diff --git a/src/cargo/core/compiler/build_config.rs b/src/cargo/core/compiler/build_config.rs
index e7964afe72b..56e3c5bc177 100644
--- a/src/cargo/core/compiler/build_config.rs
+++ b/src/cargo/core/compiler/build_config.rs
@@ -155,7 +155,8 @@ pub enum CompileMode {
     /// An example or library that will be scraped for function calls by `rustdoc`.
     Docscrape,
     /// A marker for Units that represent the execution of a `build.rs` script.
-    RunCustomBuild,
+    /// `root_mode` tells build script which `cargo` command was used to call it.
+    RunCustomBuild { root_mode: &'static str },
 }
 
 impl ser::Serialize for CompileMode {
@@ -172,7 +173,7 @@ impl ser::Serialize for CompileMode {
             Doc { .. } => "doc".serialize(s),
             Doctest => "doctest".serialize(s),
             Docscrape => "docscrape".serialize(s),
-            RunCustomBuild => "run-custom-build".serialize(s),
+            RunCustomBuild { .. } => "run-custom-build".serialize(s),
         }
     }
 }
@@ -220,7 +221,10 @@ impl CompileMode {
 
     /// Returns `true` if this is the *execution* of a `build.rs` script.
     pub fn is_run_custom_build(self) -> bool {
-        self == CompileMode::RunCustomBuild
+        match self {
+            CompileMode::RunCustomBuild { .. } => true,
+            _ => false,
+        }
     }
 
     /// Returns `true` if this mode may generate an executable.
diff --git a/src/cargo/core/compiler/build_context/target_info.rs b/src/cargo/core/compiler/build_context/target_info.rs
index 6b40d48784f..73c91bf1bb9 100644
--- a/src/cargo/core/compiler/build_context/target_info.rs
+++ b/src/cargo/core/compiler/build_context/target_info.rs
@@ -467,7 +467,7 @@ impl TargetInfo {
             CompileMode::Doc { .. }
             | CompileMode::Doctest
             | CompileMode::Docscrape
-            | CompileMode::RunCustomBuild => {
+            | CompileMode::RunCustomBuild { .. } => {
                 panic!("asked for rustc output for non-rustc mode")
             }
         }
diff --git a/src/cargo/core/compiler/context/compilation_files.rs b/src/cargo/core/compiler/context/compilation_files.rs
index 80a41cdaeff..af1505b9462 100644
--- a/src/cargo/core/compiler/context/compilation_files.rs
+++ b/src/cargo/core/compiler/context/compilation_files.rs
@@ -439,7 +439,7 @@ impl<'a, 'cfg: 'a> CompilationFiles<'a, 'cfg> {
                     flavor: FileFlavor::Normal,
                 }]
             }
-            CompileMode::RunCustomBuild => {
+            CompileMode::RunCustomBuild { .. } => {
                 // At this time, this code path does not handle build script
                 // outputs.
                 vec![]
diff --git a/src/cargo/core/compiler/custom_build.rs b/src/cargo/core/compiler/custom_build.rs
index 2ebec054791..d885ad73eb4 100644
--- a/src/cargo/core/compiler/custom_build.rs
+++ b/src/cargo/core/compiler/custom_build.rs
@@ -3,6 +3,7 @@ use super::{fingerprint, Context, LinkType, Unit};
 use crate::core::compiler::artifact;
 use crate::core::compiler::context::Metadata;
 use crate::core::compiler::job_queue::JobState;
+use crate::core::compiler::CompileMode;
 use crate::core::{profiles::ProfileRoot, PackageId, Target};
 use crate::util::errors::CargoResult;
 use crate::util::machine_message::{self, Message};
@@ -202,6 +203,14 @@ fn build_work(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Job> {
         .env("HOST", &bcx.host_triple())
         .env("RUSTC", &bcx.rustc().path)
         .env("RUSTDOC", &*bcx.config.rustdoc()?)
+        .env(
+            "CARGO_MODE",
+            if let CompileMode::RunCustomBuild { root_mode } = unit.mode {
+                root_mode
+            } else {
+                panic!("Unexpected mode {:?}", unit.mode)
+            },
+        )
         .inherit_jobserver(&cx.jobserver);
 
