)]}'
{
  "commit": "c0d4f8b735e8357d5e81730d722ecd4d5f36025d",
  "tree": "31d5a8e953b338c9fa6a68b1604d7125952fa125",
  "parents": [
    "ff14fc1112e0a8dd2c3910fb89539741cb3d3f23"
  ],
  "author": {
    "name": "Adrian Taylor",
    "email": "adetaylor@chromium.org",
    "time": "Fri Mar 25 15:05:57 2022 -0700"
  },
  "committer": {
    "name": "Commit Bot",
    "email": "commit-bot@chromium.org",
    "time": "Fri Mar 25 22:24:21 2022 +0000"
  },
  "message": "Make more variables available to scripts.\n\nThis change makes certain extra variables available to \u0027action\u0027 target types,\ni.e. scripts. Those variables are:\n* defines\n* include_dirs\n* rustenv\nThey are deduced from any configs applied to the target, so\nconfigs also now applies to \u0027action\u0027 targets.\n\nThese variables are normally exclusively used by compilers, and no compiler-\nspecific variables have been made available to scripts previously. Here\u0027s\nwhy they\u0027re needed.\n\nrustenv:\n\nThis variable represents environment variables passed to the Rust\nbuild, and in the Chromium tree this is assembled using various transitive\n\u0027config\u0027s. These environment variables are typically used to allow `env!`\ndirectives within the Rust code to work properly; often (but not always)\nthis is to support pre-existing third party Rust code (\"crates\").\n\nSuch pre-existing third party Rust code also expects the same environment\nvariables to be present when the crates\u0027 build scripts are run. In a gn\nenvironment, these build scripts are run using an \u0027action\u0027, and therefore\nwe need to make this rustenv value available to \u0027action\u0027 targets.\n\nThis is described in https://chromium/1304254.\n\ndefines/include_dirs:\n\nIn Rust development it is common to produce Rust bindings to pre-existing\nC/C++ headers. This is not built into rustc, but is instead a facility\nprovided by separate codegen tools - the most commonly known example being\nbindgen (https://rust-lang.github.io/rust-bindgen/). In a gn environment,\nthese tools are run as a gn \u0027action\u0027 target.\n\nSuch tools involve parsing C/C++ headers (using something like libclang).\nIn order to parse headers properly, such tools obviously need to be aware\nof the full list of -I and -D flags active in the current environment.\nIn Chromium, such -I and -D flags are assembled using a graph of complex\ntransitive gn \u0027config\u0027s, and there\u0027s no realistic prospect of hard-coding\nequivalent command lines.\n\n\u0027bindgen\u0027 therefore absolutely has to have access to the \u0027include_dirs\u0027\nand \u0027defines\u0027 set using gn configs. One solution here is to define a\nwhole new toolchain where we inform GN that bindgen is a compiler - this\nsolution is outlined in https://chromium/1296241 - however this is not\nideal. To parse (say) just base/cpu_info.h, we would need to depend upon\n  //base(//toolchains/fake_bindgen_toolchain)\nwhich will run a command to \u0027compile\u0027 every .cc file within //base.\nThis can be 10,000s of extra ninja rules, and even if most of them\nare just a \u0027touch\u0027 command, this is still a great deal of extra complexity.\n\nBug: chromium/1272780, chromium/1296155, chromium/1296241, chromium/1304254\nChange-Id: Ib60067b310d8508f61d04fe7ddc537a5d2c2917f\nReviewed-on: https://gn-review.googlesource.com/c/gn/+/13220\nReviewed-by: Brett Wilson \u003cbrettw@chromium.org\u003e\nCommit-Queue: Brett Wilson \u003cbrettw@chromium.org\u003e\n",
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