message: Add some comments about Layout naming and .sysroot.
diff: //! .rustc-info.json
  //!
  //! # All final artifacts are linked into this directory from `deps`.
  +//! # Note that named profiles will soon be included as separate directories
  +//! # here. They have a restricted format, similar to Rust identifiers, so
  +//! # Cargo-specific directories added in the future should use some prefix
  +//! # like `.` to avoid name collisions.
  //! debug/ # or release/
  //!
  //! # File used to lock the directory to prevent multiple cargo processes
  //! # incremental is enabled.
  //! incremental/
  //!
  +//! # The sysroot for -Zbuild-std builds. This only appears in
  +//! # target-triple directories (not host), and only if -Zbuild-std is
  +//! # enabled.
  +//! .sysroot/
  +//!
  //! # This is the location at which the output of all custom build
  //! # commands are rooted.
  //! build/
  @@ -177,6 +186,7 @@ impl Layout {
  // Compute the sysroot path for the build-std feature.
  let build_std = ws.config().cli_unstable().build_std.as_ref();
  let (sysroot, sysroot_libdir) = if let Some(tp) = build_std.and(triple_path) {
  + // This uses a leading dot to avoid collision with named profiles.
  let sysroot = dest.join(".sysroot");
  let sysroot_libdir = sysroot.join("lib").join("rustlib").join(tp).join("lib");
  (Some(sysroot), Some(sysroot_libdir))
  