fix(msrv): Error, rather than panic, on rust-version 'x'

Fixes #13768

Author: epage

crates/cargo-util-schemas/src/core/partial_version.rs

@@ -180,6 +180,12 @@ enum ErrorKind {
 }
 
 fn is_req(value: &str) -> bool {
+    if value == "x" || value == "X" {
+        // Checking for wildcard this way in case the `x` appears organically as a string so we
+        // don't error out about a req when it isn't one
+        return true;
+    }
+
     let Some(first) = value.chars().next() else {
         return false;
     };

tests/testsuite/rust_version.rs

@@ -125,7 +125,6 @@ fn rust_version_bad_pre_release() {
 }
 
 #[cargo_test]
-#[should_panic]
 fn rust_version_x_wildcard() {
     project()
         .file(
@@ -150,8 +149,8 @@ fn rust_version_x_wildcard() {
 [ERROR] unexpected version requirement, expected a version like \"1.32\"
  --> Cargo.toml:7:28
   |
-7 |             rust-version = \"^1.43\"
-  |                            ^^^^^^^
+7 |             rust-version = \"x\"
+  |                            ^^^
   |
 ",
         )