Rollup merge of #64145 - togiberlin:feature/target-features-doc, r=ehuss

Target-feature documented as unsafe

@nikomatsakis asked me to help out on the docs on this issue: https://github.com/rust-lang/rust/issues/63597

The following docs have been modified
- ```rustc -C help``` text for `target-feature`
- RustC book:

## Preview of src/doc/rustc/src/targets/index.md
![Screenshot 2019-09-17 at 12 22 45](https://user-images.githubusercontent.com/13764830/65033746-f7826700-d945-11e9-9dd2-d8f9b08f45de.png)

## Preview of src/doc/rustc/src/targets/known-issues.md
![Screenshot 2019-09-17 at 12 22 25](https://user-images.githubusercontent.com/13764830/65033774-00733880-d946-11e9-9398-90f01f3938d5.png)

Fixes #63597

src/doc/rustc/src/SUMMARY.md

@@ -14,6 +14,7 @@
 - [Targets](targets/index.md)
     - [Built-in Targets](targets/built-in.md)
     - [Custom Targets](targets/custom.md)
+    - [Known Issues](targets/known-issues.md)
 - [Profile-guided Optimization](profile-guided-optimization.md)
 - [Linker-plugin based LTO](linker-plugin-lto.md)
 - [Contributing to `rustc`](contributing.md)

src/doc/rustc/src/codegen-options/index.md

@@ -61,6 +61,8 @@ enabling or disabling a feature.
 To see the valid options and an example of use, run `rustc --print
 target-features`.
 
+Using this flag is unsafe and might result in [undefined runtime behavior](../targets/known-issues.md).
+
 ## passes
 
 This flag can be used to add extra LLVM passes to the compilation.

src/doc/rustc/src/command-line-arguments.md

@@ -145,7 +145,7 @@ of print values are:
   target CPU may be selected with the `-C target-cpu=val` flag.
 - `target-features` — List of available target features for the current
   target. Target features may be enabled with the `-C target-feature=val`
-  flag.
+  flag. This flag is unsafe. See [known issues](targets/known-issues.md) for more details.
 - `relocation-models` — List of relocation models. Relocation models may be
   selected with the `-C relocation-model=val` flag.
 - `code-models` — List of code models. Code models may be selected with the

src/doc/rustc/src/targets/index.md

@@ -11,3 +11,9 @@ To compile to a particular target, use the `--target` flag:
 ```bash
 $ rustc src/main.rs --target=wasm32-unknown-unknown
 ```
+## Target Features
+`x86`,  and `ARMv8` are two popular CPU architectures. Their instruction sets form a common baseline across most CPUs. However, some CPUs extend these with custom instruction sets, e.g. vector (`AVX`), bitwise manipulation (`BMI`) or cryptographic (`AES`).
+
+Developers, who know on which CPUs their compiled code is going to run can choose to add (or remove) CPU specific instruction sets via the `-C target-feature=val` flag.
+
+Please note, that this flag is generally considered as unsafe. More details can be found in [this section](known-issues.md).

src/doc/rustc/src/targets/known-issues.md

@@ -0,0 +1,13 @@
+# Known Issues
+This section informs you about known "gotchas". Keep in mind, that this section is (and always will be) incomplete. For suggestions and amendments, feel free to [contribute](../contributing.md) to this guide.
+
+## Target Features
+Most target-feature problems arise, when mixing code that have the target-feature _enabled_ with code that have it _disabled_. If you want to avoid undefined behavior, it is recommended to build _all code_ (including the standard library and imported crates) with a common set of target-features.
+
+By default, compiling your code with the `-C target-feature` flag will not recompile the entire standard library and/or imported crates with matching target features. Therefore, target features are generally considered as unsafe. Using `#[target_feature]` on individual functions makes the function unsafe.
+
+Examples:
+
+| Target-Feature | Issue | Seen on | Description | Details |
+| -------------- | ----- | ------- | ----------- | ------- |
+| `+soft-float` <br> and <br> `-sse` | Segfaults and ABI mismatches | `x86` and `x86-64` | The `x86` and `x86_64` architecture uses SSE registers (aka `xmm`) for floating point operations. Using software emulated floats ("soft-floats") disables usage of `xmm` registers, but parts of Rust's core libraries (e.g. `std::f32` or `std::f64`) are compiled without soft-floats and expect parameters to be passed in `xmm` registers. This leads to ABI mismatches. <br><br>  Attempting to compile with disabled SSE causes the same error, too. | [#63466](https://github.com/rust-lang/rust/issues/63466) |

src/librustc/session/config.rs

@@ -1149,7 +1149,8 @@ options! {CodegenOptions, CodegenSetter, basic_codegen_options,
     target_cpu: Option<String> = (None, parse_opt_string, [TRACKED],
         "select target processor (`rustc --print target-cpus` for details)"),
     target_feature: String = (String::new(), parse_string, [TRACKED],
-        "target specific attributes (`rustc --print target-features` for details)"),
+        "target specific attributes. (`rustc --print target-features` for details). \
+        This feature is unsafe."),
     passes: Vec<String> = (Vec::new(), parse_list, [TRACKED],
         "a list of extra LLVM passes to run (space separated)"),
     llvm_args: Vec<String> = (Vec::new(), parse_list, [TRACKED],