diff --git a/src/cargo/core/package.rs b/src/cargo/core/package.rs
index 36f612d33..c7f84f1e3 100644
--- a/src/cargo/core/package.rs
+++ b/src/cargo/core/package.rs
@@ -442,6 +442,12 @@ impl<'a, 'cfg> Downloads<'a, 'cfg> {
     /// eventually be returned from `wait_for_download`. Returns `Some(pkg)` if
     /// the package is ready and doesn't need to be downloaded.
     pub fn start(&mut self, id: PackageId) -> CargoResult<Option<&'a Package>> {
+        Ok(self
+            .start_inner(id)
+            .chain_err(|| format!("failed to download `{}`", id))?)
+    }
+
+    fn start_inner(&mut self, id: PackageId) -> CargoResult<Option<&'a Package>> {
         // First up see if we've already cached this package, in which case
         // there's nothing to do.
         let slot = self
diff --git a/src/cargo/sources/registry/index.rs b/src/cargo/sources/registry/index.rs
index a593fcc53..9b083463c 100644
--- a/src/cargo/sources/registry/index.rs
+++ b/src/cargo/sources/registry/index.rs
@@ -200,7 +200,6 @@ impl<'cfg> RegistryIndex<'cfg> {
                 ret.reserve(contents.lines().count());
                 let lines = contents.lines().map(|s| s.trim()).filter(|l| !l.is_empty());
 
-                let online = !self.config.cli_unstable().offline;
                 // Attempt forwards-compatibility on the index by ignoring
                 // everything that we ourselves don't understand, that should
                 // allow future cargo implementations to break the
@@ -215,11 +214,7 @@ impl<'cfg> RegistryIndex<'cfg> {
                             return None;
                         }
                     };
-                    if online || load.is_crate_downloaded(summary.package_id()) {
-                        Some((summary, locked))
-                    } else {
-                        None
-                    }
+                    Some((summary, locked))
                 }));
 
                 Ok(())
@@ -274,17 +269,43 @@ impl<'cfg> RegistryIndex<'cfg> {
         yanked_whitelist: &HashSet<PackageId>,
         f: &mut dyn FnMut(Summary),
     ) -> CargoResult<()> {
+        if self.config.cli_unstable().offline {
+            if self.query_inner_with_online(dep, load, yanked_whitelist, f, false)? != 0 {
+                return Ok(());
+            }
+            // If offline, and there are no matches, try again with online.
+            // This is necessary for dependencies that are not used (such as
+            // target-cfg or optional), but are not downloaded. Normally the
+            // build should succeed if they are not downloaded and not used,
+            // but they still need to resolve. If they are actually needed
+            // then cargo will fail to download and an error message
+            // indicating that the required dependency is unavailable while
+            // offline will be displayed.
+        }
+        self.query_inner_with_online(dep, load, yanked_whitelist, f, true)?;
+        Ok(())
+    }
+
+    fn query_inner_with_online(
+        &mut self,
+        dep: &Dependency,
+        load: &mut dyn RegistryData,
+        yanked_whitelist: &HashSet<PackageId>,
+        f: &mut dyn FnMut(Summary),
+        online: bool,
+    ) -> CargoResult<usize> {
         let source_id = self.source_id;
         let name = dep.package_name().as_str();
         let summaries = self.summaries(name, load)?;
         let summaries = summaries
             .iter()
             .filter(|&(summary, yanked)| {
-                !yanked || {
-                    log::debug!("{:?}", yanked_whitelist);
-                    log::debug!("{:?}", summary.package_id());
-                    yanked_whitelist.contains(&summary.package_id())
-                }
+                (online || load.is_crate_downloaded(summary.package_id()))
+                    && (!yanked || {
+                        log::debug!("{:?}", yanked_whitelist);
+                        log::debug!("{:?}", summary.package_id());
+                        yanked_whitelist.contains(&summary.package_id())
+                    })
             })
             .map(|s| s.0.clone());
 
@@ -308,9 +329,11 @@ impl<'cfg> RegistryIndex<'cfg> {
             _ => true,
         });
 
+        let mut count = 0;
         for summary in summaries {
             f(summary);
+            count += 1;
         }
-        Ok(())
+        Ok(count)
     }
 }
diff --git a/tests/testsuite/build.rs b/tests/testsuite/build.rs
index e60bf21c4..cb7d02125 100644
--- a/tests/testsuite/build.rs
+++ b/tests/testsuite/build.rs
@@ -1140,6 +1140,77 @@ fn main(){
         .run();
 }
 
+#[test]
+fn offline_unused_target_dep() {
+    // -Z offline with a target dependency that is not used and not downloaded.
+    Package::new("unused_dep", "1.0.0").publish();
+    Package::new("used_dep", "1.0.0").publish();
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [project]
+            name = "foo"
+            version = "0.1.0"
+            [dependencies]
+            used_dep = "1.0"
+            [target.'cfg(unused)'.dependencies]
+            unused_dep = "1.0"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+    // Do a build that downloads only what is necessary.
+    p.cargo("build")
+        .with_stderr_contains("[DOWNLOADED] used_dep [..]")
+        .with_stderr_does_not_contain("[DOWNLOADED] unused_dep [..]")
+        .run();
+    p.cargo("clean").run();
+    // Build offline, make sure it works.
+    p.cargo("build -Z offline")
+        .masquerade_as_nightly_cargo()
+        .run();
+}
+
+#[test]
+fn offline_missing_optional() {
+    Package::new("opt_dep", "1.0.0").publish();
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [project]
+            name = "foo"
+            version = "0.1.0"
+            [dependencies]
+            opt_dep = { version = "1.0", optional = true }
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+    // Do a build that downloads only what is necessary.
+    p.cargo("build")
+        .with_stderr_does_not_contain("[DOWNLOADED] opt_dep [..]")
+        .run();
+    p.cargo("clean").run();
+    // Build offline, make sure it works.
+    p.cargo("build -Z offline")
+        .masquerade_as_nightly_cargo()
+        .run();
+    p.cargo("build -Z offline --features=opt_dep")
+        .masquerade_as_nightly_cargo()
+        .with_stderr(
+            "\
+[ERROR] failed to download `opt_dep v1.0.0`
+
+Caused by:
+  can't make HTTP request in the offline mode
+",
+        )
+        .with_status(101)
+        .run();
+}
+
 #[test]
 fn incompatible_dependencies() {
     Package::new("bad", "0.1.0").publish();
@@ -1282,14 +1353,11 @@ fn compile_offline_while_transitive_dep_not_cached() {
         .with_status(101)
         .with_stderr(
             "\
-error: no matching package named `baz` found
-location searched: registry `[..]`
-required by package `bar v0.1.0`
-    ... which is depended on by `foo v0.0.1 ([CWD])`
-As a reminder, you're using offline mode (-Z offline) \
-which can sometimes cause surprising resolution failures, \
-if this error is too confusing you may wish to retry \
-without the offline flag.",
+[ERROR] failed to download `baz v1.0.0`
+
+Caused by:
+  can't make HTTP request in the offline mode
+",
         )
         .run();
 }
diff --git a/tests/testsuite/support/registry.rs b/tests/testsuite/support/registry.rs
index 2c302ce99..cd8e8e3b1 100644
--- a/tests/testsuite/support/registry.rs
+++ b/tests/testsuite/support/registry.rs
@@ -126,6 +126,7 @@ pub fn alt_api_url() -> Url {
 ///
 /// p.cargo("run").with_stdout("24").run();
 /// ```
+#[must_use]
 pub struct Package {
     name: String,
     vers: String,