diff --git a/src/bootstrap/CHANGELOG.md b/src/bootstrap/CHANGELOG.md
index fe426c4cec768..c9c1f90277a9e 100644
--- a/src/bootstrap/CHANGELOG.md
+++ b/src/bootstrap/CHANGELOG.md
@@ -6,7 +6,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## [Non-breaking changes since the last major version]
 
-None.
+- x.py check needs opt-in to check tests (--all-targets) [#77473](https://github.com/rust-lang/rust/pull/77473)
 
 ## [Version 2] - 2020-09-25
 
diff --git a/src/bootstrap/builder.rs b/src/bootstrap/builder.rs
index 4beeb9c87c4fd..6856cd338cfad 100644
--- a/src/bootstrap/builder.rs
+++ b/src/bootstrap/builder.rs
@@ -532,7 +532,7 @@ impl<'a> Builder<'a> {
     pub fn new(build: &Build) -> Builder<'_> {
         let (kind, paths) = match build.config.cmd {
             Subcommand::Build { ref paths } => (Kind::Build, &paths[..]),
-            Subcommand::Check { ref paths } => (Kind::Check, &paths[..]),
+            Subcommand::Check { ref paths, all_targets: _ } => (Kind::Check, &paths[..]),
             Subcommand::Clippy { ref paths } => (Kind::Clippy, &paths[..]),
             Subcommand::Fix { ref paths } => (Kind::Fix, &paths[..]),
             Subcommand::Doc { ref paths, .. } => (Kind::Doc, &paths[..]),
diff --git a/src/bootstrap/check.rs b/src/bootstrap/check.rs
index ead0bd0413b9c..371631154f72d 100644
--- a/src/bootstrap/check.rs
+++ b/src/bootstrap/check.rs
@@ -1,9 +1,12 @@
 //! Implementation of compiling the compiler and standard library, in "check"-based modes.
 
-use crate::builder::{Builder, Kind, RunConfig, ShouldRun, Step};
 use crate::compile::{add_to_sysroot, run_cargo, rustc_cargo, std_cargo};
 use crate::config::TargetSelection;
 use crate::tool::{prepare_tool_cargo, SourceType};
+use crate::{
+    builder::{Builder, Kind, RunConfig, ShouldRun, Step},
+    Subcommand,
+};
 use crate::{Compiler, Mode};
 use std::path::PathBuf;
 
@@ -74,35 +77,37 @@ impl Step for Std {
         //
         // Currently only the "libtest" tree of crates does this.
 
-        let mut cargo = builder.cargo(
-            compiler,
-            Mode::Std,
-            SourceType::InTree,
-            target,
-            cargo_subcommand(builder.kind),
-        );
-        std_cargo(builder, target, compiler.stage, &mut cargo);
-        cargo.arg("--all-targets");
+        if let Subcommand::Check { all_targets: true, .. } = builder.config.cmd {
+            let mut cargo = builder.cargo(
+                compiler,
+                Mode::Std,
+                SourceType::InTree,
+                target,
+                cargo_subcommand(builder.kind),
+            );
+            std_cargo(builder, target, compiler.stage, &mut cargo);
+            cargo.arg("--all-targets");
+
+            // Explicitly pass -p for all dependencies krates -- this will force cargo
+            // to also check the tests/benches/examples for these crates, rather
+            // than just the leaf crate.
+            for krate in builder.in_tree_crates("test") {
+                cargo.arg("-p").arg(krate.name);
+            }
 
-        // Explicitly pass -p for all dependencies krates -- this will force cargo
-        // to also check the tests/benches/examples for these crates, rather
-        // than just the leaf crate.
-        for krate in builder.in_tree_crates("test") {
-            cargo.arg("-p").arg(krate.name);
+            builder.info(&format!(
+                "Checking std test/bench/example targets ({} -> {})",
+                &compiler.host, target
+            ));
+            run_cargo(
+                builder,
+                cargo,
+                args(builder.kind),
+                &libstd_test_stamp(builder, compiler, target),
+                vec![],
+                true,
+            );
         }
-
-        builder.info(&format!(
-            "Checking std test/bench/example targets ({} -> {})",
-            &compiler.host, target
-        ));
-        run_cargo(
-            builder,
-            cargo,
-            args(builder.kind),
-            &libstd_test_stamp(builder, compiler, target),
-            vec![],
-            true,
-        );
     }
 }
 
@@ -143,7 +148,9 @@ impl Step for Rustc {
             cargo_subcommand(builder.kind),
         );
         rustc_cargo(builder, &mut cargo, target);
-        cargo.arg("--all-targets");
+        if let Subcommand::Check { all_targets: true, .. } = builder.config.cmd {
+            cargo.arg("--all-targets");
+        }
 
         // Explicitly pass -p for all compiler krates -- this will force cargo
         // to also check the tests/benches/examples for these crates, rather
@@ -205,7 +212,9 @@ macro_rules! tool_check_step {
                     &[],
                 );
 
-                cargo.arg("--all-targets");
+                if let Subcommand::Check { all_targets: true, .. } = builder.config.cmd {
+                    cargo.arg("--all-targets");
+                }
 
                 builder.info(&format!(
                     "Checking {} artifacts ({} -> {})",
diff --git a/src/bootstrap/flags.rs b/src/bootstrap/flags.rs
index 795244e7cd4a4..c1a9d4fcd23fe 100644
--- a/src/bootstrap/flags.rs
+++ b/src/bootstrap/flags.rs
@@ -47,6 +47,9 @@ pub enum Subcommand {
         paths: Vec<PathBuf>,
     },
     Check {
+        // Whether to run checking over all targets (e.g., unit / integration
+        // tests).
+        all_targets: bool,
         paths: Vec<PathBuf>,
     },
     Clippy {
@@ -250,6 +253,9 @@ To learn more about a subcommand, run `./x.py <subcommand> -h`",
                         `/<build_base>/rustfix_missing_coverage.txt`",
                 );
             }
+            "check" => {
+                opts.optflag("", "all-targets", "Check all targets");
+            }
             "bench" => {
                 opts.optmulti("", "test-args", "extra arguments", "ARGS");
             }
@@ -484,7 +490,9 @@ Arguments:
 
         let cmd = match subcommand.as_str() {
             "build" | "b" => Subcommand::Build { paths },
-            "check" | "c" => Subcommand::Check { paths },
+            "check" | "c" => {
+                Subcommand::Check { paths, all_targets: matches.opt_present("all-targets") }
+            }
             "clippy" => Subcommand::Clippy { paths },
             "fix" => Subcommand::Fix { paths },
             "test" | "t" => Subcommand::Test {