fix(build-std): remove std unsupported check

Will let `std:bool` to determine necessary deps for `-Zbuild-std`,
instead of erroring out.

src/cargo/core/compiler/standard_lib.rs

@@ -15,7 +15,7 @@ use std::path::PathBuf;
 
 use super::BuildConfig;
 
-fn std_crates<'a>(crates: &'a [String], units: Option<&[Unit]>) -> HashSet<&'a str> {
+fn std_crates<'a>(crates: &'a [String], units: &[Unit]) -> HashSet<&'a str> {
     let mut crates = HashSet::from_iter(crates.iter().map(|s| s.as_str()));
     // This is a temporary hack until there is a more principled way to
     // declare dependencies in Cargo.toml.
@@ -30,13 +30,11 @@ fn std_crates<'a>(crates: &'a [String], units: Option<&[Unit]>) -> HashSet<&'a s
         crates.insert("compiler_builtins");
         // Only build libtest if it looks like it is needed (libtest depends on libstd)
         // If we know what units we're building, we can filter for libtest depending on the jobs.
-        if let Some(units) = units {
-            if units
-                .iter()
-                .any(|unit| unit.mode.is_rustc_test() && unit.target.harness())
-            {
-                crates.insert("test");
-            }
+        if units
+            .iter()
+            .any(|unit| unit.mode.is_rustc_test() && unit.target.harness())
+        {
+            crates.insert("test");
         }
     } else if crates.contains("core") {
         crates.insert("compiler_builtins");
@@ -50,27 +48,13 @@ pub fn resolve_std<'gctx>(
     ws: &Workspace<'gctx>,
     target_data: &mut RustcTargetData<'gctx>,
     build_config: &BuildConfig,
-    crates: &[String],
 ) -> CargoResult<(PackageSet<'gctx>, Resolve, ResolvedFeatures)> {
-    let crates = std_crates(crates, None);
-
     if build_config.build_plan {
         ws.gctx()
             .shell()
             .warn("-Zbuild-std does not currently fully support --build-plan")?;
     }
 
-    // check that targets support building std
-    if crates.contains("std") {
-        let unsupported_targets = target_data.get_unsupported_std_targets();
-        if !unsupported_targets.is_empty() {
-            anyhow::bail!(
-                "building std is not supported on the following targets: {}",
-                unsupported_targets.join(", ")
-            )
-        }
-    }
-
     let src_path = detect_sysroot_src_path(target_data)?;
     let std_ws_manifest_path = src_path.join("Cargo.toml");
     let gctx = ws.gctx();
@@ -129,7 +113,7 @@ pub fn generate_std_roots(
     profiles: &Profiles,
     target_data: &RustcTargetData<'_>,
 ) -> CargoResult<HashMap<CompileKind, Vec<Unit>>> {
-    let std_ids = std_crates(crates, Some(units))
+    let std_ids = std_crates(crates, units)
         .iter()
         .map(|crate_name| std_resolve.query(crate_name))
         .collect::<CargoResult<Vec<PackageId>>>()?;

src/cargo/ops/cargo_compile/mod.rs

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

src/cargo/ops/cargo_fetch.rs

@@ -64,9 +64,8 @@ pub fn fetch<'a>(
     }
 
     // If -Zbuild-std was passed, download dependencies for the standard library.
-    if let Some(crates) = gctx.cli_unstable().build_std.as_ref() {
-        let (std_package_set, _, _) =
-            standard_lib::resolve_std(ws, &mut data, &build_config, &crates)?;
+    if gctx.cli_unstable().build_std.is_some() {
+        let (std_package_set, _, _) = standard_lib::resolve_std(ws, &mut data, &build_config)?;
         packages.add_set(std_package_set);
     }
 

tests/testsuite/standard_lib.rs

@@ -389,8 +389,29 @@ fn check_core() {
         .run();
 }
 
-#[cargo_test(build_std_mock)]
-fn test_std_on_unsupported_target() {
+#[cargo_test(build_std_mock, requires = "rustup")]
+fn build_std_with_no_arg_for_core_only_target() {
+    let has_rustup_aarch64_unknown_none = std::process::Command::new("rustup")
+        .args(["target", "list", "--installed"])
+        .output()
+        .ok()
+        .map(|output| {
+            String::from_utf8(output.stdout)
+                .map(|stdout| stdout.contains("aarch64-unknown-none"))
+                .unwrap_or_default()
+        })
+        .unwrap_or_default();
+    if !has_rustup_aarch64_unknown_none {
+        let msg =
+            "to run this test, run `rustup target add aarch64-unknown-none --toolchain nightly`";
+        if cargo_util::is_ci() {
+            panic!("{msg}");
+        } else {
+            eprintln!("{msg}");
+        }
+        return;
+    }
+
     let setup = setup();
 
     let p = project()
@@ -405,13 +426,14 @@ fn test_std_on_unsupported_target() {
         )
         .build();
 
-    p.cargo("build")
+    p.cargo("build -v")
         .arg("--target=aarch64-unknown-none")
-        .arg("--target=x86_64-unknown-none")
         .build_std(&setup)
         .with_status(101)
         .with_stderr_data(str![[r#"
-[ERROR] building std is not supported on the following targets: [..]
+...
+error[E0463]: can't find crate for `std`
+...
 "#]])
         .run();
 }