fix(build-std): make Resolve align to what to build

src/cargo/core/compiler/build_context/target_info.rs

@@ -619,6 +619,16 @@ impl TargetInfo {
             .iter()
             .any(|sup| sup.as_str() == split.as_str())
     }
+
+    /// Checks if a target maybe support std.
+    ///
+    /// If no explictly stated in target spec json, we treat it as "maybe support".
+    ///
+    /// This is only useful for `-Zbuild-std` to determine the default set of
+    /// crates it is going to build.
+    pub fn maybe_support_std(&self) -> bool {
+        matches!(self.supports_std, Some(true) | None)
+    }
 }
 
 /// Takes rustc output (using specialized command line args), and calculates the file prefix and

src/cargo/core/compiler/standard_lib.rs

@@ -44,10 +44,14 @@ fn std_crates<'a>(crates: &'a [String], default: &'static str, units: &[Unit]) -
 }
 
 /// Resolve the standard library dependencies.
+///
+/// * `crates` is the arg value from `-Zbuild-std`.
 pub fn resolve_std<'gctx>(
     ws: &Workspace<'gctx>,
     target_data: &mut RustcTargetData<'gctx>,
     build_config: &BuildConfig,
+    crates: &[String],
+    kinds: &[CompileKind],
 ) -> CargoResult<(PackageSet<'gctx>, Resolve, ResolvedFeatures)> {
     if build_config.build_plan {
         ws.gctx()
@@ -65,10 +69,21 @@ pub fn resolve_std<'gctx>(
     // `[dev-dependencies]`. No need for us to generate a `Resolve` which has
     // those included because we'll never use them anyway.
     std_ws.set_require_optional_deps(false);
-    // `sysroot` + the default feature set below should give us a good default
-    // Resolve, which includes `libtest` as well.
-    let specs = Packages::Packages(vec!["sysroot".into()]);
-    let specs = specs.to_package_id_specs(&std_ws)?;
+    let specs = {
+        // If there is anything looks like needing std, resolve with it.
+        // If not, we assume only `core` maye be needed, as `core the most fundamental crate.
+        //
+        // This may need a UI overhaul if `build-std` wants to fully support multi-targets.
+        let maybe_std = kinds
+            .iter()
+            .any(|kind| target_data.info(*kind).maybe_support_std());
+        let mut crates = std_crates(crates, if maybe_std { "std" } else { "core" }, &[]);
+        // `sysroot` is not in the default set because it is optional, but it needs
+        // to be part of the resolve in case we do need it or `libtest`.
+        crates.insert("sysroot");
+        let specs = Packages::Packages(crates.into_iter().map(Into::into).collect());
+        specs.to_package_id_specs(&std_ws)?
+    };
     let features = match &gctx.cli_unstable().build_std_features {
         Some(list) => list.clone(),
         None => vec![
@@ -115,10 +130,10 @@ pub fn generate_std_roots(
 ) -> CargoResult<HashMap<CompileKind, Vec<Unit>>> {
     // Generate a map of Units for each kind requested.
     let mut ret = HashMap::new();
-    let (core_only, maybe_std): (Vec<&CompileKind>, Vec<_>) = kinds.iter().partition(|kind|
-        // Only include targets that explicitly don't support std
-        target_data.info(**kind).supports_std == Some(false));
-    for (default_crate, kinds) in [("core", core_only), ("std", maybe_std)] {
+    let (maybe_std, maybe_core): (Vec<&CompileKind>, Vec<_>) = kinds
+        .iter()
+        .partition(|kind| target_data.info(**kind).maybe_support_std());
+    for (default_crate, kinds) in [("core", maybe_core), ("std", maybe_std)] {
         if kinds.is_empty() {
             continue;
         }

src/cargo/ops/cargo_compile/mod.rs

@@ -289,9 +289,14 @@ pub fn create_bcx<'a, 'gctx>(
         resolved_features,
     } = resolve;
 
-    let std_resolve_features = if gctx.cli_unstable().build_std.is_some() {
-        let (std_package_set, std_resolve, std_features) =
-            standard_lib::resolve_std(ws, &mut target_data, &build_config)?;
+    let std_resolve_features = if let Some(crates) = &gctx.cli_unstable().build_std {
+        let (std_package_set, std_resolve, std_features) = standard_lib::resolve_std(
+            ws,
+            &mut target_data,
+            &build_config,
+            crates,
+            &build_config.requested_kinds,
+        )?;
         pkg_set.add_set(std_package_set);
         Some((std_resolve, std_features))
     } else {

src/cargo/ops/cargo_fetch.rs

@@ -64,8 +64,14 @@ pub fn fetch<'a>(
     }
 
     // If -Zbuild-std was passed, download dependencies for the standard library.
-    if gctx.cli_unstable().build_std.is_some() {
-        let (std_package_set, _, _) = standard_lib::resolve_std(ws, &mut data, &build_config)?;
+    if let Some(crates) = &gctx.cli_unstable().build_std {
+        let (std_package_set, _, _) = standard_lib::resolve_std(
+            ws,
+            &mut data,
+            &build_config,
+            crates,
+            &build_config.requested_kinds,
+        )?;
         packages.add_set(std_package_set);
     }
 

tests/build-std/main.rs

@@ -25,9 +25,11 @@ use cargo_test_support::{basic_manifest, paths, project, rustc_host, str, Execs}
 use std::env;
 use std::path::Path;
 
