chore: add vendor dependencies for kauma build

vendor/proc-macro2/build/probe.rs

@@ -0,0 +1,41 @@
+// This code exercises the surface area that we expect of Span's unstable API.
+// If the current toolchain is able to compile it, then proc-macro2 is able to
+// offer these APIs too.
+
+#![feature(proc_macro_span)]
+
+extern crate proc_macro;
+
+use core::ops::{Range, RangeBounds};
+use proc_macro::{Literal, Span};
+
+pub fn byte_range(this: &Span) -> Range<usize> {
+    this.byte_range()
+}
+
+pub fn start(this: &Span) -> Span {
+    this.start()
+}
+
+pub fn end(this: &Span) -> Span {
+    this.end()
+}
+
+pub fn line(this: &Span) -> usize {
+    this.line()
+}
+
+pub fn column(this: &Span) -> usize {
+    this.column()
+}
+
+pub fn join(this: &Span, other: Span) -> Option<Span> {
+    this.join(other)
+}
+
+pub fn subspan<R: RangeBounds<usize>>(this: &Literal, range: R) -> Option<Span> {
+    this.subspan(range)
+}
+
+// Include in sccache cache key.
+const _: Option<&str> = option_env!("RUSTC_BOOTSTRAP");