fix: add error message for github PR url in dep

Prior to this, using a github PR URL would cause cargo to attempt to
fetch from an incorrect URL several times before failing.
Providing a github pull request url now fails with an error message
that shows how to fix the problem.

E.g.:
```toml
bar = { git = "https://github.com/foo/bar/pull/123" }
```
Now gives the following error message:
```
dependency (bar) specifies a GitHub pull request link. If you were trying to specify a specific github PR, replace the URL with the git URL (e.g. `git = "https://github.com/foo/bar.git"`) and add `rev = "refs/pull/123/head"` in the dependency declaration.
```

Fixes: rust-lang#15001

src/cargo/util/toml/mod.rs

@@ -2144,6 +2144,8 @@ fn to_dependency_source_id<P: ResolveToPath + Clone>(
                 .unwrap_or(GitReference::DefaultBranch);
             let loc = git.into_url()?;
 
+            bail_if_github_pull_request(&name_in_toml, &loc)?;
+
             if let Some(fragment) = loc.fragment() {
                 let msg = format!(
                     "URL fragment `#{fragment}` in git URL is ignored for dependency ({name_in_toml}). \
@@ -2182,6 +2184,26 @@ fn to_dependency_source_id<P: ResolveToPath + Clone>(
     }
 }
 
+/// Checks if the URL is a GitHub pull request URL.
+///
+/// If the URL is a GitHub pull request URL, an error is returned with a message that explains
+/// how to specify a specific git revision.
+fn bail_if_github_pull_request(name_in_toml: &str, url: &Url) -> CargoResult<()> {
+    if url.host_str() != Some("github.com") {
+        return Ok(());
+    }
+    let path_components = url.path().split('/').collect::<Vec<_>>();
+    if let ["", owner, repo, "pull", pr_number, ..] = path_components[..] {
+        bail!(
+            "dependency ({name_in_toml}) specifies a GitHub pull request link. \
+                If you were trying to specify a specific github PR, replace the URL with the git \
+                URL (e.g. `git = \"https://github.com/{owner}/{repo}.git\"`) \
+                and add `rev = \"refs/pull/{pr_number}/head\"` in the dependency declaration.",
+        );
+    }
+    Ok(())
+}
+
 pub(crate) fn lookup_path_base<'a>(
     base: &PathBaseName,
     gctx: &GlobalContext,

tests/testsuite/bad_config.rs

@@ -2151,6 +2151,37 @@ Caused by:
         .run();
 }
 
+#[cargo_test]
+fn github_pull_request_url() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.0.0"
+                edition = "2015"
+                authors = []
+
+                [dependencies.bar]
+                git = "https://github.com/foo/bar/pull/123"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("check -v")
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[ERROR] failed to parse manifest at `[ROOT]/foo/Cargo.toml`
+
+Caused by:
+  dependency (bar) specifies a GitHub pull request link. If you were trying to specify a specific github PR, replace the URL with the git URL (e.g. `git = "https://github.com/foo/bar.git"`) and add `rev = "refs/pull/123/head"` in the dependency declaration.
+
+"#]])
+        .run();
+}
+
 #[cargo_test]
 fn fragment_in_git_url() {
     let p = project()