Rollup merge of #129863 - RalfJung:target-spec-features, r=wesleywiser

update comment regarding TargetOptions.features

The claim that `-Ctarget-features` cannot disable these features set in the target spec is definitely wrong -- I tried it for `x86_64-pc-windows-gnu`, which enables SSE3 that way. Building with `-Ctarget-feature=-sse3` works fine, and `cfg!(target_feature = "sse3")` is `false` in that build.

There are also some indications that these are actually intended to be overwritten:

https://github.com/rust-lang/rust/blob/3b14526cead4105f82c398d8d4c7954efa3bab6b/compiler/rustc_target/src/spec/targets/i686_unknown_uefi.rs#L22-L23

https://github.com/rust-lang/rust/blob/84ac80f1921afc243d71fd0caaa4f2838c294102/compiler/rustc_target/src/spec/targets/x86_64h_apple_darwin.rs#L18-L23

So... let's update the comment to match reality, I guess?

The claim that they overwrite `-Ctarget-cpu` is based on
- for `native`, the comment in the apple target spec quoted above
- for other CPU strings, the assumption that `LLVMRustCreateTargetMachine` will apply these features after doing whatever the base CPU model does. I am not sure how to check that, I hope some LLVM backend people can chime in. :)

compiler/rustc_target/src/spec/mod.rs

@@ -2097,9 +2097,10 @@ pub struct TargetOptions {
     /// Default CPU to pass to LLVM. Corresponds to `llc -mcpu=$cpu`. Defaults
     /// to "generic".
     pub cpu: StaticCow<str>,
-    /// Default target features to pass to LLVM. These features will *always* be
-    /// passed, and cannot be disabled even via `-C`. Corresponds to `llc
-    /// -mattr=$features`.
+    /// Default target features to pass to LLVM. These features overwrite
+    /// `-Ctarget-cpu` but can be overwritten with `-Ctarget-features`.
+    /// Corresponds to `llc -mattr=$features`.
+    /// Note that these are LLVM feature names, not Rust feature names!
     pub features: StaticCow<str>,
     /// Direct or use GOT indirect to reference external data symbols
     pub direct_access_external_data: Option<bool>,