Add a regression test for --require-hashes with unpinned build requirements (#15147)

Test case for https://github.com/astral-sh/uv/issues/15146

crates/uv/tests/it/pip_install.rs

@@ -6667,6 +6667,48 @@ fn require_hashes() -> Result<()> {
     Ok(())
 }
 
+/// Use `--require-hashes` when there are no hashes for build dependencies.
+#[test]
+fn require_hashes_build_dependencies() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    // Write to a requirements file.
+    let requirements_txt = context.temp_dir.child("requirements.txt");
+    requirements_txt.write_str(indoc::indoc! {r"
+        anyio==4.0.0 \
+            --hash=sha256:cfdb2b588b9fc25ede96d8db56ed50848b0b649dca3dd1df0b11f683bb9e0b5f \
+            --hash=sha256:f7ed51751b2c2add651e5747c891b47e26d2a21be5d32d9311dfe9692f3e5d7a
+        idna==3.6 \
+            --hash=sha256:9ecdbbd083b06798ae1e86adcbfe8ab1479cf864e4ee30fe4e46a003d12491ca \
+            --hash=sha256:c05567e9c24a6b9faaa835c4821bad0590fbb9d5779e7caa6e1cc4978e7eb24f
+            # via anyio
+        sniffio==1.3.1 \
+            --hash=sha256:2f6da418d1f1e0fddd844478f41680e794e6051915791a034ff65e5f100525a2 \
+            --hash=sha256:f4324edc670a0f49750a81b895f35c3adb843cca46f0530f79fc1babb23789dc
+            # via anyio
+    "})?;
+
+    uv_snapshot!(context.pip_install()
+        .arg("--no-binary").arg(":all:")
+        .arg("-r")
+        .arg("requirements.txt")
+        .arg("--require-hashes"), @r"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 3 packages in [TIME]
+      × Failed to download and build `idna==3.6`
+      ├─▶ Failed to resolve requirements from `build-system.requires`
+      ├─▶ No solution found when resolving: `flit-core>=3.2, <4`
+      ╰─▶ In `--require-hashes` mode, all requirements must be pinned upfront with `==`, but found: `flit-core`
+    "
+    );
+
+    Ok(())
+}
+
 /// Omit hashes for dependencies with `--require-hashes`, which is allowed with `--no-deps`.
 #[test]
 fn require_hashes_no_deps() -> Result<()> {