Auto merge of #4017 - Xanewok:wip-disambiguate-extern, r=oli-obk

compiletest: Disambiguate extern crate deps shared with the compiler

Fixes #4015.

changelog: Handle deps shared with the compiler in the internal compiletest suite

Attempts to fix the multiple matching crates error using the `--extern dep=path` disambiguation. This only includes `serde` at the moment because it's the only problematic dep right now (is inside Rust sysroot and pulled via `extern crate` in the test suite).

I'm not exactly sure this is the right approach (FWIW it fixes the issue locally), please do tell if this should be done differently.

Author: bors

tests/compile-test.rs

@@ -46,9 +46,32 @@ fn config(mode: &str, dir: PathBuf) -> compiletest::Config {
         config.run_lib_path = rustc_lib_path();
         config.compile_lib_path = rustc_lib_path();
     }
+
+    // When we'll want to use `extern crate ..` for a dependency that is used
+    // both by the crate and the compiler itself, we can't simply pass -L flags
+    // as we'll get a duplicate matching versions. Instead, disambiguate with
+    // `--extern dep=path`.
+    // See https://github.com/rust-lang/rust-clippy/issues/4015.
+    let needs_disambiguation = ["serde"];
+    // This assumes that deps are compiled (they are for Cargo integration tests).
+    let deps = std::fs::read_dir(host_libs().join("deps")).unwrap();
+    let disambiguated = deps
+        .filter_map(|dep| {
+            let path = dep.ok()?.path();
+            let name = path.file_name()?.to_string_lossy();
+            // NOTE: This only handles a single dep
+            // https://github.com/laumann/compiletest-rs/issues/101
+            needs_disambiguation
+                .iter()
+                .find(|dep| name.starts_with(&format!("lib{}-", dep)))
+                .map(|dep| format!("--extern {}={}", dep, path.display()))
+        })
+        .collect::<Vec<_>>();
+
     config.target_rustcflags = Some(format!(
-        "-L {0} -L {0}/deps -Dwarnings -Zui-testing",
-        host_libs().display()
+        "-L {0} -L {0}/deps -Dwarnings -Zui-testing {1}",
+        host_libs().display(),
+        disambiguated.join(" ")
     ));
 
     config.mode = cfg_mode;