Handle pipelined tests of libraries

The previous implementation of pipelining accidentally forgot to account
for rlibs being compiled in `--test` mode. The compilations would be
pipelined to where all the dependencies of an rlib might not be
available yet, but `--test` actually performs linking in rustc!

This commit fixes the issue by refactoring slightly and removing
`Target::requires_upstream_objects` (moving it to `TargetKind`) and then
making the source of truth a `Unit::requires_upstream_objects` method
which takes into account the value from `TargetKind` as well as the
`CompileMode`.

Closes #6993

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

@@ -420,7 +420,7 @@ impl<'a, 'cfg: 'a> CompilationFiles<'a, 'cfg> {
                     )?;
                 }
                 let path = out_dir.join(format!("lib{}.rmeta", file_stem));
-                if !unit.target.requires_upstream_objects() {
+                if !unit.requires_upstream_objects() {
                     ret.push(OutputFile {
                         path,
                         hardlink: None,

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

@@ -475,11 +475,11 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
         self.pipelining
             // We're only a candidate for requiring an `rmeta` file if we
             // ourselves are building an rlib,
-            && !parent.target.requires_upstream_objects()
+            && !parent.requires_upstream_objects()
             && parent.mode == CompileMode::Build
             // Our dependency must also be built as an rlib, otherwise the
             // object code must be useful in some fashion
-            && !dep.target.requires_upstream_objects()
+            && !dep.requires_upstream_objects()
             && dep.mode == CompileMode::Build
     }
 

src/cargo/core/compiler/job_queue.rs

@@ -195,7 +195,7 @@ impl<'a, 'cfg> JobQueue<'a, 'cfg> {
         // the target as well. This should ensure that edges changed to
         // `Metadata` propagate upwards `All` dependencies to anything that
         // transitively contains the `Metadata` edge.
-        if unit.target.requires_upstream_objects() {
+        if unit.requires_upstream_objects() {
             for dep in dependencies.iter() {
                 depend_on_deps_of_deps(cx, &mut queue_deps, dep);
             }

src/cargo/core/compiler/mod.rs

@@ -827,7 +827,7 @@ fn build_base_args<'a, 'cfg>(
 
     if unit.mode.is_check() {
         cmd.arg("--emit=dep-info,metadata");
-    } else if !unit.target.requires_upstream_objects() {
+    } else if !unit.requires_upstream_objects() {
         // Always produce metdata files for rlib outputs. Metadata may be used
         // in this session for a pipelined compilation, or it may be used in a
         // future Cargo session as part of a pipelined compile.

src/cargo/core/compiler/unit.rs

@@ -50,6 +50,18 @@ pub struct UnitInner<'a> {
     pub mode: CompileMode,
 }
 
+impl UnitInner<'_> {
+    /// Returns whether compilation of this unit requires all upstream artifacts
+    /// to be available.
+    ///
+    /// This effectively means that this unit is a synchronization point (if the
+    /// return value is `true`) that all previously pipelined units need to
+    /// finish in their entirety before this one is started.
+    pub fn requires_upstream_objects(&self) -> bool {
+        self.mode.is_any_test() || self.target.kind().requires_upstream_objects()
+    }
+}
+
 impl<'a> Unit<'a> {
     pub fn buildkey(&self) -> String {
         format!("{}-{}", self.pkg.name(), short_hash(self))

src/cargo/core/manifest.rs

@@ -206,6 +206,20 @@ impl TargetKind {
             TargetKind::CustomBuild => "build-script",
         }
     }
+
+    /// Returns whether production of this artifact requires the object files
+    /// from dependencies to be available.
+    ///
+    /// This only returns `false` when all we're producing is an rlib, otherwise
+    /// it will return `true`.
+    pub fn requires_upstream_objects(&self) -> bool {
+        match self {
+            TargetKind::Lib(kinds) | TargetKind::ExampleLib(kinds) => {
+                kinds.iter().any(|k| k.requires_upstream_objects())
+            }
+            _ => true,
+        }
+    }
 }
 
 /// Information about a binary, a library, an example, etc. that is part of the
@@ -821,20 +835,6 @@ impl Target {
         }
     }
 
-    /// Returns whether production of this artifact requires the object files
-    /// from dependencies to be available.
-    ///
-    /// This only returns `false` when all we're producing is an rlib, otherwise
-    /// it will return `true`.
-    pub fn requires_upstream_objects(&self) -> bool {
-        match &self.kind {
-            TargetKind::Lib(kinds) | TargetKind::ExampleLib(kinds) => {
-                kinds.iter().any(|k| k.requires_upstream_objects())
-            }
-            _ => true,
-        }
-    }
-
     pub fn is_bin(&self) -> bool {
         self.kind == TargetKind::Bin
     }