Don't use $CARGO_BUILD_TARGET in cargo metadata

This commit fixes a (five year old!) regression in `cargo metadata`
where if `--filter-platform` isn't explicitly specified it will
accidentally read `$CARGO_BUILD_TARGET` (or `build.target`
configuration) and use that as the default `--filter-platform`. The
reason for this is that the calculation for targets changed in #8167
and while the shared function makes sense for other commands such as
`cargo build` the targets have a different meaning in `cargo metadata`
so a slightly different set of functionality is desired.

This commit fixes the issue by introducing a new constructor for the
list of `CompileKind` variants where the fallback of "if nothing is
specified" is explicitly chosen.

src/cargo/core/compiler/compile_kind.rs

@@ -29,6 +29,20 @@ pub enum CompileKind {
     Target(CompileTarget),
 }
 
+/// Fallback behavior in the
+/// [`CompileKind::from_requested_targets_with_fallback`] function when
+/// no targets are specified.
+pub enum CompileKindFallback {
+    /// The build configuration is consulted to find the default target, such as
+    /// `$CARGO_BUILD_TARGET` or reading `build.target`.
+    BuildConfig,
+
+    /// Only the host should be returned when targets aren't explicitly
+    /// specified. This is used by `cargo metadata` for example where "only
+    /// host" has a special meaning in terms of the returned metadata.
+    JustHost,
+}
+
 impl CompileKind {
     pub fn is_host(&self) -> bool {
         matches!(self, CompileKind::Host)
@@ -53,6 +67,21 @@ impl CompileKind {
     pub fn from_requested_targets(
         gctx: &GlobalContext,
         targets: &[String],
+    ) -> CargoResult<Vec<CompileKind>> {
+        CompileKind::from_requested_targets_with_fallback(
+            gctx,
+            targets,
+            CompileKindFallback::BuildConfig,
+        )
+    }
+
+    /// Same as [`CompileKind::from_requested_targets`] except that if `targets`
+    /// doesn't explicitly mention anything the behavior of what to return is
+    /// controlled by the `fallback` argument.
+    pub fn from_requested_targets_with_fallback(
+        gctx: &GlobalContext,
+        targets: &[String],
+        fallback: CompileKindFallback,
     ) -> CargoResult<Vec<CompileKind>> {
         let dedup = |targets: &[String]| {
             Ok(targets
@@ -70,9 +99,11 @@ impl CompileKind {
             return dedup(targets);
         }
 
-        let kinds = match &gctx.build_config()?.target {
-            None => Ok(vec![CompileKind::Host]),
-            Some(build_target_config) => dedup(&build_target_config.values(gctx)?),
+        let kinds = match (fallback, &gctx.build_config()?.target) {
+            (_, None) | (CompileKindFallback::JustHost, _) => Ok(vec![CompileKind::Host]),
+            (CompileKindFallback::BuildConfig, Some(build_target_config)) => {
+                dedup(&build_target_config.values(gctx)?)
+            }
         };
 
         kinds

src/cargo/core/compiler/mod.rs

@@ -76,7 +76,7 @@ pub use self::build_context::{
 use self::build_plan::BuildPlan;
 pub use self::build_runner::{BuildRunner, Metadata, UnitHash};
 pub use self::compilation::{Compilation, Doctest, UnitOutput};
-pub use self::compile_kind::{CompileKind, CompileTarget};
+pub use self::compile_kind::{CompileKind, CompileKindFallback, CompileTarget};
 pub use self::crate_type::CrateType;
 pub use self::custom_build::LinkArgTarget;
 pub use self::custom_build::{BuildOutput, BuildScriptOutputs, BuildScripts};

src/cargo/ops/cargo_output_metadata.rs

@@ -1,5 +1,5 @@
 use crate::core::compiler::artifact::match_artifacts_kind_with_targets;
-use crate::core::compiler::{CompileKind, RustcTargetData};
+use crate::core::compiler::{CompileKind, CompileKindFallback, RustcTargetData};
 use crate::core::dependency::DepKind;
 use crate::core::package::SerializedPackage;
 use crate::core::resolver::{features::CliFeatures, HasDevUnits, Resolve};
@@ -132,8 +132,16 @@ fn build_resolve_graph(
 ) -> CargoResult<(Vec<SerializedPackage>, MetadataResolve)> {
     // TODO: Without --filter-platform, features are being resolved for `host` only.
     // How should this work?
-    let requested_kinds =
-        CompileKind::from_requested_targets(ws.gctx(), &metadata_opts.filter_platforms)?;
+    //
+    // Otherwise note that "just host" is used as the fallback here if
+    // `filter_platforms` is empty to intentionally avoid reading
+    // `$CARGO_BUILD_TARGET` (or `build.target`) which makes sense for other
+    // subcommands like `cargo build` but does not fit with this command.
+    let requested_kinds = CompileKind::from_requested_targets_with_fallback(
+        ws.gctx(),
+        &metadata_opts.filter_platforms,
+        CompileKindFallback::JustHost,
+    )?;
     let mut target_data = RustcTargetData::new(ws, &requested_kinds)?;
     // Resolve entire workspace.
     let specs = Packages::All(Vec::new()).to_package_id_specs(ws)?;

tests/testsuite/metadata.rs

@@ -4980,8 +4980,8 @@ fn metadata_ignores_build_target_configuration() -> anyhow::Result<()> {
         .env("CARGO_BUILD_TARGET", rustc_host())
         .exec_with_output()?;
     assert!(
-        output1.stdout != output2.stdout,
-        "metadata should change when `CARGO_BUILD_TARGET` is set",
+        output1.stdout == output2.stdout,
+        "metadata should not change when `CARGO_BUILD_TARGET` is set",
     );
     Ok(())
 }