-fn enable_build_std(e: &mut Execs, arg: Option<&str>) {
-    e.env_remove("CARGO_HOME");
-    e.env_remove("HOME");
+fn enable_build_std(e: &mut Execs, arg: Option<&str>, isolated: bool) {
+    if !isolated {
+        e.env_remove("CARGO_HOME");
+        e.env_remove("HOME");
+    }
 
     // And finally actually enable `build-std` for now
     let arg = match arg {
@@ -40,19 +42,41 @@ fn enable_build_std(e: &mut Execs, arg: Option<&str>) {
 
 // Helper methods used in the tests below
 trait BuildStd: Sized {
+    /// Set `-Zbuild-std` args and will download dependencies of the standard
+    /// library in users's `CARGO_HOME` (`~/.cargo/`) instead of isolated
+    /// environment `cargo-test-support` usually provides.
+    ///
+    /// The environment is not isolated is to avoid excessive network requests
+    /// and downloads. A side effect is `[BLOCKING]` will show up in stderr,
+    /// as a sign of package cahce lock contention when running other build-std
+    /// tests concurrently.
     fn build_std(&mut self) -> &mut Self;
+
+    /// Like [`BuildStd::build_std`] and is able to specify what crates to build.
     fn build_std_arg(&mut self, arg: &str) -> &mut Self;
+
+    /// Like [`BuildStd::build_std`] but use an isolated `CARGO_HOME` environment
+    /// to avoid package cache lock contention.
+    ///
+    /// Don't use this unless you really need to assert the full stderr
+    /// and avoid any `[BLOCKING]` message.
+    fn build_std_isolated(&mut self) -> &mut Self;
     fn target_host(&mut self) -> &mut Self;
 }
 
 impl BuildStd for Execs {
     fn build_std(&mut self) -> &mut Self {
-        enable_build_std(self, None);
+        enable_build_std(self, None, false);
         self
     }
 
     fn build_std_arg(&mut self, arg: &str) -> &mut Self {
-        enable_build_std(self, Some(arg));
+        enable_build_std(self, Some(arg), false);
+        self
+    }
+
+    fn build_std_isolated(&mut self) -> &mut Self {
+        enable_build_std(self, None, true);
         self
     }
 
@@ -107,9 +131,12 @@ fn basic() {
         )
         .build();
 
-    p.cargo("check").build_std().target_host().run();
+    // HACK: use an isolated the isolated CARGO_HOME environment (`build_std_isolated`)
+    // to avoid `[BLOCKING]` messages (from lock contention with other tests)
+    // from getting in this test's asserts
+    p.cargo("check").build_std_isolated().target_host().run();
     p.cargo("build")
-        .build_std()
+        .build_std_isolated()
         .target_host()
         // Importantly, this should not say [UPDATING]
         // There have been multiple bugs where every build triggers and update.
@@ -120,7 +147,7 @@ fn basic() {
 "#]])
         .run();
     p.cargo("run")
-        .build_std()
+        .build_std_isolated()
         .target_host()
         .with_stderr_data(str![[r#"
 [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
@@ -129,7 +156,7 @@ fn basic() {
 "#]])
         .run();
     p.cargo("test")
-        .build_std()
+        .build_std_isolated()
         .target_host()
         .with_stderr_data(str![[r#"
 [COMPILING] rustc-std-workspace-std [..]
@@ -379,17 +406,37 @@ fn test_proc_macro() {
         .file("src/lib.rs", "")
         .build();
 
-    // Download dependencies first,
-    // so we can compare  `cargo test` output without any wildcard
-    p.cargo("fetch").build_std().run();
     p.cargo("test --lib")
         .env_remove(cargo_util::paths::dylib_path_envvar())
         .build_std()
         .with_stderr_data(str![[r#"
+...
 [COMPILING] foo v0.0.0 ([ROOT]/foo)
 [FINISHED] `test` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
 [RUNNING] unittests src/lib.rs (target/debug/deps/foo-[HASH])
 
 "#]])
         .run();
 }
+
+#[cargo_test(build_std_real)]
+fn test_panic_abort() {
+    // See rust-lang/cargo#14935
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                edition = "2021"
+            "#,
+        )
+        .file("src/lib.rs", "#![no_std]")
+        .build();
+
+    p.cargo("check")
+        .build_std_arg("std,panic_abort")
+        .env("RUSTFLAGS", "-C panic=abort")
+        .arg("-Zbuild-std-features=panic_immediate_abort")
+        .run();
+}