Parse SDK settings

Previously, we did a simplistic check against the SDKROOT path to try
to figure out whether we should use the environment variable, or try to
find a better matching SDK instead.

This is brittle since the user might want their SDK to be in a
different place in the file system (that's kinda the whole idea of the
SDKROOT variable), and also didn't work properly with SDKs in the Xcode
Command Line Tools.

Instead, we now:
1. Parse the SDKSettings.json file under the SDKROOT, to determine
   whether it is suitable for the target we're compiling for (which it
   might not be when using Cargo, and targetting iOS but compiling build
   scripts on macOS).
2. If it's not, attempt to find another SDK.

This allows us to give much more detailed error messages, to better
support cross-compile scenarios, and allows using parameters from the
SDK (such as the SDK version) elsewhere in the future.

Author: madsmtm

Cargo.lock

@@ -3478,6 +3478,7 @@ dependencies = [
  "rustc_target",
  "rustc_trait_selection",
  "rustc_type_ir",
+ "serde",
  "serde_json",
  "smallvec",
  "tempfile",

compiler/rustc_codegen_ssa/Cargo.toml

@@ -37,6 +37,7 @@ rustc_target = { path = "../rustc_target" }
 rustc_trait_selection = { path = "../rustc_trait_selection" }
 rustc_type_ir = { path = "../rustc_type_ir" }
 serde_json = "1.0.59"
+serde = { version = "1", features = [ "derive" ]}
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
 tempfile = "3.2"
 thin-vec = "0.2.12"

compiler/rustc_codegen_ssa/messages.ftl

@@ -8,9 +8,17 @@ codegen_ssa_apple_deployment_target_invalid =
 codegen_ssa_apple_deployment_target_too_low =
     deployment target in {$env_var} was set to {$version}, but the minimum supported by `rustc` is {$os_min}
 
+codegen_ssa_apple_deployment_target_too_high =
+    deployment target was set to {$version}, but the maximum supported by the current SDK is {$sdk_max}
+
+    Use the {$env_var} environment variable to set it to something lower.
+
 codegen_ssa_apple_sdk_error_failed_reading =
     failed reading `{$path}` while looking for SDK root: {$error}
 
+codegen_ssa_apple_sdk_error_invalid_sdk_settings_json =
+    failed parsing SDK settings from `{$path}`: {$error}
+
 codegen_ssa_apple_sdk_error_missing =
     failed finding SDK for platform `{$sdk_name}`. It looks like you have not installed Xcode?
 
@@ -37,6 +45,11 @@ codegen_ssa_apple_sdk_error_missing_developer_dir =
     - `{$sdkroot}`
     - `{$sdkroot_bare}`
 
+codegen_ssa_apple_sdk_error_missing_sdk_settings =
+    failed finding `SDKSettings.json` or `SDKSettings.plist` in `{$sdkroot}`.
+
+    Are you sure this is a valid SDK?
+
 codegen_ssa_apple_sdk_error_missing_xcode =
     failed finding SDK at `{$sdkroot}` in Xcode installation.
 
@@ -49,6 +62,49 @@ codegen_ssa_apple_sdk_error_missing_xcode_select =
 
     Consider using `sudo xcode-select --switch path/to/Xcode.app` or `sudo xcode-select --reset` to select a valid path.
 
+codegen_ssa_apple_sdk_error_not_sdk_path =
+    failed parsing SDK at `{$path}`.
+
+    The SDK did not seem to contain `SDKSettings.json`, and hence `rustc` falled back to parsing required SDK details from the SDK name itself.
+
+    This name must be of the form `[SDKName][major].[minor].[patch].sdk`, for example `MacOSX10.13.sdk` or `iPhoneOS11.0.sdk`.
+
+codegen_ssa_apple_sdk_error_sdk_does_not_support_arch =
+    the SDK at `{$sdkroot}` does not support the {$arch} architecture.
+
+codegen_ssa_apple_sdk_error_sdk_does_not_support_os =
+    the SDK at `{$sdkroot}` does not support { $abi ->
+        [macabi] Mac Catalyst
+        [sim] the {$os} simulator
+        *[normal] {$os}
+    }.
+
+    Use `xcode-select --switch` or one of the `SDKROOT` and `DEVELOPER_DIR` environment variables to select { $abi ->
+        [macabi] a newer version of Xcode that has support for it
+        *[others] another SDK that has support for it
+    }.
+
+    { $abi ->
+        [macabi] Also remember that it is the macOS SDK that supports Mac Catalyst, not the iOS SDK.
+        *[others] {""}
+    }
+
+codegen_ssa_apple_sdk_error_sdk_root_ignored =
+    the SDKROOT environment variable has been ignored, and `rustc` will attempt to find a suitable SDK
+
+codegen_ssa_apple_sdk_error_sdk_root_is_root_path =
+    the path SDKROOT was set to the root path `/`.
+
+    This will work poorly with other tooling, and as such the value has been ignored.
+
+codegen_ssa_apple_sdk_error_sdk_root_missing =
+    the path `{$sdkroot}` specified in SDKROOT did not exist, so it has been ignored
+
+codegen_ssa_apple_sdk_error_sdk_root_not_absolute =
+    the path `{$sdkroot}` specified in SDKROOT was not an absolute path.
+
+    This will work poorly with other tooling, and as such the value has been ignored.
+
 codegen_ssa_archive_build_failure = failed to build archive at `{$path}`: {$error}
 
 codegen_ssa_atomic_compare_exchange = Atomic compare-exchange intrinsic missing failure memory ordering