Check build target supports std when building with -Zbuild-std=std

Running cargo with "-Zbuild-std=std" should check whether the build
target supports building the standard library. This information can be
obtained from rustc with the target-spec-json "--print" option. When
'std' is false for the build target, cargo should not attempt to build
the standard library.

This avoids the "use of unstable library" errors, as this check is
performed before Cargo starts trying to build restricted_std code.

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

@@ -44,6 +44,8 @@ pub struct TargetInfo {
     crate_types: RefCell<HashMap<CrateType, Option<(String, String)>>>,
     /// `cfg` information extracted from `rustc --print=cfg`.
     cfg: Vec<Cfg>,
+    /// `target_spec` information extracted from `rustc --print=target-spec-json`
+    pub target_spec: TargetSpec,
     /// Supported values for `-Csplit-debuginfo=` flag, queried from rustc
     support_split_debuginfo: Vec<String>,
     /// Path to the sysroot.
@@ -62,6 +64,16 @@ pub struct TargetInfo {
     pub support_check_cfg: bool,
 }
 
+#[derive(Deserialize, Clone)]
+pub struct Metadata {
+    pub std: Option<bool>,
+}
+
+#[derive(Deserialize, Clone)]
+pub struct TargetSpec {
+    pub metadata: Metadata,
+}
+
 /// Kind of each file generated by a Unit, part of `FileType`.
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub enum FileFlavor {
@@ -175,9 +187,11 @@ impl TargetInfo {
             let mut process = rustc.workspace_process();
             apply_env_config(gctx, &mut process)?;
             process
+                .env("RUSTC_BOOTSTRAP", &"1".to_string())
                 .arg("-")
                 .arg("--crate-name")
                 .arg("___")
+                .arg("-Zunstable-options")
                 .arg("--print=file-names")
                 .args(&rustflags)
                 .env_remove("RUSTC_LOG");
@@ -215,6 +229,7 @@ impl TargetInfo {
             process.arg("--print=sysroot");
             process.arg("--print=split-debuginfo");
             process.arg("--print=crate-name"); // `___` as a delimiter.
+            process.arg("--print=target-spec-json");
             process.arg("--print=cfg");
 
             let (output, error) = rustc
@@ -266,6 +281,36 @@ impl TargetInfo {
                 res
             };
 
+            let target_spec_json = {
+                let mut res: String = String::new();
+                loop {
+                    match lines.next() {
+                        Some(line) if line == "}" => {
+                            res.push_str("}");
+                            break;
+                        }
+                        Some(line) => res.push_str(line.into()),
+                        None => {
+                            return error_missing_print_output(
+                                "target-spec-json",
+                                &process,
+                                &output,
+                                &error,
+                            )
+                        }
+                    }
+                }
+                res
+            };
+
+            let target_spec: TargetSpec = serde_json::from_str(&target_spec_json.as_str())
+                .with_context(|| {
+                    format!(
+                    "failed to parse target spec from `rustc --print=target-spec-json`, got:\n{}",
+                    output
+                )
+                })?;
+
             let cfg = lines
                 .map(|line| Ok(Cfg::from_str(line)?))
                 .filter(TargetInfo::not_user_specific_cfg)
@@ -322,6 +367,7 @@ impl TargetInfo {
                     Flags::Rustdoc,
                 )?,
                 cfg,
+                target_spec,
                 support_split_debuginfo,
                 support_check_cfg,
             });
@@ -1026,6 +1072,11 @@ impl<'gctx> RustcTargetData<'gctx> {
     pub fn script_override(&self, lib_name: &str, kind: CompileKind) -> Option<&BuildOutput> {
         self.target_config(kind).links_overrides.get(lib_name)
     }
+
+    /// Gets the target info hashmap from the target data.
+    pub fn target_info(&self) -> &HashMap<CompileTarget, TargetInfo> {
+        &self.target_info
+    }
 }
 
 /// Structure used to deal with Rustdoc fingerprinting

src/cargo/core/compiler/standard_lib.rs

@@ -73,6 +73,18 @@ pub fn resolve_std<'gctx>(
             .warn("-Zbuild-std does not currently fully support --build-plan")?;
     }
 
+    // check that targets support building std
+    if crates.contains(&"std".to_string()) {
+        for (target, target_info) in target_data.target_info() {
+            if target_info.target_spec.metadata.std == Some(false) {
+                return Err(anyhow::Error::msg(format!(
+                    "building std is not supported on this target: {:?}",
+                    target.short_name()
+                )));
+            }
+        }
+    }
+
     let src_path = detect_sysroot_src_path(target_data)?;
     let to_patch = [
         "rustc-std-workspace-core",