fix(manifest): Improve rust-version error messages

Since we have tests for a couple of cases, I figured we could
improve the error messages for them.

src/cargo/util/semver_ext.rs

@@ -136,9 +136,18 @@ impl std::str::FromStr for PartialVersion {
         // HACK: `PartialVersion` is a subset of the `VersionReq` syntax that only ever
         // has one comparator with a required minor and optional patch, and uses no
         // other features.
+        if is_req(value) {
+            anyhow::bail!("unexpected version requirement, expected a version like \"1.32\"")
+        }
         let version_req = match semver::VersionReq::parse(value) {
             // Exclude semver operators like `^` and pre-release identifiers
             Ok(req) if value.chars().all(|c| c.is_ascii_digit() || c == '.') => req,
+            Err(_) if value.contains('+') => {
+                anyhow::bail!("unexpected build field, expected a version like \"1.32\"")
+            }
+            Err(_) if value.contains('-') => {
+                anyhow::bail!("unexpected prerelease field, expected a version like \"1.32\"")
+            }
             _ => anyhow::bail!("expected a version like \"1.32\""),
         };
         assert_eq!(
@@ -211,6 +220,13 @@ impl<'de> serde::Deserialize<'de> for PartialVersion {
     }
 }
 
+fn is_req(value: &str) -> bool {
+    let Some(first) = value.chars().next() else {
+        return false;
+    };
+    "<>=^~".contains(first) || value.contains('*') || value.contains(',')
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

tests/testsuite/rust_version.rs

@@ -48,7 +48,7 @@ fn rust_version_bad_caret() {
 error: failed to parse manifest at `[..]`
 
 Caused by:
-  expected a version like \"1.32\"
+  unexpected version requirement, expected a version like \"1.32\"
   in `package.rust-version`",
         )
         .run();
@@ -78,7 +78,7 @@ fn rust_version_bad_pre_release() {
 error: failed to parse manifest at `[..]`
 
 Caused by:
-  expected a version like \"1.32\"
+  unexpected prerelease field, expected a version like \"1.32\"
   in `package.rust-version`",
         )
         .run();