     // Find all artifact dependencies and make their file and containing directory discoverable using environment variables.
diff --git a/src/cargo/core/compiler/job_queue.rs b/src/cargo/core/compiler/job_queue.rs
index 2d902cca140..1c9f59b6534 100644
--- a/src/cargo/core/compiler/job_queue.rs
+++ b/src/cargo/core/compiler/job_queue.rs
@@ -935,7 +935,7 @@ impl<'cfg> DrainState<'cfg> {
         let target_name = unit.target.name();
         match unit.mode {
             CompileMode::Doc { .. } => format!("{}(doc)", pkg_name),
-            CompileMode::RunCustomBuild => format!("{}(build)", pkg_name),
+            CompileMode::RunCustomBuild { .. } => format!("{}(build)", pkg_name),
             CompileMode::Test | CompileMode::Check { test: true } => match unit.target.kind() {
                 TargetKind::Lib(_) => format!("{}(test)", target_name),
                 TargetKind::CustomBuild => panic!("cannot test build script"),
diff --git a/src/cargo/core/compiler/timings.rs b/src/cargo/core/compiler/timings.rs
index 77dcec94f6d..b925c527134 100644
--- a/src/cargo/core/compiler/timings.rs
+++ b/src/cargo/core/compiler/timings.rs
@@ -167,7 +167,7 @@ impl<'cfg> Timings<'cfg> {
             CompileMode::Doc { .. } => target.push_str(" (doc)"),
             CompileMode::Doctest => target.push_str(" (doc test)"),
             CompileMode::Docscrape => target.push_str(" (doc scrape)"),
-            CompileMode::RunCustomBuild => target.push_str(" (run)"),
+            CompileMode::RunCustomBuild { .. } => target.push_str(" (run)"),
         }
         let unit_time = UnitTime {
             unit,
diff --git a/src/cargo/core/compiler/unit_dependencies.rs b/src/cargo/core/compiler/unit_dependencies.rs
index 857436e9f90..4fa54549797 100644
--- a/src/cargo/core/compiler/unit_dependencies.rs
+++ b/src/cargo/core/compiler/unit_dependencies.rs
@@ -811,6 +811,20 @@ fn dep_build_script(
                 unit_for.is_for_host_features(),
                 unit_for.root_compile_kind(),
             );
+            // Root mode is used to tell build script which cargo command invoked it
+            let root_mode = match unit.mode {
+                CompileMode::Test => "test",
+                CompileMode::Build => "build",
+                CompileMode::Check { .. } => "check",
+                CompileMode::Bench => "bench",
+                CompileMode::Doc { .. } => "doc",
+
+                // FIXME: Are Doctest and Docscrape possible here, and if so, what should the values be?
+                CompileMode::Doctest => "doctest",
+                CompileMode::Docscrape => "docscrape",
+
+                CompileMode::RunCustomBuild { .. } => panic!("Nested {:?}", unit.mode),
+            };
             new_unit_dep_with_profile(
                 state,
                 unit,
@@ -818,7 +832,7 @@ fn dep_build_script(
                 t,
                 script_unit_for,
                 unit.kind,
-                CompileMode::RunCustomBuild,
+                CompileMode::RunCustomBuild { root_mode },
                 profile,
                 IS_NO_ARTIFACT_DEP,
             )
@@ -934,7 +948,7 @@ fn connect_run_custom_build_deps(state: &mut State<'_, '_>) {
         let mut reverse_deps_map = HashMap::new();
         for (unit, deps) in state.unit_dependencies.iter() {
             for dep in deps {
-                if dep.unit.mode == CompileMode::RunCustomBuild {
+                if dep.unit.mode.is_run_custom_build() {
                     reverse_deps_map
                         .entry(dep.unit.clone())
                         .or_insert_with(HashSet::new)
@@ -955,7 +969,7 @@ fn connect_run_custom_build_deps(state: &mut State<'_, '_>) {
         for unit in state
             .unit_dependencies
             .keys()
-            .filter(|k| k.mode == CompileMode::RunCustomBuild)
+            .filter(|k| k.mode.is_run_custom_build())
         {
             // This list of dependencies all depend on `unit`, an execution of
             // the build script.
@@ -1004,7 +1018,7 @@ fn connect_run_custom_build_deps(state: &mut State<'_, '_>) {
                 .filter_map(|other| {
                     state.unit_dependencies[&other.unit]
                         .iter()
-                        .find(|other_dep| other_dep.unit.mode == CompileMode::RunCustomBuild)
+                        .find(|other_dep| other_dep.unit.mode.is_run_custom_build())
                         .cloned()
                 })
                 .collect::<HashSet<_>>();
diff --git a/src/cargo/core/profiles.rs b/src/cargo/core/profiles.rs
index f15f48e3338..9a0ffa372f8 100644
--- a/src/cargo/core/profiles.rs
+++ b/src/cargo/core/profiles.rs
@@ -313,7 +313,9 @@ impl Profiles {
                         )
                     }
                 }
-                CompileMode::Build | CompileMode::Check { .. } | CompileMode::RunCustomBuild => {
+                CompileMode::Build
+                | CompileMode::Check { .. }
+                | CompileMode::RunCustomBuild { .. } => {
                     // Note: `RunCustomBuild` doesn't normally use this code path.
                     // `build_unit_profiles` normally ensures that it selects the
                     // ancestor's profile. However, `cargo clean -p` can hit this
diff --git a/src/cargo/ops/cargo_compile.rs b/src/cargo/ops/cargo_compile.rs
index 3b247c9f7a7..6215ec9e7bf 100644
--- a/src/cargo/ops/cargo_compile.rs
+++ b/src/cargo/ops/cargo_compile.rs
@@ -348,7 +348,7 @@ pub fn create_bcx<'a, 'cfg>(
         | CompileMode::Build
         | CompileMode::Check { .. }
         | CompileMode::Bench
-        | CompileMode::RunCustomBuild => {
+        | CompileMode::RunCustomBuild { .. } => {
             if std::env::var("RUST_FLAGS").is_ok() {
                 config.shell().warn(
                     "Cargo does not read `RUST_FLAGS` environment variable. Did you mean `RUSTFLAGS`?",
@@ -890,7 +890,7 @@ impl CompileFilter {
                     ..
                 } => examples.is_specific() || tests.is_specific() || benches.is_specific(),
             },
-            CompileMode::RunCustomBuild => panic!("Invalid mode"),
+            CompileMode::RunCustomBuild { .. } => panic!("Invalid mode"),
         }
     }
 
@@ -1482,7 +1482,7 @@ fn filter_default_targets(targets: &[Target], mode: CompileMode) -> Vec<&Target>
                 })
                 .collect()
         }
-        CompileMode::Doctest | CompileMode::Docscrape | CompileMode::RunCustomBuild => {
+        CompileMode::Doctest | CompileMode::Docscrape | CompileMode::RunCustomBuild { .. } => {
             panic!("Invalid mode {:?}", mode)
         }
     }