Remove -Zthinlto

[bjorn3]

This flag enables local ThinLTO, not full ThinLTO. It seems to be a remnant of when ThinLTO was originally introduced before -Clto=thin was supported. Local ThinLTO is already the default in release mode when -Clto is not passed and there is more than 1 codegen units. In debug mode or with 1 codegen unit local ThinLTO doesn't have any benefit, so there is no reason to support enabling it in that case anyway.

[rustbot]

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[bjorn3]

This has a minor conflict with #144738 in a test.

[rust-log-analyzer]

The job aarch64-gnu-llvm-19-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

---- [ui] tests/ui/lto/dylib-works.rs stdout ----

error: auxiliary build of /checkout/tests/ui/lto/auxiliary/dylib.rs failed to compile: 
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/lto/auxiliary/dylib.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/lto/dylib-works/auxiliary" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "-Clto=thin" "-C" "codegen-units=8" "--crate-type" "dylib" "-L" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/lto/dylib-works/auxiliary"
stdout: none
--- stderr -------------------------------
error: lto cannot be used for `dylib` crate type without `-Zdylib-lto`

error: aborting due to 1 previous error
------------------------------------------


---- [ui] tests/ui/lto/msvc-imp-present.rs stdout ----

error: auxiliary build of /checkout/tests/ui/lto/auxiliary/msvc-imp-present.rs failed to compile: 
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/lto/auxiliary/msvc-imp-present.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/lto/msvc-imp-present/auxiliary" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "-Clto=thin" "-C" "codegen-units=8" "-C" "prefer-dynamic" "-L" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/lto/msvc-imp-present/auxiliary"
stdout: none
--- stderr -------------------------------
error: lto cannot be used for `dylib` crate type without `-Zdylib-lto`

error: aborting due to 1 previous error
------------------------------------------


---- [ui] tests/ui/lto/thin-lto-global-allocator.rs stdout ----

error: test compilation failed although it shouldn't!
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/lto/thin-lto-global-allocator.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "-O" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/lto/thin-lto-global-allocator/a" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "-Clto=thin" "-C" "codegen-units=2"
stdout: none
--- stderr -------------------------------
error: cannot prefer dynamic linking when performing LTO
   |
   = note: only 'staticlib', 'bin', and 'cdylib' outputs are supported with LTO

error: aborting due to 1 previous error
------------------------------------------


---- [ui] tests/ui/lto/thin-lto-inlines.rs stdout ----

error: test compilation failed although it shouldn't!
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/lto/thin-lto-inlines.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/lto/thin-lto-inlines/a" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "-Clto=thin" "-C" "codegen-units=8" "-O"
stdout: none
--- stderr -------------------------------
error: cannot prefer dynamic linking when performing LTO
   |
   = note: only 'staticlib', 'bin', and 'cdylib' outputs are supported with LTO

error: aborting due to 1 previous error
------------------------------------------


---- [ui] tests/ui/lto/weak-works.rs stdout ----

error: test compilation failed although it shouldn't!
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/lto/weak-works.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "-O" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/lto/weak-works/a" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "-C" "codegen-units=8" "-Clto=thin"
stdout: none
--- stderr -------------------------------
error: cannot prefer dynamic linking when performing LTO
   |
   = note: only 'staticlib', 'bin', and 'cdylib' outputs are supported with LTO

error: aborting due to 1 previous error
------------------------------------------

[nikic]

To preserve behavior, shouldn't these drop the flag entirely? Otherwise they will switch from thin-local to thin?

[bjorn3]

I did then also have to pass -Copt-level=1 or higher. Otherwise LTO is disabled entirely.

[bjorn3]

I think changing some tests from local ThinLTO to ThinLTO is a bit risky after all.