test(miner): migrate backfill tests off hand-typed existing_versions

c9ee104 added existing_versions()/ExistingVersionsQuery as the intended
producer for is_backfill()'s existing_versions argument, but left the
two miner.rs tests that motivated it still hand-typing the expected
Vec<String> (vec!["4.2.1".to_string()] / vec![]) instead of calling it
against a real Candidate universe. Migrates both to build a small
universe and query it, so the tests exercise the actual production path
instead of asserting against a value nothing computes.

Adds three unit tests for existing_versions() itself in eb_parse.rs
(returns every match at the queried generation, empty for an absent
package, excludes other toolchain generations of the same package) --
narrower scenarios than c9ee104's own
existing_versions_filters_by_name_and_generation_newest_first, not a
duplicate of it.

Field names and types checked by hand against domain.rs's real
Candidate/Toolchain definitions; not verified by a local build or test
run (no cargo build on this laptop, no reachable remote builder this
session -- CI on push is the actual verification here).

Author: HaoZeke

src/eb_parse.rs

@@ -2165,6 +2165,82 @@ mod tests {
             .any(|c| c.name == "OpenMPI" && c.version == "4.1.6"));
     }
 
+    fn existing_versions_universe() -> Vec<Candidate> {
+        let gcccore = Toolchain {
+            name: "GCCcore".into(),
+            version: "15.2.0".into(),
+        };
+        let older = Toolchain {
+            name: "GCCcore".into(),
+            version: "14.3.0".into(),
+        };
+        let mk = |name: &str, version: &str, tc: &Toolchain| Candidate {
+            name: name.into(),
+            version: version.into(),
+            toolchain: tc.clone(),
+            versionsuffix: None,
+            easyconfig_path: String::new(),
+            dependencies: vec![],
+            builddependencies: vec![],
+            exts_list: vec![],
+        };
+        vec![
+            mk("CMake", "4.2.1", &gcccore),
+            mk("CMake", "3.31.10", &gcccore),
+            mk("CMake", "3.30.0", &older),
+            mk("Boost", "1.88.0", &older),
+        ]
+    }
+
+    #[test]
+    fn existing_versions_returns_every_version_at_the_queried_generation() {
+        let universe = existing_versions_universe();
+        let generation = Toolchain {
+            name: "GCCcore".into(),
+            version: "15.2.0".into(),
+        };
+        let mut got = existing_versions(&ExistingVersionsQuery {
+            universe: &universe,
+            name: "CMake",
+            generation: &generation,
+        });
+        got.sort();
+        assert_eq!(got, vec!["3.31.10".to_string(), "4.2.1".to_string()]);
+    }
+
+    #[test]
+    fn existing_versions_is_empty_for_a_package_not_in_the_universe() {
+        let universe = existing_versions_universe();
+        let generation = Toolchain {
+            name: "GCCcore".into(),
+            version: "15.2.0".into(),
+        };
+        assert!(existing_versions(&ExistingVersionsQuery {
+            universe: &universe,
+            name: "LLVM",
+            generation: &generation,
+        })
+        .is_empty());
+    }
+
+    #[test]
+    fn existing_versions_excludes_other_generations_of_the_same_package() {
+        let universe = existing_versions_universe();
+        let generation = Toolchain {
+            name: "GCCcore".into(),
+            version: "15.2.0".into(),
+        };
+        let got = existing_versions(&ExistingVersionsQuery {
+            universe: &universe,
+            name: "Boost",
+            generation: &generation,
+        });
+        assert!(
+            got.is_empty(),
+            "Boost 1.88.0 sits at GCCcore-14.3.0 and must not answer a GCCcore-15.2.0 query, got {got:?}"
+        );
+    }
+
     #[test]
     fn strip_comments_preserves_hashes_in_strings() {
         let s = "name = 'foo#bar'\n# comment\nversion = '1.0'\n";

src/miner.rs

@@ -431,7 +431,21 @@ fn raw_line_diff(emitted: &str, target: &str) -> Vec<DiffLine> {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::eb_parse::resolve_easyconfig_str;
+    use crate::domain::{Candidate, Toolchain};
+    use crate::eb_parse::{existing_versions, resolve_easyconfig_str, ExistingVersionsQuery};
+
+    fn backfill_candidate(name: &str, version: &str, toolchain: &Toolchain) -> Candidate {
+        Candidate {
+            name: name.into(),
+            version: version.into(),
+            toolchain: toolchain.clone(),
+            versionsuffix: None,
+            easyconfig_path: String::new(),
+            dependencies: vec![],
+            builddependencies: vec![],
+            exts_list: vec![],
+        }
+    }
 
     /// gompi-2026.1.eb, easybuilders/easybuild-easyconfigs PR #25613,
     /// merge commit f929f1f2ef4c15feb19692f33da59e9e1e072572. Real
@@ -506,7 +520,16 @@ moduleclass = 'mpi'
         // CMake-4.2.1-GCCcore-15.2.0.eb from an earlier PR. A plain string
         // compare of "3.31.11" vs "4.2.1" would (wrongly) rank 4.2.1 lower;
         // cmp_version must not make that mistake.
-        let existing = vec!["4.2.1".to_string()];
+        let generation = Toolchain {
+            name: "GCCcore".into(),
+            version: "15.2.0".into(),
+        };
+        let universe = vec![backfill_candidate("CMake", "4.2.1", &generation)];
+        let existing = existing_versions(&ExistingVersionsQuery {
+            universe: &universe,
+            name: "CMake",
+            generation: &generation,
+        });
         assert!(
             is_backfill("3.31.11", &existing),
             "3.31.11 merging after 4.2.1 already exists at the same generation must be flagged as a backfill"
@@ -519,7 +542,20 @@ moduleclass = 'mpi'
         // GCC-15.2.0, with only 1.88.0 present at the prior generation
         // (GCC-14.3.0) in the merge-base tree, nothing newer already at
         // 15.2.0.
-        let existing: Vec<String> = vec![];
+        let generation = Toolchain {
+            name: "GCC".into(),
+            version: "15.2.0".into(),
+        };
+        let prior_generation = Toolchain {
+            name: "GCC".into(),
+            version: "14.3.0".into(),
+        };
+        let universe = vec![backfill_candidate("Boost", "1.88.0", &prior_generation)];
+        let existing = existing_versions(&ExistingVersionsQuery {
+            universe: &universe,
+            name: "Boost",
+            generation: &generation,
+        });
         assert!(
             !is_backfill("1.90.0", &existing),
             "a version with nothing newer already in the tree at that generation is a clean bump, not a backfill"