.
-        let meta = Scalar::from_target_usize(u64::try_from(bytes.len()).unwrap(), self);
+        let meta = Scalar::from_target_usize(u64::try_from(len).unwrap(), self);
 
         // Get layout for Rust's str type.
-        let layout = self.layout_of(self.tcx.types.str_).unwrap();
+        let layout = self.layout_of(slice_ty).unwrap();
 
-        // Combine pointer and metadata into a wide pointer.
-        interp_ok(self.ptr_with_meta_to_mplace(
+        self.ptr_with_meta_to_mplace(
             ptr.into(),
             MemPlaceMeta::Meta(meta),
             layout,
             /*unaligned*/ false,
+        )
+    }
+
+    pub fn allocate_byte_slice_dedup(
+        &mut self,
+        bytes: &[u8],
+    ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::Provenance>> {
+        let ptr = self.allocate_bytes_dedup(bytes)?;
+        interp_ok(self.slice_from_ptr_and_len(
+            ptr,
+            bytes.len(),
+            Ty::new_slice(*self.tcx, self.tcx.types.u8),
         ))
     }
 
+    /// Allocates a string in the interpreter's memory, returning it as a (wide) place.
+    /// This is allocated in immutable global memory and deduplicated.
+    pub fn allocate_str_dedup(
+        &mut self,
+        str: &str,
+    ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::Provenance>> {
+        let bytes = str.as_bytes();
+        let ptr = self.allocate_bytes_dedup(bytes)?;
+        interp_ok(self.slice_from_ptr_and_len(ptr, bytes.len(), self.tcx.types.str_))
+    }
+
     pub fn raw_const_to_mplace(
         &self,
         raw: mir::ConstAlloc<'tcx>,

compiler/rustc_const_eval/src/util/caller_location.rs

@@ -16,13 +16,19 @@ fn alloc_caller_location<'tcx>(
     line: u32,
     col: u32,
 ) -> MPlaceTy<'tcx> {
+    // Ensure that the filename itself does not contain nul bytes.
+    // This isn't possible via POSIX or Windows, but we should ensure no one
+    // ever does such a thing.
+    assert!(!filename.as_str().as_bytes().contains(&0));
+
     let loc_details = ecx.tcx.sess.opts.unstable_opts.location_detail;
     // This can fail if rustc runs out of memory right here. Trying to emit an error would be
     // pointless, since that would require allocating more memory than these short strings.
     let file = if loc_details.file {
-        ecx.allocate_str_dedup(filename.as_str()).unwrap()
+        let filename_with_nul = filename.as_str().to_owned() + "\0";
+        ecx.allocate_byte_slice_dedup(filename_with_nul.as_bytes()).unwrap()
     } else {
-        ecx.allocate_str_dedup("<redacted>").unwrap()
+        ecx.allocate_byte_slice_dedup("<redacted>\0".as_bytes()).unwrap()
     };
     let file = file.map_provenance(CtfeProvenance::as_immutable);
     let line = if loc_details.line { Scalar::from_u32(line) } else { Scalar::from_u32(0) };

library/core/src/panic/location.rs

@@ -1,3 +1,5 @@
+#[cfg(not(bootstrap))]
+use crate::ffi::CStr;
 use crate::fmt;
 
 /// A struct containing information about the location of a panic.
@@ -32,7 +34,16 @@ use crate::fmt;
 #[derive(Copy, Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
 #[stable(feature = "panic_hooks", since = "1.10.0")]
 pub struct Location<'a> {
+    #[cfg(bootstrap)]
     file: &'a str,
+
+    // Note: this filename will have exactly one nul byte at its end, but otherwise
+    // it must never contain interior nul bytes. This is relied on for the conversion
+    // to `CStr` below.
+    //
+    // The prefix of the string without the trailing nul byte will be a regular UTF8 `str`.
+    #[cfg(not(bootstrap))]
+    file_bytes_with_nul: &'a [u8],
     line: u32,
     col: u32,
 }
@@ -122,12 +133,43 @@ impl<'a> Location<'a> {
     ///
     /// panic!("Normal panic");
     /// ```
+    ///
+    /// # C/C++ compatibility
+    ///
+    /// Although `file` returns an `&str`, the characters of `file` are guaranteed to be followed
+    /// by a nul-terminator. This allows for greater interoperabilty with C and C++ code using
+    /// `__FILE__` or `std::source_location::file_name`, both of which return nul-terminated
+    /// `const char*`.
     #[must_use]
     #[stable(feature = "panic_hooks", since = "1.10.0")]
     #[rustc_const_stable(feature = "const_location_fields", since = "1.79.0")]
-    #[inline]
     pub const fn file(&self) -> &str {
-        self.file
+        #[cfg(bootstrap)]
+        {
+            self.file
+        }
+
+        #[cfg(not(bootstrap))]
+        {
+            let str_len = self.file_bytes_with_nul.len() - 1;
+            // SAFETY: `file_bytes_with_nul` without the trailing nul byte is guaranteed to be
+            // valid UTF8.
+            unsafe { crate::str::from_raw_parts(self.file_bytes_with_nul.as_ptr(), str_len) }
+        }
+    }
+
+    /// Returns the name of the source file as a nul-terminated `CStr`.
+    ///
+    /// This is useful for interop with APIs that expect C/C++ `__FILE__` or
+    /// `std::source_location::file_name`, both of which return a nul-terminated `const char*`.
+    #[cfg(not(bootstrap))]
+    #[must_use]
+    #[unstable(feature = "file_cstr", issue = "none")]
+    #[inline]
+    pub const fn file_cstr(&self) -> &CStr {
+        // SAFETY: `file_bytes_with_nul` is guaranteed to have a trailing nul byte and no
+        // interior nul bytes.
+        unsafe { CStr::from_bytes_with_nul_unchecked(self.file_bytes_with_nul) }
     }
 
     /// Returns the line number from which the panic originated.
@@ -181,22 +223,10 @@ impl<'a> Location<'a> {
     }
 }
 
-#[unstable(
-    feature = "panic_internals",
-    reason = "internal details of the implementation of the `panic!` and related macros",
-    issue = "none"
-)]
-impl<'a> Location<'a> {
-    #[doc(hidden)]
-    pub const fn internal_constructor(file: &'a str, line: u32, col: u32) -> Self {
-        Location { file, line, col }
-    }
-}
-
 #[stable(feature = "panic_hook_display", since = "1.26.0")]
 impl fmt::Display for Location<'_> {
     #[inline]
     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(formatter, "{}:{}:{}", self.file, self.line, self.col)
+        write!(formatter, "{}:{}:{}", self.file(), self.line, self.col)
     }
 }

tests/ui/rfcs/rfc-2091-track-caller/file-is-nul-terminated.rs

@@ -0,0 +1,25 @@
+//@ run-pass
+#![feature(file_cstr)]
+
+#[track_caller]
+const fn assert_file_has_trailing_zero() {
+    let caller = core::panic::Location::caller();
+    let file_str = caller.file();
+    let file_cstr = caller.file_cstr();
+    if file_str.len() != file_cstr.count_bytes() {
+        panic!("mismatched lengths");
+    }
+    let trailing_byte: core::ffi::c_char = unsafe {
+        *file_cstr.as_ptr().offset(file_cstr.count_bytes() as _)
+    };
+    if trailing_byte != 0 {
+        panic!("trailing byte was nonzero")
+    }
+}
+
+#[allow(dead_code)]
+const _: () = assert_file_has_trailing_zero();
+
+fn main() {
+    assert_file_has_trailing_zero();
+}