Auto merge of rust-lang#2283 - RalfJung:color, r=RalfJung

revert --color=always changes

They [cause problems](rust-lang/miri#2277) and they completely break rendering on the playground:
```
   Compiling playground v0.0.1 (/playground)
    Finished dev [unoptimized + debuginfo] target(s) in 0.47s
     Running `/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo-miri target/miri/x86_64-unknown-linux-gnu/debug/playground`
�[0m�[1m�[38;5;9merror[E0277]�[0m�[0m�[1m: `*mut std::sync::atomic::AtomicUsize` cannot be sent between threads safely�[0m
�[0m   �[0m�[0m�[1m�[38;5;12m--> �[0m�[0msrc/main.rs:17:18�[0m
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```

Sorry `@saethlin,` I think we need to go back to start here and consider another solution.

Fixes rust-lang/miri#2277

Author: bors

cargo-miri/bin.rs

@@ -952,17 +952,8 @@ fn phase_runner(binary: &Path, binary_args: env::Args, phase: RunnerPhase) {
             assert!(suffix.starts_with('='));
             // Drop this argument.
         } else if let Some(suffix) = arg.strip_prefix(json_flag) {
-            let suffix = suffix.strip_prefix('=').unwrap();
-            // This is how we pass through --color=always. We detect that Cargo is detecting rustc
-            // to emit the diagnostic structure that Cargo would consume from rustc to emit colored
-            // diagnostics, and ask rustc to emit them.
-            // See https://github.com/rust-lang/miri/issues/2037
-            // First skip over the leading `=`, then check for diagnostic-rendered-ansi in the
-            // comma-separated list
-            if suffix.split(',').any(|a| a == "diagnostic-rendered-ansi") {
-                cmd.arg("--color=always");
-            }
-            // But aside from remembering that colored output was requested, drop this argument.
+            assert!(suffix.starts_with('='));
+            // Drop this argument.
         } else {
             cmd.arg(arg);
         }