fix: Only fix "primary" packages by default

The previous heuristic for fixing packages was to fix all packages in a
workspace, aka those with path dependencies. Instead this commit switches cargo
over to only fixing the "primary" package, or those requested on the command
line or implicitly via cwd.

This will later help us identify which packages are being targeted so we can
provide tailored warnings and errors for mixed up transition steps.

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

@@ -96,6 +96,7 @@ pub struct Context<'a, 'cfg: 'a> {
     pub links: Links<'a>,
     pub used_in_plugin: HashSet<Unit<'a>>,
     pub jobserver: Client,
+    primary_packages: HashSet<&'a PackageId>,
     unit_dependencies: HashMap<Unit<'a>, Vec<Unit<'a>>>,
     files: Option<CompilationFiles<'a, 'cfg>>,
 }
@@ -129,6 +130,7 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
             jobserver,
             build_script_overridden: HashSet::new(),
 
+            primary_packages: HashSet::new(),
             unit_dependencies: HashMap::new(),
             files: None,
         })
@@ -321,6 +323,7 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
             Some(target) => Some(Layout::new(self.bcx.ws, Some(target), dest)?),
             None => None,
         };
+        self.primary_packages.extend(units.iter().map(|u| u.pkg.package_id()));
 
         build_unit_dependencies(units, self.bcx, &mut self.unit_dependencies)?;
         self.build_used_in_plugin_map(units)?;
@@ -487,6 +490,10 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
         let dir = self.files().layout(unit.kind).incremental().display();
         Ok(vec!["-C".to_string(), format!("incremental={}", dir)])
     }
+
+    pub fn is_primary_package(&self, unit: &Unit<'a>) -> bool {
+        self.primary_packages.contains(unit.pkg.package_id())
+    }
 }
 
 #[derive(Default)]

src/cargo/core/compiler/mod.rs

@@ -161,11 +161,14 @@ fn compile<'a, 'cfg: 'a>(
 }
 
 fn rustc<'a, 'cfg>(
-    mut cx: &mut Context<'a, 'cfg>,
+    cx: &mut Context<'a, 'cfg>,
     unit: &Unit<'a>,
     exec: &Arc<Executor>,
 ) -> CargoResult<Work> {
     let mut rustc = prepare_rustc(cx, &unit.target.rustc_crate_types(), unit)?;
+    if cx.is_primary_package(unit) {
+        rustc.env("CARGO_PRIMARY_PACKAGE", "1");
+    }
     let build_plan = cx.bcx.build_config.build_plan;
 
     let name = unit.pkg.name().to_string();
@@ -195,7 +198,7 @@ fn rustc<'a, 'cfg>(
     } else {
         root.join(&cx.files().file_stem(unit))
     }.with_extension("d");
-    let dep_info_loc = fingerprint::dep_info_loc(&mut cx, unit);
+    let dep_info_loc = fingerprint::dep_info_loc(cx, unit);
 
     rustc.args(&cx.bcx.rustflags_args(unit)?);
     let json_messages = cx.bcx.build_config.json_messages();

src/cargo/ops/fix.rs

@@ -134,7 +134,7 @@ pub fn fix_maybe_exec_rustc() -> CargoResult<bool> {
     // not the best heuristic but matches what Cargo does today at least.
     let mut fixes = FixedCrate::default();
     if let Some(path) = filename {
-        if !Path::new(&path).is_absolute() {
+        if env::var("CARGO_PRIMARY_PACKAGE").is_ok() {
             trace!("start rustfixing {:?}", path);
             fixes = rustfix_crate(&lock_addr, rustc.as_ref(), &path)?;
         }

tests/testsuite/fix.rs

@@ -196,7 +196,7 @@ fn fix_path_deps() {
         .build();
 
     assert_that(
-        p.cargo("fix --allow-no-vcs")
+        p.cargo("fix --allow-no-vcs -p foo -p bar")
             .env("__CARGO_FIX_YOLO", "1"),
         execs()
             .with_status(0)