Incremental ThinLTO doesn't reuse unchanged CGUs originating form upstream crates

[bjorn3]

When rustc is in incremental mode, cg_llvm is supposed to ask ThinLTO for every CGU if it needs to be re-optimized or if the artifacts from the previous compilation session can be re-used. This works fine for local CGUs, but for CGUs originating from upstream crates we incorrectly recompile them unconditionally:

Somewhat verbose logs

$ echo 'fn main() {}' | RUSTC_LOG=rustc_codegen_llvm::back rustc - -Cincremental=i -Clto=thin
warning: some trace filter directives would enable traces that are disabled statically
 | `rustc_codegen_llvm::back=trace` would enable the TRACE level for the `rustc_codegen_llvm::back` target
 = note: the static max level is `info`
 = help: to enable DEBUG logging, remove the `max_level_info` feature from the `tracing` crate
  INFO rustc_codegen_llvm::back::lto adding bitcode from std-97ad18f5ff73ac59.std.eb0a984909e3e9db-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto adding bitcode from panic_unwind-6d74918d8c9fc8eb.panic_unwind.2c8f5aefaa401768-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto adding bitcode from object-93fe4a6732f49df4.object.23ed47b63b254e1f-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto adding bitcode from memchr-6d799c7a867f65ec.memchr.9857587667506101-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto adding bitcode from addr2line-25929983526f04fa.addr2line.c0ce80aede3d4387-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto adding bitcode from gimli-45b43a86c98f869a.gimli.4bcf315d396003be-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto adding bitcode from cfg_if-3367568ca42d58f6.cfg_if.24f6f58b1e6c0512-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto adding bitcode from rustc_demangle-917b4137dcc16c1a.rustc_demangle.faed9c870cf8c981-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto adding bitcode from std_detect-89da0f264c038cfd.std_detect.3472883a54d5ea88-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto adding bitcode from hashbrown-708d53ebeba09f31.hashbrown.75685bee96dce093-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto adding bitcode from rustc_std_workspace_alloc-e9dc45f98c8ec199.rustc_std_workspace_alloc.2a9edd1fe8ec284e-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto adding bitcode from miniz_oxide-ed184832220c0641.miniz_oxide.aa3c4a081c6dccb-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto adding bitcode from adler2-336aefd7b842107c.adler2.17cc26489f12f344-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto adding bitcode from unwind-d8feb668298a034e.unwind.4ba8e0ce54b89e2a-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto adding bitcode from libc-93b6ac19591830f2.libc.5723d27d479e3139-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto adding bitcode from rustc_std_workspace_core-5d9a775410d678c7.rustc_std_workspace_core.38ed106ace1adde5-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto adding bitcode from alloc-63de78ca4b0f7b13.alloc.c8499e84c8ebed9c-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto adding bitcode from core-0b74e43f28b113df.core.65ad41b0d205847a-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto going for that thin, thin LTO
  INFO rustc_codegen_llvm::back::lto local module: 0 - 6lae4cjdhmree6e3dytjzcywj
  INFO rustc_codegen_llvm::back::lto upstream or cached module "std-97ad18f5ff73ac59.std.eb0a984909e3e9db-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "panic_unwind-6d74918d8c9fc8eb.panic_unwind.2c8f5aefaa401768-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "object-93fe4a6732f49df4.object.23ed47b63b254e1f-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "memchr-6d799c7a867f65ec.memchr.9857587667506101-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "addr2line-25929983526f04fa.addr2line.c0ce80aede3d4387-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "gimli-45b43a86c98f869a.gimli.4bcf315d396003be-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "cfg_if-3367568ca42d58f6.cfg_if.24f6f58b1e6c0512-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "rustc_demangle-917b4137dcc16c1a.rustc_demangle.faed9c870cf8c981-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "std_detect-89da0f264c038cfd.std_detect.3472883a54d5ea88-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "hashbrown-708d53ebeba09f31.hashbrown.75685bee96dce093-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "rustc_std_workspace_alloc-e9dc45f98c8ec199.rustc_std_workspace_alloc.2a9edd1fe8ec284e-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "miniz_oxide-ed184832220c0641.miniz_oxide.aa3c4a081c6dccb-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "adler2-336aefd7b842107c.adler2.17cc26489f12f344-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "unwind-d8feb668298a034e.unwind.4ba8e0ce54b89e2a-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "libc-93b6ac19591830f2.libc.5723d27d479e3139-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "rustc_std_workspace_core-5d9a775410d678c7.rustc_std_workspace_core.38ed106ace1adde5-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "alloc-63de78ca4b0f7b13.alloc.c8499e84c8ebed9c-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "core-0b74e43f28b113df.core.65ad41b0d205847a-cgu.0.rcgu.o"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "c70u0a76x19t4gww0jvj6yao8"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "chne6zhcy9x85cp0h2x5aqu0m"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "18jjsdd37c3gq3h2osg79u26t"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "d8ox0gwb3a4do5jzgumfggb4p"
  INFO rustc_codegen_llvm::back::lto upstream or cached module "cjc4ymmspqlie6c93a6aq3hdm"
  INFO rustc_codegen_llvm::back::lto thin LTO data created
  INFO rustc_codegen_llvm::back::lto thin LTO cache key map loaded
  INFO rustc_codegen_llvm::back::lto prev_key_map: Some(
     ThinLTOKeysMap {
         keys: {
             "18jjsdd37c3gq3h2osg79u26t": "1A07C247D9E640DDB4C6A4C3768F0A91B6076465",
             "6lae4cjdhmree6e3dytjzcywj": "F1EDCCF55AABA8D5D1973CEF29B2460EACED894A",
             "addr2line-25929983526f04fa.addr2line.c0ce80aede3d4387-cgu.0.rcgu.o": "58BD95A74B698B630367B8D9C8003AFCBFA41B06",
             "adler2-336aefd7b842107c.adler2.17cc26489f12f344-cgu.0.rcgu.o": "D1359BD89B4B7F6DFBED758F4F8803183970EC04",
             "alloc-63de78ca4b0f7b13.alloc.c8499e84c8ebed9c-cgu.0.rcgu.o": "296C7F99478A15A1BFE17A7B96827D30EAB8272E",
             "c70u0a76x19t4gww0jvj6yao8": "C73EA2E7CAC6B5C328B89BE28B317C845A7756F2",
             "cfg_if-3367568ca42d58f6.cfg_if.24f6f58b1e6c0512-cgu.0.rcgu.o": "B94F0408A018DEFB82E2ADBE52ADB9A3973C1A84",
             "chne6zhcy9x85cp0h2x5aqu0m": "3EE6A4F3F7D88032ADAFA4A6EAEE161ABC76CE8F",
             "cjc4ymmspqlie6c93a6aq3hdm": "E672F1DB0CBBDC0E3AE6D54360B2203847B5DA89",
             "core-0b74e43f28b113df.core.65ad41b0d205847a-cgu.0.rcgu.o": "726553BB6C60FF201B5516502FF256FD9FC7D924",
             "d8ox0gwb3a4do5jzgumfggb4p": "5313E5DEF9C845E41259478DA6CE8BE9D67B8B6A",
             "gimli-45b43a86c98f869a.gimli.4bcf315d396003be-cgu.0.rcgu.o": "F336E621AE74B9F2AE13E23FC3DACF15FF1F17C8",
             "hashbrown-708d53ebeba09f31.hashbrown.75685bee96dce093-cgu.0.rcgu.o": "36C19A9D0EB535EE0E9DC12AD424312E9B3A34C6",
             "libc-93b6ac19591830f2.libc.5723d27d479e3139-cgu.0.rcgu.o": "AEC2BF1AC99E3536ACECD3E33E5E9608DD5FEB0A",
             "memchr-6d799c7a867f65ec.memchr.9857587667506101-cgu.0.rcgu.o": "F09F9EC99D94A8ECE980A2CC8C6950A701D712EA",
             "miniz_oxide-ed184832220c0641.miniz_oxide.aa3c4a081c6dccb-cgu.0.rcgu.o": "41433802828A60FEEDE0A7D9E7D1F3051097416B",
             "object-93fe4a6732f49df4.object.23ed47b63b254e1f-cgu.0.rcgu.o": "389CBB69C7A8769E499E97ECF9AFF0E58460D865",
             "panic_unwind-6d74918d8c9fc8eb.panic_unwind.2c8f5aefaa401768-cgu.0.rcgu.o": "0C1456DF798DA045B018BC714EC1E7B7D7A80AF2",
             "rustc_demangle-917b4137dcc16c1a.rustc_demangle.faed9c870cf8c981-cgu.0.rcgu.o": "A09E27B2414E8104906E8DD8B85D24CA46441FC5",
             "rustc_std_workspace_alloc-e9dc45f98c8ec199.rustc_std_workspace_alloc.2a9edd1fe8ec284e-cgu.0.rcgu.o": "2DA83DC76E47D24DCB43BE759CCC56CCD5DDA312",
             "rustc_std_workspace_core-5d9a775410d678c7.rustc_std_workspace_core.38ed106ace1adde5-cgu.0.rcgu.o": "0BA0C636CC5F9DB300FD3715B2330C6F4B9AA074",
