error message for target specification that couldn't be queried from …

…rustc

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

@@ -938,10 +938,22 @@ impl<'cfg> RustcTargetData<'cfg> {
     }
 
     /// Information about the given target platform, learned by querying rustc.
+    ///
+    /// # Panics
+    ///
+    /// Panics, if the target platform described by `kind` can't be found.
+    /// See [`get_info`](Self::get_info) for a non-panicking alternative.
     pub fn info(&self, kind: CompileKind) -> &TargetInfo {
+        self.get_info(kind).unwrap()
+    }
+
+    /// Information about the given target platform, learned by querying rustc.
+    ///
+    /// Returns `None` if the target platform described by `kind` can't be found.
+    pub fn get_info(&self, kind: CompileKind) -> Option<&TargetInfo> {
         match kind {
-            CompileKind::Host => &self.host_info,
-            CompileKind::Target(s) => &self.target_info[&s],
+            CompileKind::Host => Some(&self.host_info),
+            CompileKind::Target(s) => self.target_info.get(&s),
         }
     }
 

src/cargo/core/compiler/compilation.rs

@@ -129,16 +129,7 @@ impl<'cfg> Compilation<'cfg> {
                 .info(CompileKind::Host)
                 .sysroot_host_libdir
                 .clone(),
-            sysroot_target_libdir: bcx
-                .all_kinds
-                .iter()
-                .map(|&kind| {
-                    (
-                        kind,
-                        bcx.target_data.info(kind).sysroot_target_libdir.clone(),
-                    )
-                })
-                .collect(),
+            sysroot_target_libdir: get_sysroot_target_libdir(bcx)?,
             tests: Vec::new(),
             binaries: Vec::new(),
             cdylibs: Vec::new(),
@@ -384,6 +375,48 @@ fn fill_rustc_tool_env(mut cmd: ProcessBuilder, unit: &Unit) -> ProcessBuilder {
     cmd
 }
 
+fn get_sysroot_target_libdir(
+    bcx: &BuildContext<'_, '_>,
+) -> CargoResult<HashMap<CompileKind, PathBuf>> {
+    bcx.all_kinds
+        .iter()
+        .map(|&kind| {
+            Ok((
+                kind,
+                bcx.target_data
+                    .get_info(kind)
+                    .ok_or_else(|| {
+                        let target = match kind {
+                            CompileKind::Host => "host".to_owned(),
+                            CompileKind::Target(s) => s.short_name().to_owned(),
+                        };
+
+                        let dependency = bcx
+                            .unit_graph
+                            .iter()
+                            .find_map(|(k, _)| {
+                                if k.kind == kind {
+                                    let summary = k.pkg.manifest().summary();
+
+                                    Some(format!("{} v{}", summary.name(), summary.version()))
+                                } else {
+                                    None
+                                }
+                            })
+                            .unwrap();
+
+                        anyhow::anyhow!(
+                            "could not find specification for target \"{target}\".\n  \
+                            Dependency `{dependency}` requires to build for target \"{target}\"."
+                        )
+                    })?
+                    .sysroot_target_libdir
+                    .clone(),
+            ))
+        })
+        .collect::<CargoResult<HashMap<_, _>>>()
+}
+
 fn target_runner(
     bcx: &BuildContext<'_, '_>,
     kind: CompileKind,

tests/testsuite/artifact_dep.rs

@@ -2880,7 +2880,7 @@ fn check_transitive_artifact_dependency_with_different_target() {
                 version = "0.0.0"
 
                 [dependencies]
-                baz = { path = "baz/", artifact = "bin", target = "custom" }
+                baz = { path = "baz/", artifact = "bin", target = "custom-target" }
             "#,
         )
         .file("bar/src/lib.rs", "")
@@ -2899,7 +2899,10 @@ fn check_transitive_artifact_dependency_with_different_target() {
 
     p.cargo("check -Z bindeps")
         .masquerade_as_nightly_cargo(&["bindeps"])
-        .with_stderr_contains(r#"thread 'main' panicked at 'no entry found for key'[..]"#)
+        .with_stderr_contains(
+            "error: could not find specification for target \"custom-target\".\n  \
+            Dependency `baz v0.0.0` requires to build for target \"custom-target\".",
+        )
         .with_status(101)
         .run();
 }