ci: Bump minimum Rust version to v1.31.0

Now that we no longer support el7, let's peg our minimum `rustc` version
to the latest Rust module available in el8 instead. That's 1.31.0 right
now.

And drop use of deprecated `trim_left_matches()` (it was deprecated in
1.33.0, which is currently used in the other testsuites that pull
`rustc` from the Fedora repos; we might want to eventually just use the
`RUST_MIN_VERSION` in all the testsuites to not get caught in the middle
like this again).

Closes: #1787
Approved by: cgwalters

.papr.yml

@@ -160,8 +160,8 @@ container:
   image: registry.fedoraproject.org/fedora:29
 
 env:
-  # this corresponds to the DTS rustc version we want to support
-  RUST_MIN_VERSION: 1.29.2
+  # this corresponds to the latest Rust module available in el8
+  RUST_MIN_VERSION: 1.31.0
 
 tests:
   - ci/installdeps.sh

rust/src/treefile.rs

@@ -396,7 +396,7 @@ fn treefile_parse_recurse<P: AsRef<Path>>(
 // Similar to the importer check but just checks for prefixes since
 // they're files, and also allows /etc since it's before conversion
 fn add_files_path_is_valid(path: &str) -> bool {
-    let path = path.trim_left_matches("/");
+    let path = path.trim_start_matches("/");
     (path.starts_with("usr/") && !path.starts_with("usr/local/"))
         || path.starts_with("etc/")
         || path.starts_with("bin/")