             "std-97ad18f5ff73ac59.std.eb0a984909e3e9db-cgu.0.rcgu.o": "534E18DF0ED9E1B7311A57313B1A4AF5237D27CA",
             "std_detect-89da0f264c038cfd.std_detect.3472883a54d5ea88-cgu.0.rcgu.o": "9AF87D9E47AD3C1CB15D036C9FCB07A48CE68D70",
             "unwind-d8feb668298a034e.unwind.4ba8e0ce54b89e2a-cgu.0.rcgu.o": "7F2936A470B7EE86B6E04DA189E9CFB137D29DED",
         },
     },
 )
  INFO rustc_codegen_llvm::back::lto curr_key_map: ThinLTOKeysMap {
     keys: {
         "18jjsdd37c3gq3h2osg79u26t": "1A07C247D9E640DDB4C6A4C3768F0A91B6076465",
         "6lae4cjdhmree6e3dytjzcywj": "F1EDCCF55AABA8D5D1973CEF29B2460EACED894A",
         "addr2line-25929983526f04fa.addr2line.c0ce80aede3d4387-cgu.0.rcgu.o": "58BD95A74B698B630367B8D9C8003AFCBFA41B06",
         "adler2-336aefd7b842107c.adler2.17cc26489f12f344-cgu.0.rcgu.o": "D1359BD89B4B7F6DFBED758F4F8803183970EC04",
         "alloc-63de78ca4b0f7b13.alloc.c8499e84c8ebed9c-cgu.0.rcgu.o": "296C7F99478A15A1BFE17A7B96827D30EAB8272E",
         "c70u0a76x19t4gww0jvj6yao8": "C73EA2E7CAC6B5C328B89BE28B317C845A7756F2",
         "cfg_if-3367568ca42d58f6.cfg_if.24f6f58b1e6c0512-cgu.0.rcgu.o": "B94F0408A018DEFB82E2ADBE52ADB9A3973C1A84",
         "chne6zhcy9x85cp0h2x5aqu0m": "3EE6A4F3F7D88032ADAFA4A6EAEE161ABC76CE8F",
         "cjc4ymmspqlie6c93a6aq3hdm": "E672F1DB0CBBDC0E3AE6D54360B2203847B5DA89",
         "core-0b74e43f28b113df.core.65ad41b0d205847a-cgu.0.rcgu.o": "726553BB6C60FF201B5516502FF256FD9FC7D924",
         "d8ox0gwb3a4do5jzgumfggb4p": "5313E5DEF9C845E41259478DA6CE8BE9D67B8B6A",
         "gimli-45b43a86c98f869a.gimli.4bcf315d396003be-cgu.0.rcgu.o": "F336E621AE74B9F2AE13E23FC3DACF15FF1F17C8",
         "hashbrown-708d53ebeba09f31.hashbrown.75685bee96dce093-cgu.0.rcgu.o": "36C19A9D0EB535EE0E9DC12AD424312E9B3A34C6",
         "libc-93b6ac19591830f2.libc.5723d27d479e3139-cgu.0.rcgu.o": "AEC2BF1AC99E3536ACECD3E33E5E9608DD5FEB0A",
         "memchr-6d799c7a867f65ec.memchr.9857587667506101-cgu.0.rcgu.o": "F09F9EC99D94A8ECE980A2CC8C6950A701D712EA",
         "miniz_oxide-ed184832220c0641.miniz_oxide.aa3c4a081c6dccb-cgu.0.rcgu.o": "41433802828A60FEEDE0A7D9E7D1F3051097416B",
         "object-93fe4a6732f49df4.object.23ed47b63b254e1f-cgu.0.rcgu.o": "389CBB69C7A8769E499E97ECF9AFF0E58460D865",
         "panic_unwind-6d74918d8c9fc8eb.panic_unwind.2c8f5aefaa401768-cgu.0.rcgu.o": "0C1456DF798DA045B018BC714EC1E7B7D7A80AF2",
         "rustc_demangle-917b4137dcc16c1a.rustc_demangle.faed9c870cf8c981-cgu.0.rcgu.o": "A09E27B2414E8104906E8DD8B85D24CA46441FC5",
         "rustc_std_workspace_alloc-e9dc45f98c8ec199.rustc_std_workspace_alloc.2a9edd1fe8ec284e-cgu.0.rcgu.o": "2DA83DC76E47D24DCB43BE759CCC56CCD5DDA312",
         "rustc_std_workspace_core-5d9a775410d678c7.rustc_std_workspace_core.38ed106ace1adde5-cgu.0.rcgu.o": "0BA0C636CC5F9DB300FD3715B2330C6F4B9AA074",
         "std-97ad18f5ff73ac59.std.eb0a984909e3e9db-cgu.0.rcgu.o": "534E18DF0ED9E1B7311A57313B1A4AF5237D27CA",
         "std_detect-89da0f264c038cfd.std_detect.3472883a54d5ea88-cgu.0.rcgu.o": "9AF87D9E47AD3C1CB15D036C9FCB07A48CE68D70",
         "unwind-d8feb668298a034e.unwind.4ba8e0ce54b89e2a-cgu.0.rcgu.o": "7F2936A470B7EE86B6E04DA189E9CFB137D29DED",
     },
 }
  INFO rustc_codegen_llvm::back::lto checking which modules can be-reused and which have to be re-optimized.
  INFO rustc_codegen_llvm::back::lto  - 6lae4cjdhmree6e3dytjzcywj: re-compiled
  INFO rustc_codegen_llvm::back::lto  - std-97ad18f5ff73ac59.std.eb0a984909e3e9db-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - panic_unwind-6d74918d8c9fc8eb.panic_unwind.2c8f5aefaa401768-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - object-93fe4a6732f49df4.object.23ed47b63b254e1f-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - memchr-6d799c7a867f65ec.memchr.9857587667506101-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - addr2line-25929983526f04fa.addr2line.c0ce80aede3d4387-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - gimli-45b43a86c98f869a.gimli.4bcf315d396003be-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - cfg_if-3367568ca42d58f6.cfg_if.24f6f58b1e6c0512-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - rustc_demangle-917b4137dcc16c1a.rustc_demangle.faed9c870cf8c981-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - std_detect-89da0f264c038cfd.std_detect.3472883a54d5ea88-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - hashbrown-708d53ebeba09f31.hashbrown.75685bee96dce093-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - rustc_std_workspace_alloc-e9dc45f98c8ec199.rustc_std_workspace_alloc.2a9edd1fe8ec284e-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - miniz_oxide-ed184832220c0641.miniz_oxide.aa3c4a081c6dccb-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - adler2-336aefd7b842107c.adler2.17cc26489f12f344-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - unwind-d8feb668298a034e.unwind.4ba8e0ce54b89e2a-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - libc-93b6ac19591830f2.libc.5723d27d479e3139-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - rustc_std_workspace_core-5d9a775410d678c7.rustc_std_workspace_core.38ed106ace1adde5-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - alloc-63de78ca4b0f7b13.alloc.c8499e84c8ebed9c-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - core-0b74e43f28b113df.core.65ad41b0d205847a-cgu.0.rcgu.o: re-compiled
  INFO rustc_codegen_llvm::back::lto  - c70u0a76x19t4gww0jvj6yao8: re-used
  INFO rustc_codegen_llvm::back::lto  - chne6zhcy9x85cp0h2x5aqu0m: re-used
  INFO rustc_codegen_llvm::back::lto  - 18jjsdd37c3gq3h2osg79u26t: re-used
  INFO rustc_codegen_llvm::back::lto  - d8ox0gwb3a4do5jzgumfggb4p: re-used
  INFO rustc_codegen_llvm::back::lto  - cjc4ymmspqlie6c93a6aq3hdm: re-used
  INFO rustc_codegen_llvm::back::lto running thin lto passes over adler2-336aefd7b842107c.adler2.17cc26489f12f344-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto running thin lto passes over libc-93b6ac19591830f2.libc.5723d27d479e3139-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto running thin lto passes over hashbrown-708d53ebeba09f31.hashbrown.75685bee96dce093-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto running thin lto passes over rustc_std_workspace_core-5d9a775410d678c7.rustc_std_workspace_core.38ed106ace1adde5-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto running thin lto passes over rustc_std_workspace_alloc-e9dc45f98c8ec199.rustc_std_workspace_alloc.2a9edd1fe8ec284e-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto running thin lto passes over cfg_if-3367568ca42d58f6.cfg_if.24f6f58b1e6c0512-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto running thin lto passes over unwind-d8feb668298a034e.unwind.4ba8e0ce54b89e2a-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto running thin lto passes over std_detect-89da0f264c038cfd.std_detect.3472883a54d5ea88-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto running thin lto passes over miniz_oxide-ed184832220c0641.miniz_oxide.aa3c4a081c6dccb-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto running thin lto passes over memchr-6d799c7a867f65ec.memchr.9857587667506101-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto running thin lto passes over object-93fe4a6732f49df4.object.23ed47b63b254e1f-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto running thin lto passes over rustc_demangle-917b4137dcc16c1a.rustc_demangle.faed9c870cf8c981-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto running thin lto passes over 6lae4cjdhmree6e3dytjzcywj
  INFO rustc_codegen_llvm::back::lto running thin lto passes over panic_unwind-6d74918d8c9fc8eb.panic_unwind.2c8f5aefaa401768-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto running thin lto passes over addr2line-25929983526f04fa.addr2line.c0ce80aede3d4387-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto running thin lto passes over gimli-45b43a86c98f869a.gimli.4bcf315d396003be-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto running thin lto passes over alloc-63de78ca4b0f7b13.alloc.c8499e84c8ebed9c-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto running thin lto passes over core-0b74e43f28b113df.core.65ad41b0d205847a-cgu.0.rcgu.o
  INFO rustc_codegen_llvm::back::lto running thin lto passes over std-97ad18f5ff73ac59.std.eb0a984909e3e9db-cgu.0.rcgu.o

