rust-status: add trace-level logging for Rust status fallback for debugging

The added `log` crate is already a sub-dependency.

Differential Revision: https://phab.mercurial-scm.org/D8300

rust/Cargo.lock

@@ -253,6 +253,7 b' dependencies = ['
  "cpython 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "hg-core 0.1.0",
  "libc 0.2.67 (registry+https://github.com/rust-lang/crates.io-index)",
+ "log 0.4.8 (registry+https://github.com/rust-lang/crates.io-index)",
  "simple_logger 1.6.0 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 

rust/hg-cpython/Cargo.toml

@@ -24,6 +24,7 b' python3-bin = ["cpython/python3-sys"]'
 [dependencies]
 hg-core = { path = "../hg-core"}
 libc = '*'
+log = "0.4.8"
 simple_logger = "1.6.0"
 
 [dependencies.cpython]

rust/hg-cpython/src/dirstate/status.rs

@@ -84,7 +84,10 b' fn collect_bad_matches('
 fn handle_fallback(py: Python, err: StatusError) -> PyErr {
     match err {
         StatusError::Pattern(e) => {
-            PyErr::new::<FallbackError, _>(py, e.to_string())
+            let as_string = e.to_string();
+            log::trace!("Rust status fallback: `{}`", &as_string);
+
+            PyErr::new::<FallbackError, _>(py, &as_string)
         }
         e => PyErr::new::<ValueError, _>(py, e.to_string()),
     }