Add opt-for-size core lib feature flag

library/alloc/Cargo.toml

@@ -37,4 +37,6 @@ compiler-builtins-no-asm = ["compiler_builtins/no-asm"]
 compiler-builtins-mangled-names = ["compiler_builtins/mangled-names"]
 compiler-builtins-weak-intrinsics = ["compiler_builtins/weak-intrinsics"]
 # Make panics and failed asserts immediately abort without formatting any message
-panic_immediate_abort = []
+panic_immediate_abort = ["core/panic_immediate_abort"]
+# Choose algorithms that are optimized for binary size instead of runtime performance
+optimize_for_size = ["core/optimize_for_size"]

library/core/Cargo.toml

@@ -31,6 +31,8 @@ rand_xorshift = { version = "0.3.0", default-features = false }
 [features]
 # Make panics and failed asserts immediately abort without formatting any message
 panic_immediate_abort = []
+# Choose algorithms that are optimized for binary size instead of runtime performance
+optimize_for_size = []
 # Make `RefCell` store additional debugging information, which is printed out when
 # a borrow error occurs
 debug_refcell = []

library/std/Cargo.toml

@@ -81,6 +81,8 @@ system-llvm-libunwind = ["unwind/system-llvm-libunwind"]
 
 # Make panics and failed asserts immediately abort without formatting any message
 panic_immediate_abort = ["core/panic_immediate_abort", "alloc/panic_immediate_abort"]
+# Choose algorithms that are optimized for binary size instead of runtime performance
+optimize_for_size = ["core/optimize_for_size", "alloc/optimize_for_size"]
 
 # Enable std_detect default features for stdarch/crates/std_detect:
 # https://github.com/rust-lang/stdarch/blob/master/crates/std_detect/Cargo.toml

library/sysroot/Cargo.toml

@@ -22,6 +22,7 @@ llvm-libunwind = ["std/llvm-libunwind"]
 system-llvm-libunwind = ["std/system-llvm-libunwind"]
 panic-unwind = ["std/panic_unwind"]
 panic_immediate_abort = ["std/panic_immediate_abort"]
+optimize_for_size = ["std/optimize_for_size"]
 profiler = ["std/profiler"]
 std_detect_file_io = ["std/std_detect_file_io"]
 std_detect_dlsym_getauxval = ["std/std_detect_dlsym_getauxval"]

src/ci/docker/scripts/x86_64-gnu-llvm.sh

@@ -6,10 +6,15 @@ set -ex
 if [[ -z "${PR_CI_JOB}" ]]; then
     # When running gcc backend tests, we need to install `libgccjit` and to not run llvm codegen
     # tests as it will fail them.
+    # The tests are run a second time with the size optimizations enabled.
     if [[ "${ENABLE_GCC_CODEGEN}" == "1" ]]; then
         ../x.py --stage 1 test --skip src/tools/tidy --skip tests/codegen
+        ../x.py --stage 1 test --skip src/tools/tidy --skip tests/codegen \
+            --rustc-args "--cfg feature=\"optimize_for_size\""
     else
         ../x.py --stage 1 test --skip src/tools/tidy
+        ../x.py --stage 1 test --skip src/tools/tidy --rustc-args \
+            "--cfg feature=\"optimize_for_size\""
     fi
 
     # Run the `mir-opt` tests again but this time for a 32-bit target.