> ## Documentation Index
> Fetch the complete documentation index at: https://bazel.build/llms.txt
> Use this file to discover all available pages before exploring further.

# Toolchain Resolution Implementation Details

**Note:** This section is intended for Bazel developers, and is not needed by
rule authors.

Several SkyFunction classes implement the [toolchain resolution](/versions/6.5.0/docs/toolchains) process:

1. [`RegisteredToolchainsFunction`](https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/skyframe/RegisteredToolchainsFunction.java) and
   [`RegisteredExecutionPlatformsFunction`](https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/skyframe/RegisteredExecutionPlatformsFunction.java)
   find available toolchains and execution platforms, based on the current
   configuration and WORKSPACE file.

2. [`SingleToolchainResolutionFunction`](https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/skyframe/SingleToolchainResolutionFunction.java)
   resolves a single toolchain type for every execution platform. That is, for
   every execution platform it finds the best registered toolchain to use based
   on the following criteria:

   1. Make sure the toolchain and target platform are compatible, by checking
      the `target_compatible_with` attribute.
   2. Make sure the toolchain and execution platform are compatible, by
      checking the `exec_compatible_with` attribute.
   3. If multiple toolchains are left, choose the highest-priority one (the
      one that was registered first).

3. [`ToolchainResolutionFunction`](https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/skyframe/ToolchainResolutionFunction.java) calls
   `SingleToolchainResolutionFunction` for each requested toolchain type, and
   then determines the best execution platform to use.

   1. First, remove any execution platform that does not have a valid
      toolchain for each requested toolchain type.
   2. If multiple execution platforms are left, choose the highest-priority
      one (the one that was registered first).
      1. If the execution platform is already set by the toolchain
         transition, it will be selected first as described below.

As discussed in [Toolchains and Configurations](/versions/6.5.0/docs/toolchains#toolchains_and_configurations),
the dependency from a target to a toolchain uses a special configuration that
forces the execution platform to be the same for both. Despite the name
"toolchain transition", this is not implemented as a configuration
transition, but instead as a special subclass of
[`ConfiguredTargetKey`](https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/skyframe/ConfiguredTargetKey.java), called
[`ToolchainDependencyConfiguredTargetKey`](https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/skyframe/ConfiguredTargetKey.java;bpv=1;bpt=1;l=164?ss=bazel\&q=ConfiguredTargetKey\&gsn=ToolchainDependencyConfiguredTargetKey\&gs=kythe%3A%2F%2Fgithub.com%2Fbazelbuild%2Fbazel%3Flang%3Djava%3Fpath%3Dcom.google.devtools.build.lib.skyframe.ConfiguredTargetKey.ToolchainDependencyConfiguredTargetKey%2336c7e68f8cd5ea0b5a21b3769e63e6b2d489b9ca8c6f79798839e7f40cf2a19e).
In addition to the other data in `ConfiguredTargetKey`, this subclass also holds
the label of the execution platform. When `ToolchainResolutionFunction` is
considering which execution platform to use, if the forced execution platform
from the `ToolchainDependencyConfiguredTargetKey` is valid, it will be used even
if it is not the highest-priority.

**Note:** If the forced execution platform is not valid (because there are no
valid toolchains, or because of execution constraints from the rule or target),
then the highest-priority valid execution platform will be used instead.