fix(doctest): Include all search paths with new build layout (#16348)

### What does this PR try to resolve?

When compilation was updated for the new build layout, doctests were
overlooked. This extracts the search path (`-L`) creation and switches
doctests to reuse it.

Part of #15010

### How to test and review this PR?

Author: epage

src/cargo/core/compiler/build_runner/mod.rs

@@ -234,6 +234,7 @@ impl<'a, 'gctx> BuildRunner<'a, 'gctx> {
             if unit.mode.is_doc_test() {
                 let mut unstable_opts = false;
                 let mut args = compiler::extern_args(&self, unit, &mut unstable_opts)?;
+                args.extend(compiler::lib_search_paths(&self, unit)?);
                 args.extend(compiler::lto_args(&self, unit));
                 args.extend(compiler::features_args(unit));
                 args.extend(compiler::check_cfg_args(unit));

src/cargo/core/compiler/mod.rs

@@ -1700,37 +1700,9 @@ fn build_deps_args(
     unit: &Unit,
 ) -> CargoResult<()> {
     let bcx = build_runner.bcx;
-    if build_runner.bcx.gctx.cli_unstable().build_dir_new_layout {
-        let mut map = BTreeMap::new();
-
-        // Recursively add all dependency args to rustc process
-        add_dep_arg(&mut map, build_runner, unit);
-
-        let paths = map.into_iter().map(|(_, path)| path).sorted_unstable();
 
-        for path in paths {
-            cmd.arg("-L").arg(&{
-                let mut deps = OsString::from("dependency=");
-                deps.push(path);
-                deps
-            });
-        }
-    } else {
-        cmd.arg("-L").arg(&{
-            let mut deps = OsString::from("dependency=");
-            deps.push(build_runner.files().deps_dir(unit));
-            deps
-        });
-    }
-
-    // Be sure that the host path is also listed. This'll ensure that proc macro
-    // dependencies are correctly found (for reexported macros).
-    if !unit.kind.is_host() {
-        cmd.arg("-L").arg(&{
-            let mut deps = OsString::from("dependency=");
-            deps.push(build_runner.files().host_deps(unit));
-            deps
-        });
+    for arg in lib_search_paths(build_runner, unit)? {
+        cmd.arg(arg);
     }
 
     let deps = build_runner.unit_deps(unit);
@@ -1847,6 +1819,42 @@ fn add_custom_flags(
     Ok(())
 }
 
+/// Generate a list of `-L` arguments
+pub fn lib_search_paths(
+    build_runner: &BuildRunner<'_, '_>,
+    unit: &Unit,
+) -> CargoResult<Vec<OsString>> {
+    let mut lib_search_paths = Vec::new();
+    if build_runner.bcx.gctx.cli_unstable().build_dir_new_layout {
+        let mut map = BTreeMap::new();
+
+        // Recursively add all dependency args to rustc process
+        add_dep_arg(&mut map, build_runner, unit);
+
+        let paths = map.into_iter().map(|(_, path)| path).sorted_unstable();
+
+        for path in paths {
+            let mut deps = OsString::from("dependency=");
+            deps.push(path);
+            lib_search_paths.extend(["-L".into(), deps]);
+        }
+    } else {
+        let mut deps = OsString::from("dependency=");
+        deps.push(build_runner.files().deps_dir(unit));
+        lib_search_paths.extend(["-L".into(), deps]);
+    }
+
+    // Be sure that the host path is also listed. This'll ensure that proc macro
+    // dependencies are correctly found (for reexported macros).
+    if !unit.kind.is_host() {
+        let mut deps = OsString::from("dependency=");
+        deps.push(build_runner.files().host_deps(unit));
+        lib_search_paths.extend(["-L".into(), deps]);
+    }
+
+    Ok(lib_search_paths)
+}
+
 /// Generates a list of `--extern` arguments.
 pub fn extern_args(
     build_runner: &BuildRunner<'_, '_>,

src/cargo/ops/cargo_test.rs

@@ -229,15 +229,6 @@ fn run_doc_tests(
             p.arg("-C").arg(format!("panic={}", unit.profile.panic));
         }
 
-        for &rust_dep in &[
-            &compilation.deps_output[&unit.kind],
-            &compilation.deps_output[&CompileKind::Host],
-        ] {
-            let mut arg = OsString::from("dependency=");
-            arg.push(rust_dep);
-            p.arg("-L").arg(arg);
-        }
-
         for native_dep in compilation.native_dirs.iter() {
             p.arg("-L").arg(native_dep);
         }

tests/testsuite/test.rs

@@ -2562,6 +2562,76 @@ fn doctest_dev_dep() {
     p.cargo("test -v").run();
 }
 
+#[cargo_test]
+fn doctest_dep() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.0.1"
+                edition = "2015"
+                authors = []
+
+                [dependencies]
+                b = { path = "b" }
+            "#,
+        )
+        .file(
+            "src/lib.rs",
+            r#"
+                /// ```
+                /// foo::foo();
+                /// ```
+                pub fn foo() {
+                    b::bar();
+                }
+            "#,
+        )
+        .file("b/Cargo.toml", &basic_manifest("b", "0.0.1"))
+        .file("b/src/lib.rs", "pub fn bar() {}")
+        .build();
+
+    p.cargo("test -v").run();
+}
+
+#[cargo_test]
+fn doctest_dep_new_layout() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.0.1"
+                edition = "2015"
+                authors = []
+
+                [dependencies]
+                b = { path = "b" }
+            "#,
+        )
+        .file(
+            "src/lib.rs",
+            r#"
+                /// ```
+                /// foo::foo();
+                /// ```
+                pub fn foo() {
+                    b::bar();
+                }
+            "#,
+        )
+        .file("b/Cargo.toml", &basic_manifest("b", "0.0.1"))
+        .file("b/src/lib.rs", "pub fn bar() {}")
+        .build();
+
+    p.cargo("-Zbuild-dir-new-layout test")
+        .masquerade_as_nightly_cargo(&["new build-dir layout"])
+        .run();
+}
+
 #[cargo_test]
 fn filter_no_doc_tests() {
     let p = project()