do not run miri for the dependency build

and reset MIRIFLAGS as well just to be safe

src/tools/miri/cargo-miri/src/phases.rs

@@ -63,6 +63,9 @@ pub fn phase_cargo_miri(mut args: impl Iterator<Item = String>) {
         "setup" => MiriCommand::Setup,
         "test" | "t" | "run" | "r" | "nextest" => MiriCommand::Forward(subcommand),
         "clean" => MiriCommand::Clean,
+        // For use by the `./miri test` dependency builder.
+        "build" if env::var_os("MIRI_BUILD_TEST_DEPS").is_some() =>
+            MiriCommand::Forward("build".into()),
         _ => {
             // Check for version and help flags.
             if has_arg_flag("--help") || has_arg_flag("-h") {

src/tools/miri/tests/deps/src/main.rs

@@ -1 +1,3 @@
-fn main() {}
+fn main() {
+    unreachable!()
+}

src/tools/miri/tests/ui.rs

@@ -132,14 +132,19 @@ fn miri_config(
                     // (It's a separate crate, so we don't get an env var from cargo.)
                     program: miri_path()
                         .with_file_name(format!("cargo-miri{}", env::consts::EXE_SUFFIX)),
-                    // There is no `cargo miri build` so we just use `cargo miri run`.
                     // Add `-Zbinary-dep-depinfo` since it is needed for bootstrap builds (and doesn't harm otherwise).
-                    args: ["miri", "run", "--quiet", "-Zbinary-dep-depinfo"]
+                    args: ["miri", "build", "-Zbinary-dep-depinfo"]
                         .into_iter()
                         .map(Into::into)
                         .collect(),
-                    // Reset `RUSTFLAGS` to work around <https://github.com/rust-lang/rust/pull/119574#issuecomment-1876878344>.
-                    envs: vec![("RUSTFLAGS".into(), None)],
+                    envs: vec![
+                        // Reset `RUSTFLAGS` to work around <https://github.com/rust-lang/rust/pull/119574#issuecomment-1876878344>.
+                        ("RUSTFLAGS".into(), None),
+                        // Reset `MIRIFLAGS` because it caused trouble in the past and should not be needed.
+                        ("MIRIFLAGS".into(), None),
+                        // Allow `cargo miri build`.
+                        ("MIRI_BUILD_TEST_DEPS".into(), Some("1".into())),
+                    ],
                     ..CommandBuilder::cargo()
                 },
                 crate_manifest_path: Path::new("tests/deps").join("Cargo.toml"),