Update installed compiler dependencies
there is a soft-error for older toolchains
from https://reviews.llvm.org/D57264 in preparation for the
hard error in https://reviews.llvm.org/D66188
diff --git a/README.md b/README.md
index 724bc36..2df9e6a 100644
--- a/README.md
+++ b/README.md
@@ -26,7 +26,7 @@
### Building on *nix
1. Make sure you have installed the dependencies:
- * `g++` 4.7 or later or `clang++` 3.x or later
+ * `g++` 5.1 or later or `clang++` 3.5 or later
* `python` 2.7 (but not 3.x)
* GNU `make` 3.81 or later
* `cmake` 3.4.3 or later
@@ -148,6 +148,17 @@
> python x.py build
```
+### Building rustc with older host toolchains
+It is still possible to build Rust with the older toolchain versions listed below, but only if the
+LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN option is set to true in the config.toml file.
+
+* Clang 3.1
+* Apple Clang 3.1
+* GCC 4.8
+* Visual Studio 2015 (Update 3)
+
+Toolchain versions older than what is listed above cannot be used to build rustc.
+
#### Specifying an ABI
Each specific ABI can also be used from either environment (for example, using
