rev = "refs/pull/𑑛/head"

src/cargo/sources/git/utils.rs

@@ -799,13 +799,17 @@ pub fn fetch(
             refspecs.push(String::from("HEAD:refs/remotes/origin/HEAD"));
         }
 
-        // For `rev` dependencies we don't know what the rev will point to. To
-        // handle this situation we fetch all branches and tags, and then we
-        // pray it's somewhere in there.
-        GitReference::Rev(_) => {
-            refspecs.push(String::from("refs/heads/*:refs/remotes/origin/*"));
-            refspecs.push(String::from("HEAD:refs/remotes/origin/HEAD"));
-            tags = true;
+        GitReference::Rev(rev) => {
+            if rev.starts_with("refs/") {
+                refspecs.push(format!("{0}:{0}", rev));
+            } else {
+                // We don't know what the rev will point to. To handle this
+                // situation we fetch all branches and tags, and then we pray
+                // it's somewhere in there.
+                refspecs.push(String::from("refs/heads/*:refs/remotes/origin/*"));
+                refspecs.push(String::from("HEAD:refs/remotes/origin/HEAD"));
+                tags = true;
+            }
         }
     }
 
@@ -1025,9 +1029,13 @@ fn github_up_to_date(
         GitReference::Branch(branch) => branch,
         GitReference::Tag(tag) => tag,
         GitReference::DefaultBranch => "HEAD",
-        GitReference::Rev(_) => {
-            debug!("can't use github fast path with `rev`");
-            return Ok(false);
+        GitReference::Rev(rev) => {
+            if rev.starts_with("refs/") {
+                rev
+            } else {
+                debug!("can't use github fast path with `rev = \"{}\"`", rev);
+                return Ok(false);
+            }
         }
     };
 

src/doc/src/reference/specifying-dependencies.md

@@ -147,6 +147,13 @@ the latest commit on a branch named `next`:
 regex = { git = "/~https://github.com/rust-lang/regex", branch = "next" }
 ```
 
+Anything that is not a branch or tag falls under `rev`. This can be a commit
+hash like `rev = "4c59b707"`, or a named reference exposed by the remote
+repository such as `rev = "refs/pull/493/head"`. What references are available
+varies by where the repo is hosted; GitHub in particular exposes a reference to
+the most recent commit of every pull request as shown, but other git hosts often
+provide something equivalent, possibly under a different naming scheme.
+
 Once a `git` dependency has been added, Cargo will lock that dependency to the
 latest commit at the time. New commits will not be pulled down automatically
 once the lock is in place. However, they can be pulled down manually with

tests/testsuite/git.rs

@@ -230,6 +230,68 @@ fn cargo_compile_git_dep_tag() {
     project.cargo("build").run();
 }
 
+#[cargo_test]
+fn cargo_compile_git_dep_pull_request() {
+    let project = project();
+    let git_project = git::new("dep1", |project| {
+        project
+            .file("Cargo.toml", &basic_lib_manifest("dep1"))
+            .file(
+                "src/dep1.rs",
+                r#"
+                    pub fn hello() -> &'static str {
+                        "hello world"
+                    }
+                "#,
+            )
+    });
+
+    // Make a reference in GitHub's pull request ref naming convention.
+    let repo = git2::Repository::open(&git_project.root()).unwrap();
+    let oid = repo.refname_to_id("HEAD").unwrap();
+    let force = false;
+    let log_message = "open pull request";
+    repo.reference("refs/pull/330/head", oid, force, log_message)
+        .unwrap();
+
+    let project = project
+        .file(
+            "Cargo.toml",
+            &format!(
+                r#"
+                    [project]
+                    name = "foo"
+                    version = "0.0.0"
+
+                    [dependencies]
+                    dep1 = {{ git = "{}", rev = "refs/pull/330/head" }}
+                "#,
+                git_project.url()
+            ),
+        )
+        .file(
+            "src/main.rs",
+            &main_file(r#""{}", dep1::hello()"#, &["dep1"]),
+        )
+        .build();
+
+    let git_root = git_project.root();
+
+    project
+        .cargo("build")
+        .with_stderr(&format!(
+            "[UPDATING] git repository `{}`\n\
+             [COMPILING] dep1 v0.5.0 ({}?rev=refs/pull/330/head#[..])\n\
+             [COMPILING] foo v0.0.0 ([CWD])\n\
+             [FINISHED] dev [unoptimized + debuginfo] target(s) in [..]\n",
+            path2url(&git_root),
+            path2url(&git_root),
+        ))
+        .run();
+
+    assert!(project.bin("foo").is_file());
+}
+
 #[cargo_test]
 fn cargo_compile_with_nested_paths() {
     let git_project = git::new("dep1", |project| {