Check build target supports std when building with -Zbuild-std=std (#…

…14183)

**What does this PR try to resolve?**

Ensures that Cargo first verifies whether a given target supports
building the standard library when the `-Zbuild-std=std` option is
passed to Cargo ([see issue
here](rust-lang/wg-cargo-std-aware#87)). This
information can be obtained by querying `rustc
--print=target-spec-json`. The target spec "metadata" contains an
`Option<bool>` determining whether the target supports building std.

In the case this value is `false`, cargo will stop the build. This
avoids the numerous "use of unstable library" errors, giving a cleaner,
and simpler, "building std is not supported on this target".

**How should we test and review this PR?**

It can be manually tested by running `cargo build --target <target>
-Zbuild-std=std`. If a target who's target-spec marks std as false is
passed, cargo will exit. This works with multiple `--target`'s, and if
any of them don't support std, cargo will exit.

**Additional Information**

This change relies on two things:
* The target-spec metadata in rustc needs to be filled out. Currently,
most fields are None, which is treated as OK with this change. Meaning
this can be merged before the rustc changes.
* The new test case added with this change will fail without at least
`aarch64-unknown-none` having it's target-spec metadata completed.
* Some targets have std support marked as "?". If this state is properly
represented in the target-spec in the future, it needs to be determined
whether this is allowed, so the build can continue, or whether it fails.

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

@@ -43,6 +43,8 @@ pub struct TargetInfo {
     crate_types: RefCell<HashMap<CrateType, Option<(String, String)>>>,
     /// `cfg` information extracted from `rustc --print=cfg`.
     cfg: Vec<Cfg>,
+    /// `supports_std` information extracted from `rustc --print=target-spec-json`
+    pub supports_std: Option<bool>,
     /// Supported values for `-Csplit-debuginfo=` flag, queried from rustc
     support_split_debuginfo: Vec<String>,
     /// Path to the sysroot.
@@ -294,6 +296,44 @@ impl TargetInfo {
                 gctx.shell().warn("non-trivial mutual dependency between target-specific configuration and RUSTFLAGS")?;
             }
 
+            let mut supports_std: Option<bool> = None;
+
+            // The '--print=target-spec-json' is an unstable option of rustc, therefore only
+            // try to fetch this information if rustc allows nightly features. Additionally,
+            // to avoid making two rustc queries when not required, only try to fetch the
+            // target-spec when the '-Zbuild-std' option is passed.
+            if gctx.cli_unstable().build_std.is_some() {
+                let mut target_spec_process = rustc.workspace_process();
+                apply_env_config(gctx, &mut target_spec_process)?;
+                target_spec_process
+                    .arg("--print=target-spec-json")
+                    .arg("-Zunstable-options")
+                    .args(&rustflags)
+                    .env_remove("RUSTC_LOG");
+
+                if let CompileKind::Target(target) = kind {
+                    target_spec_process
+                        .arg("--target")
+                        .arg(target.rustc_target());
+                }
+
+                #[derive(Deserialize)]
+                struct Metadata {
+                    pub std: Option<bool>,
+                }
+
+                #[derive(Deserialize)]
+                struct TargetSpec {
+                    pub metadata: Metadata,
+                }
+
+                if let Ok(output) = target_spec_process.output() {
+                    if let Ok(spec) = serde_json::from_slice::<TargetSpec>(&output.stdout) {
+                        supports_std = spec.metadata.std;
+                    }
+                }
+            }
+
             return Ok(TargetInfo {
                 crate_type_process,
                 crate_types: RefCell::new(map),
@@ -310,6 +350,7 @@ impl TargetInfo {
                 )?
                 .into(),
                 cfg,
+                supports_std,
                 support_split_debuginfo,
             });
         }
@@ -1026,6 +1067,16 @@ impl<'gctx> RustcTargetData<'gctx> {
             CompileKind::Target(s) => &self.target_config[&s],
         }
     }
+
+    pub fn get_unsupported_std_targets(&self) -> Vec<&str> {
+        let mut unsupported = Vec::new();
+        for (target, target_info) in &self.target_info {
+            if target_info.supports_std == Some(false) {
+                unsupported.push(target.short_name());
+            }
+        }
+        unsupported
+    }
 }
 
 /// Structure used to deal with Rustdoc fingerprinting

src/cargo/core/compiler/standard_lib.rs

@@ -72,6 +72,17 @@ 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()) {
+        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();

tests/testsuite/standard_lib.rs

@@ -389,6 +389,32 @@ 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")
+        .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: [..]
+"#]])
+        .run();
+}
+
 #[cargo_test(build_std_mock)]
 fn depend_same_as_std() {
     let setup = setup();