Rollup merge of rust-lang#72094 - petrochenkov:overfeature, r=nikic

cmdline: Make target features individually overridable

Fixes rust-lang#56527

Previously `-C target-feature=+avx2 -C target-feature=+fma` was equivalent to `-C target-feature=+fma` because the later `-C target-feature` option fully overridden previous `-C target-feature`.
With this PR `-C target-feature=+avx2 -C target-feature=+fma` is equivalent to `-C target-feature=+avx2,+fma` and the options are combined.

I'm not sure where the comma-separated features in a single option came from (clang uses a scheme with single feature per-option), but logically these features are entirely independent options.
So they should be overridable individually as well to be more useful in hierarchical build system, and more consistent with other rustc options and clang behavior as well.

Target feature options have a few other issues (rust-lang#44815), but fixing those is going to be a bit more invasive.

Author: RalfJung

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

@@ -464,7 +464,15 @@ machine. Each target has a default base CPU.
 
 Individual targets will support different features; this flag lets you control
 enabling or disabling a feature. Each feature should be prefixed with a `+` to
-enable it or `-` to disable it. Separate multiple features with commas.
+enable it or `-` to disable it.
+
+Features from multiple `-C target-feature` options are combined. \
+Multiple features can be specified in a single option by separating them
+with commas - `-C target-feature=+x,-y`. \
+If some feature is specified more than once with both `+` and `-`,
+then values passed later override values passed earlier. \
+For example, `-C target-feature=+x,-y,+z -Ctarget-feature=-x,+y`
+is equivalent to `-C target-feature=-x,+y,+z`.
 
 To see the valid options and an example of use, run `rustc --print
 target-features`.

src/librustc_session/options.rs

@@ -271,6 +271,7 @@ macro_rules! options {
             "one of supported relocation models (`rustc --print relocation-models`)";
         pub const parse_tls_model: &str =
             "one of supported TLS models (`rustc --print tls-models`)";
+        pub const parse_target_feature: &str = parse_string;
     }
 
     #[allow(dead_code)]
@@ -647,6 +648,19 @@ macro_rules! options {
             }
             true
         }
+
+        fn parse_target_feature(slot: &mut String, v: Option<&str>) -> bool {
+            match v {
+                Some(s) => {
+                    if !slot.is_empty() {
+                        slot.push_str(",");
+                    }
+                    slot.push_str(s);
+                    true
+                }
+                None => false,
+            }
+        }
     }
 ) }
 
@@ -742,7 +756,7 @@ options! {CodegenOptions, CodegenSetter, basic_codegen_options,
         "use soft float ABI (*eabihf targets only) (default: no)"),
     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_feature: String = (String::new(), parse_target_feature, [TRACKED],
         "target specific attributes. (`rustc --print target-features` for details). \
         This feature is unsafe."),
 

src/test/codegen/target-feature-multiple.rs

@@ -0,0 +1,9 @@
+// only-x86_64
+// compile-flags: -C target-feature=+sse2,-avx,+avx2 -C target-feature=+avx,-avx2
+
+#![crate_type = "lib"]
+
+#[no_mangle]
+pub fn foo() {
+    // CHECK: attributes #0 = { {{.*}}"target-features"="+sse2,-avx,+avx2,+avx,-avx2"{{.*}} }
+}