hive-forge: explain the pr-blocked-by-issue dependency failure instead of a raw 500

hive-forge/src/verbs/dependency.rs

@@ -17,7 +17,7 @@ use forgejo_api::structs::IssueMeta;
 use serde_json::Value;
 
 use crate::client::{Client, index};
-use crate::verbs::{dependency_summaries, print_json};
+use crate::verbs::{dependency_summaries, is_pr, print_json};
 
 #[derive(ClapArgs)]
 pub struct Args {
@@ -152,6 +152,10 @@ fn apply_and_verify(
                 "hive-forge: warning: dependency on #{dep} converged despite a reported \
                  error (server-side issue after the write, not a real failure): {e:#}"
             );
+        } else if let Some(hint) =
+            pr_blocked_by_issue_hint(client, owner, name, number, dep, adding)
+        {
+            real_failures.push(format!("#{dep}: {e:#}\n  {hint}"));
         } else {
             real_failures.push(format!("#{dep}: {e:#}"));
         }
@@ -168,6 +172,37 @@ fn apply_and_verify(
     Ok(())
 }
 
+/// A hint appended to a genuine add-failure when it matches the one known
+/// unsupported shape: a **PR** depending on a plain **issue**.
+///
+/// Confirmed via the full four-way matrix against the live forge:
+/// issue→issue and issue→PR both work, PR→issue 500s every time with the
+/// edge genuinely absent on read-back. `hive-forge`'s own request is
+/// identical across all four directions, so this is an upstream Forgejo
+/// limitation, not a bug this client can route around — there is no other
+/// side to add the edge from. Best-effort: the two extra classification
+/// GETs are swallowed on error rather than obscuring the real failure with
+/// a diagnostic-aid failure.
+fn pr_blocked_by_issue_hint(
+    client: &Client,
+    owner: &str,
+    name: &str,
+    number: u64,
+    dep: u64,
+    adding: bool,
+) -> Option<&'static str> {
+    if !adding {
+        return None;
+    }
+    let source_is_pr = is_pr(client, owner, name, number).ok()?;
+    let target_is_pr = is_pr(client, owner, name, dep).ok()?;
+    (source_is_pr && !target_is_pr).then_some(
+        "known limitation: a PR cannot be marked as depending on a plain issue (an upstream \
+         Forgejo limitation, not a hive-forge bug). Record the hold as prose in the PR \
+         body/comment instead; there's no dependency-edge mechanism for this direction.",
+    )
+}
+
 /// Build the `IssueMeta` body the create/remove endpoints want.
 ///
 /// `owner`/`repo` are filled in with the *same* repo the request URL

hive-forge/src/verbs/mod.rs

@@ -131,20 +131,28 @@ pub(crate) enum Kind {
     Issue,
 }
 
-/// Verify `number` is the expected kind before a kind-namespaced verb (one
-/// of the generics that work on both — close/comment/labels/…) acts on it —
-/// the validation win the `pr <verb>` / `issue <verb>` split buys over the
-/// old generic verbs. Forgejo's `/issues/{n}` endpoint serves both issues and
-/// PRs and marks PRs with a non-null `pull_request` field, so one GET
-/// classifies it. Errors with a "use the other command" message on mismatch.
-pub(crate) fn assert_kind(client: &Client, number: u64, expected: Kind) -> Result<()> {
-    let (owner, name) = client.owner_repo()?;
+/// Whether `number` is a PR rather than a plain issue. Forgejo's
+/// `/issues/{n}` endpoint serves both and marks PRs with a non-null
+/// `pull_request` field, so one GET classifies it — shared by
+/// [`assert_kind`] and by `dependency`'s failure-diagnosis path, which
+/// needs the same classification without wanting an error on mismatch.
+pub(crate) fn is_pr(client: &Client, owner: &str, name: &str, number: u64) -> Result<bool> {
     let issue = client
         .api()
         .issue_get_issue(owner, name, index(number)?)
         .send()?;
-    let is_pr = issue.pull_request.is_some();
-    match (expected, is_pr) {
+    Ok(issue.pull_request.is_some())
+}
+
+/// Verify `number` is the expected kind before a kind-namespaced verb (one
+/// of the generics that work on both — close/comment/labels/…) acts on it —
+/// the validation win the `pr <verb>` / `issue <verb>` split buys over the
+/// old generic verbs. Errors with a "use the other command" message on
+/// mismatch.
+pub(crate) fn assert_kind(client: &Client, number: u64, expected: Kind) -> Result<()> {
+    let (owner, name) = client.owner_repo()?;
+    let is_pr_result = is_pr(client, owner, name, number)?;
+    match (expected, is_pr_result) {
         (Kind::Pr, false) => {
             anyhow::bail!(
                 "#{number} is an issue, not a PR — use `hive-forge issue <verb> {number}`"