Fix build script outputs ending up in the wrong place

src/cargo/core/compiler/context/compilation_files.rs

@@ -200,7 +200,7 @@ impl<'a, 'cfg: 'a> CompilationFiles<'a, 'cfg> {
     /// Note that some units may share the same directory, so care should be
     /// taken in those cases!
     fn pkg_dir(&self, unit: &Unit) -> String {
-        let name = unit.pkg.package_id().name();
+        let name = unit.pkg.package_id().file_safe_name();
         match self.metas[unit] {
             Some(ref meta) => format!("{}-{}", name, meta),
             None => format!("{}-{}", name, self.target_short_hash(unit)),

src/cargo/core/package.rs

@@ -19,7 +19,7 @@ use serde::Serialize;
 
 use crate::core::compiler::{CompileKind, RustcTargetData};
 use crate::core::dependency::DepKind;
-use crate::core::manifest::{SUBCRATE_DELIMETER, SUBCRATE_DELIMETER_FILENAME_REPLACEMENT};
+use crate::core::manifest::SUBCRATE_DELIMETER;
 use crate::core::resolver::features::ForceAllTargets;
 use crate::core::resolver::{HasDevUnits, Resolve};
 use crate::core::source::MaybePackage;
@@ -142,11 +142,7 @@ impl Package {
     }
 
     pub fn file_safe_name(&self) -> InternedString {
-        let i_str = self
-            .package_id()
-            .name()
-            .replace(SUBCRATE_DELIMETER, SUBCRATE_DELIMETER_FILENAME_REPLACEMENT);
-        i_str.into()
+        self.package_id().file_safe_name()
     }
 
     /// Gets the name of the package that can be used in URLs and filenames

src/cargo/core/package_id.rs

@@ -13,6 +13,8 @@ use crate::core::source::SourceId;
 use crate::util::interning::InternedString;
 use crate::util::{CargoResult, ToSemver};
 
+use super::manifest::{SUBCRATE_DELIMETER, SUBCRATE_DELIMETER_FILENAME_REPLACEMENT};
+
 lazy_static::lazy_static! {
     static ref PACKAGE_ID_CACHE: Mutex<HashSet<&'static PackageIdInner>> =
         Mutex::new(HashSet::new());
@@ -139,6 +141,12 @@ impl PackageId {
     pub fn name(self) -> InternedString {
         self.inner.name
     }
+    pub fn file_safe_name(self) -> InternedString {
+        let i_str = self
+            .name()
+            .replace(SUBCRATE_DELIMETER, SUBCRATE_DELIMETER_FILENAME_REPLACEMENT);
+        i_str.into()
+    }
     pub fn version(self) -> &'static semver::Version {
         &self.inner.version
     }

tests/testsuite/build_subcrate.rs

@@ -295,3 +295,36 @@ To learn more, run the command again with --verbose.\n",
         .run();
     assert!(p.root().join("Cargo.lock").is_file());
 }
+
+#[cargo_test]
+fn cargo_compile_with_build_script_puts_output_in_correct_folder() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo/bar"
+                authors = []
+                version = "0.0.0"
+            "#,
+        )
+        .file("src/lib.rs", "pub fn bar() {}")
+        .file("build.rs", "pub fn main() { println!(\"Hello world\"); }")
+        .build();
+    p.cargo("build").run();
+
+    let build_script_dir = p.target_debug_dir().join("build");
+    let matches: Vec<std::fs::DirEntry> = build_script_dir
+        .read_dir()
+        .expect("build dir did not exist")
+        .map(|dir| dir.unwrap())
+        .filter(|dir| dir.file_name().to_string_lossy().starts_with("foo_bar"))
+        .filter(|dir| dir.path().join("output").exists())
+        .collect();
+
+    assert!(matches.len() >= 1);
+
+    let output_contents = std::fs::read_to_string(matches[0].path().join("output"))
+        .unwrap_or_else(|e| panic!("could not read file {}: {}", matches[0].path().display(), e));
+    assert_eq!(output_contents, "Hello world\n");
+}