diff --git a/src/cargo/core/compiler/mod.rs b/src/cargo/core/compiler/mod.rs
index 79d8c47abed..0fc9a8145d9 100644
--- a/src/cargo/core/compiler/mod.rs
+++ b/src/cargo/core/compiler/mod.rs
@@ -530,6 +530,11 @@ fn prepare_rustc(
     let mut base = cx
         .compilation
         .rustc_process(unit, is_primary, is_workspace)?;
+
+    if is_primary {
+        base.env("CARGO_PRIMARY_PACKAGE", "1");
+    }
+
     if cx.bcx.config.cli_unstable().jobserver_per_rustc {
         let client = cx.new_jobserver()?;
         base.inherit_jobserver(&client);
diff --git a/src/doc/src/reference/environment-variables.md b/src/doc/src/reference/environment-variables.md
index eec0d0aac04..4e9f36bc20e 100644
--- a/src/doc/src/reference/environment-variables.md
+++ b/src/doc/src/reference/environment-variables.md
@@ -265,6 +265,12 @@ let out_dir = env::var("OUT_DIR").unwrap();
                       Currently Cargo doesn't set the `MAKEFLAGS` variable,
                       but it's free for build scripts invoking GNU Make
                       to set it to the contents of `CARGO_MAKEFLAGS`.
+* `CARGO_PRIMARY_PACKAGE` — This environment variable will be set if the package being
+                            built is primary. Primary packages are the ones the user 
+                            selected on the command-line, either with `-p` flags or 
+                            the defaults based on the current directory and the default 
+                            workspace members.
+                            This environment variable will not be set when building dependencies.
 * `CARGO_FEATURE_<name>` — For each activated feature of the package being
                            built, this environment variable will be present
                            where `<name>` is the name of the feature uppercased
diff --git a/tests/testsuite/build.rs b/tests/testsuite/build.rs
index 268ddbdf36f..a350286d55c 100644
--- a/tests/testsuite/build.rs
+++ b/tests/testsuite/build.rs
@@ -10,7 +10,7 @@ use cargo::{
 use cargo_test_support::paths::{root, CargoPathExt};
 use cargo_test_support::registry::Package;
 use cargo_test_support::{
-    basic_bin_manifest, basic_lib_manifest, basic_manifest, is_nightly, lines_match_unordered,
+    basic_bin_manifest, basic_lib_manifest, basic_manifest, git, is_nightly, lines_match_unordered,
     main_file, paths, project, rustc_host, sleep_ms, symlink_supported, t, Execs, ProjectBuilder,
 };
 use std::env;
@@ -5179,3 +5179,68 @@ fn build_script_o0_default_even_with_release() {
         .with_stderr_does_not_contain("[..]build_script_build[..]opt-level[..]")
         .run();
 }
+
+#[cargo_test]
+fn primary_package_env_var() {
+    // Test that CARGO_PRIMARY_PACKAGE is enabled only for "foo" and not for any dependency.
+
+    let is_primary_package = r#"
+        pub fn is_primary_package() -> bool {{
+            option_env!("CARGO_PRIMARY_PACKAGE").is_some()
+        }}
+    "#;
+
+    Package::new("qux", "0.1.0")
+        .file("src/lib.rs", is_primary_package)
+        .publish();
+
+    let baz = git::new("baz", |project| {
+        project
+            .file("Cargo.toml", &basic_manifest("baz", "0.1.0"))
+            .file("src/lib.rs", is_primary_package)
+    });
+
+    let foo = project()
+        .file(
+            "Cargo.toml",
+            &format!(
+                r#"
+                    [package]
+                    name = "foo"
+                    version = "0.1.0"
+
+                    [dependencies]
+                    bar = {{ path = "bar" }}
+                    baz = {{ git = '{}' }}
+                    qux = "0.1"
+                "#,
+                baz.url()
+            ),
+        )
+        .file(
+            "src/lib.rs",
+            &format!(
+                r#"
+                    extern crate bar;
+                    extern crate baz;
+                    extern crate qux;
+
+                    {}
+
+                    #[test]
+                    fn verify_primary_package() {{
+                        assert!(!bar::is_primary_package());
+                        assert!(!baz::is_primary_package());
+                        assert!(!qux::is_primary_package());
+                        assert!(is_primary_package());
+                    }}
+                "#,
+                is_primary_package
+            ),
+        )
+        .file("bar/Cargo.toml", &basic_manifest("bar", "0.1.0"))
+        .file("bar/src/lib.rs", is_primary_package)
+        .build();
+
+    foo.cargo("test").run();
+}
diff --git a/tests/testsuite/workspaces.rs b/tests/testsuite/workspaces.rs
index c9ca991708d..a20ef11ba70 100644
--- a/tests/testsuite/workspaces.rs
+++ b/tests/testsuite/workspaces.rs
@@ -2319,3 +2319,80 @@ Caused by:
         )
         .run();
 }
+
+#[cargo_test]
+fn simple_primary_package_env_var() {
+    let is_primary_package = r#"
+        #[test]
+        fn verify_primary_package() {{
+            assert!(option_env!("CARGO_PRIMARY_PACKAGE").is_some());
+        }}
+    "#;
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [project]
+                name = "foo"
+                version = "0.1.0"
+                authors = []
+
+                [workspace]
+                members = ["bar"]
+            "#,
+        )
+        .file("src/lib.rs", is_primary_package)
+        .file(
+            "bar/Cargo.toml",
+            r#"
+                [project]
+                name = "bar"
+                version = "0.1.0"
+                authors = []
+                workspace = ".."
+            "#,
+        )
+        .file("bar/src/lib.rs", is_primary_package);
+    let p = p.build();
+
+    p.cargo("test").run();
+
+    // Again, this time selecting a specific crate
+    p.cargo("clean").run();
+    p.cargo("test -p bar").run();
+
+    // Again, this time selecting all crates
+    p.cargo("clean").run();
+    p.cargo("test --all").run();
+}
+
+#[cargo_test]
+fn virtual_primary_package_env_var() {
+    let is_primary_package = r#"
+        #[test]
+        fn verify_primary_package() {{
+            assert!(option_env!("CARGO_PRIMARY_PACKAGE").is_some());
+        }}
+    "#;
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [workspace]
+                members = ["foo", "bar"]
+            "#,
+        )
+        .file("foo/Cargo.toml", &basic_manifest("foo", "0.1.0"))
+        .file("foo/src/lib.rs", is_primary_package)
+        .file("bar/Cargo.toml", &basic_manifest("bar", "0.1.0"))
+        .file("bar/src/lib.rs", is_primary_package);
+    let p = p.build();
+
+    p.cargo("test").run();
+
+    // Again, this time selecting a specific crate
+    p.cargo("clean").run();
+    p.cargo("test -p foo").run();
+}