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

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",

tests/testsuite/artifact_dep.rs

@@ -149,7 +149,7 @@ fn check_with_invalid_target_triple() {
 [ERROR] failed to run `rustc` to learn about target-specific information
 
 Caused by:
-  process didn't exit successfully: `rustc - --crate-name ___ --print=file-names --target unknown-target-triple [..]` ([EXIT_STATUS]: 1)
+  process didn't exit successfully: `rustc - --crate-name ___ -Zunstable-options --print=file-names --target unknown-target-triple [..]` ([EXIT_STATUS]: 1)
   --- stderr
   [ERROR] Error loading target specification: Could not find specification for target "unknown-target-triple". Run `rustc --print target-list` for a list of built-in targets
 

tests/testsuite/bad_config.rs

@@ -425,7 +425,7 @@ fn bad_crate_type() {
 [ERROR] failed to run `rustc` to learn about crate-type bad_type information
 
 Caused by:
-  process didn't exit successfully: `rustc - --crate-name ___ --print=file-names --crate-type bad_type` ([EXIT_STATUS]: 1)
+  process didn't exit successfully: `rustc - --crate-name ___ -Zunstable-options --print=file-names --crate-type bad_type` ([EXIT_STATUS]: 1)
   --- stderr
   [ERROR] unknown crate type: `bad_type`
 

tests/testsuite/build.rs

@@ -5199,7 +5199,7 @@ fn rustc_wrapper_queries() {
         .env("RUSTC_WRAPPER", &wrapper)
         .with_stderr_contains("[..]running [..]rustc-echo-wrapper[EXE] rustc -vV[..]")
         .with_stderr_contains(
-            "[..]running [..]rustc-echo-wrapper[EXE] rustc - --crate-name ___ --print[..]",
+            "[..]running [..]rustc-echo-wrapper[EXE] rustc - --crate-name ___ -Zunstable-options --print[..]",
         )
         .run();
     p.build_dir().rm_rf();
@@ -5208,7 +5208,7 @@ fn rustc_wrapper_queries() {
         .env("RUSTC_WORKSPACE_WRAPPER", &wrapper)
         .with_stderr_contains("[..]running [..]rustc-echo-wrapper[EXE] rustc -vV[..]")
         .with_stderr_contains(
-            "[..]running [..]rustc-echo-wrapper[EXE] rustc - --crate-name ___ --print[..]",
+            "[..]running [..]rustc-echo-wrapper[EXE] rustc - --crate-name ___ -Zunstable-options --print[..]",
         )
         .run();
 }

tests/testsuite/cfg.rs

@@ -387,6 +387,17 @@ fn bad_cfg_discovery() {
                     }
                 };
 
+                if mode == "no-target-spec-json" {
+                    return;
+                }
+                loop {
+                    let line = lines.next().unwrap();
+                    println!("{line}");
+                    if line == "}" {
+                        break;
+                    }
+                }
+
                 if mode != "bad-cfg" {
                     panic!("unexpected");
                 }
@@ -471,10 +482,34 @@ command was: `[..]compiler[..]--crate-type [..]`
 
     p.cargo("check")
         .env("RUSTC", &funky_rustc)
-        .env("FUNKY_MODE", "bad-cfg")
+        .env("FUNKY_MODE", "no-target-spec-json")
         .with_status(101)
         .with_stderr(
             "\
+[ERROR] output of --print=target-spec-json missing when learning about target-specific information from rustc
+command was: `[..]compiler[..]--crate-type [..]`
+
+--- stdout
+[..]___[..]
+[..]___[..]
+[..]___[..]
+[..]___[..]
+[..]___[..]
+[..]___[..]
+[..]
+[..]
+___
+
+",
+        )
+        .run();
+
+    p.cargo("check")
+        .env("RUSTC", &funky_rustc)
+        .env("FUNKY_MODE", "bad-cfg")
+        .with_status(101)
+        .with_stderr_contains(
+            "\
 [ERROR] failed to parse the cfg from `rustc --print=cfg`, got:
 [..]___[..]
 [..]___[..]
@@ -483,8 +518,13 @@ command was: `[..]compiler[..]--crate-type [..]`
 [..]___[..]
 [..]___[..]
 [..]
-[..],[..]
+[..]
 ___
+{
+",
+        )
+        .with_stderr_contains(
+            "\
 123
 
 

tests/testsuite/standard_lib.rs

@@ -162,6 +162,7 @@ trait BuildStd: Sized {
     fn build_std(&mut self, setup: &Setup) -> &mut Self;
     fn build_std_arg(&mut self, setup: &Setup, arg: &str) -> &mut Self;
     fn target_host(&mut self) -> &mut Self;
+    fn target(&mut self, target: &str) -> &mut Self;
 }
 
 impl BuildStd for Execs {
@@ -181,6 +182,11 @@ impl BuildStd for Execs {
         self.arg("--target").arg(rustc_host());
         self
     }
+
+    fn target(&mut self, target: &str) -> &mut Self {
+        self.arg("--target").arg(target);
+        self
+    }
 }
 
 #[cargo_test(build_std_mock)]
@@ -321,6 +327,33 @@ fn check_core() {
         .run();
 }
 
+#[cargo_test(build_std_mock)]
+fn test_std_on_unsupported_target() {
+    let setup = setup();
+
+    let p = project()
+        .file(
+            "src/main.rs",
+            r#"
+        fn main() {
+            println!("hello");
+        }
+        "#,
+        )
+        .build();
+
+    p.cargo("build")
+        .build_std(&setup)
+        .target("aarch64-unknown-none")
+        .with_status(101)
+        .with_stderr_data(
+            "\
+error: building std is not supported on this target: [..]
+",
+        )
+        .run();
+}
+
 #[cargo_test(build_std_mock)]
 fn depend_same_as_std() {
     let setup = setup();