Pass compile mode to the build script

Closes rust-lang#4001

This PR adds code to track and pass compile mode to the build scripts. Since the unit's mode is always `RunCustomBuild`, I had to add tracking for the parent's compile mode, and pass it around.

  - [ ] env var naming. I used `CARGO_MODE`, but perhaps another name is better?
  - [ ] env var values naming. I used: `Test`, `Build`, `Check_test/Check`, `Bench`, `Doc_with_deps/Doc`, `Doctest`, `Docscrape`, `RunCustomBuild`
  - [ ] how to represent bool values for `Check` and `Doc`. I created two separate values, but perhaps the `test` and `deps` values should be ignored?
  - [ ] figure out why `cargo bench` sets env var to `Test`
  - [ ] add unit tests

src/cargo/core/compiler/custom_build.rs

@@ -1,5 +1,6 @@
 use super::job::{Freshness, Job, Work};
 use super::{fingerprint, Context, LinkType, Unit};
+use crate::core::compiler::CompileMode;
 use crate::core::compiler::context::Metadata;
 use crate::core::compiler::job_queue::JobState;
 use crate::core::{profiles::ProfileRoot, PackageId, Target};
@@ -201,6 +202,19 @@ 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",
+            match unit.root_mode {
+                CompileMode::Test => "Test",
+                CompileMode::Build => "Build",
+                CompileMode::Check { test } => if test { "Check_test" } else { "Check" },
+                CompileMode::Bench => "Bench",
+                CompileMode::Doc { deps }  => if deps { "Doc_with_deps" } else { "Doc" },
+                CompileMode::Doctest => "Doctest",
+                CompileMode::Docscrape => "Docscrape",
+                CompileMode::RunCustomBuild => "RunCustomBuild",
+            },
+        )
         .inherit_jobserver(&cx.jobserver);
 
     if let Some(linker) = &bcx.target_data.target_config(unit.kind).linker {

src/cargo/core/compiler/standard_lib.rs

@@ -176,6 +176,7 @@ pub fn generate_std_roots(
                 profile,
                 *kind,
                 mode,
+                mode,
                 features.clone(),
                 /*is_std*/ true,
                 /*dep_hash*/ 0,

src/cargo/core/compiler/unit.rs

@@ -52,6 +52,8 @@ pub struct UnitInner {
     pub kind: CompileKind,
     /// The "mode" this unit is being compiled for. See [`CompileMode`] for more details.
     pub mode: CompileMode,
+    /// The "mode" of the root unit. Required when unit's mode is `CompileMode::RunCustomBuild`
+    pub root_mode: CompileMode,
     /// The `cfg` features to enable for this unit.
     /// This must be sorted.
     pub features: Vec<InternedString>,
@@ -176,6 +178,7 @@ impl UnitInterner {
         profile: Profile,
         kind: CompileKind,
         mode: CompileMode,
+        root_mode: CompileMode,
         features: Vec<InternedString>,
         is_std: bool,
         dep_hash: u64,
@@ -207,6 +210,7 @@ impl UnitInterner {
             profile,
             kind,
             mode,
+            root_mode,
             features,
             is_std,
             dep_hash,

src/cargo/core/compiler/unit_dependencies.rs

@@ -641,7 +641,7 @@ fn new_unit_dep_with_profile(
     let features = state.activated_features(pkg.package_id(), features_for);
     let unit = state
         .interner
-        .intern(pkg, target, profile, kind, mode, features, state.is_std, 0);
+        .intern(pkg, target, profile, kind, mode, parent.mode, features, state.is_std, 0);
     Ok(UnitDep {
         unit,
         unit_for,

src/cargo/ops/cargo_compile.rs

@@ -1048,6 +1048,7 @@ fn generate_targets(
                     profile,
                     kind.for_target(target),
                     target_mode,
+                    target_mode,
                     features.clone(),
                     /*is_std*/ false,
                     /*dep_hash*/ 0,
@@ -1609,6 +1610,7 @@ fn traverse_and_share(
         unit.profile,
         new_kind,
         unit.mode,
+        unit.root_mode,
         unit.features.clone(),
         unit.is_std,
         new_dep_hash,