The cause for this is that we don't load WorkProducts for upstream modules before starting ThinLTO and don't track if the pre-lto module was unchanged and thus don't know what artifacts we can copy.

[cezarbbb]

I think we could integrate previous_work_products into CodegenContext, and add a branch within the loop—if the module is not in green_modules, the key remains the same, and there is a corresponding prev work product, then use the existing CopyPostLtoArtifacts cached object. The local logic would remain unchanged, and if a key or work product is missing, it would still revert to re-compiled.
Would this solve the problem?

[cezarbbb]

I'm not sure if there's anything missing. If not, I can add this content and supplement the corresponding tests.

[bjorn3]

Does the ThinLTO key uniquely identify the optimized module that would be produced or does it only uniquely identify the imported data while still requiring you to know if the pre-opt module was unchanged? If the former, we can probably ignore cgu names and work products entirely for the lookup and use the ThinLTO key as sole key for looking up existing artifacts in the incr comp cache and would be able to get rid of the distinction between the Green and Red variants of ThinLtoInput. Currently that would still require work products for tracking outdated artifacts, but I'm working on a PR to remove outdated artifacts without needing work products tracking them.

[cezarbbb]

The key uniquely identifies the optimized output. This is because it seems the key explicitly includes the optimized module's own IR hash (getModuleHash(ModuleID)).
I'll wait until you submit your PR and then see what I can do.

[bjorn3]

but I'm working on a PR to remove outdated artifacts without needing work products tracking them.

This is #159287, but still blocked on another PR landing first.