Rename CLI option to target_api_feature, rework handling

- Rename to singular form, allowing comma separation and multiple occurrences
- Allow usage of `target_api_feature` even if the
  target has no defined `target_api_kind`

Author: seritools

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

@@ -480,11 +480,14 @@ point instructions in software. It takes one of the following values:
 * `y`, `yes`, `on`, or no value: use soft floats.
 * `n`, `no`, or `off`: use hardware floats (the default).
 
-## target-api-features
+## target-api-feature
 
 This option tells `rustc` which operating system APIs are expected to be supported for the target.
 Implied features are added as well.
 
+API features are added as a comma-separated list (`-C target_api_feature=a,b`). The option can be
+specified more than once to add more features the the list.
+
 #### Windows
 
 Currently, this option is only used for Windows targets, where the feature

src/librustc_interface/util.rs

@@ -54,19 +54,25 @@ pub fn add_configuration(
         cfg.insert((tf, Some(sym::crt_dash_static)));
     }
 
+    let ta = sym::target_api_feature;
     let target_options = &sess.target.target.options;
+    let mut selected_target_apis = sess.opts.cg.target_api_feature.as_ref();
+
     if let Some(target_api_kind) = &target_options.target_api_kind {
         let target_api_defaults = &target_options.target_api_default_features;
-        let selected_target_apis =
-            sess.opts.cg.target_api_features.as_ref().unwrap_or(target_api_defaults);
+
+        let api_features_to_check = selected_target_apis.unwrap_or(target_api_defaults);
+        selected_target_apis = Some(api_features_to_check);
 
         let apis = rustc_target::api::get_enabled_target_api_features(
             target_api_kind,
-            selected_target_apis,
+            api_features_to_check,
         );
 
-        let ta = sym::target_api_feature;
         cfg.extend(apis.into_iter().map(|api| (ta, Some(Symbol::intern(api)))));
+    }
+
+    if let Some(selected_target_apis) = selected_target_apis {
         cfg.extend(selected_target_apis.into_iter().map(|api| (ta, Some(Symbol::intern(api)))));
     }
 }

src/librustc_session/options.rs

@@ -273,6 +273,7 @@ macro_rules! options {
         pub const parse_tls_model: &str =
             "one of supported TLS models (`rustc --print tls-models`)";
         pub const parse_target_feature: &str = parse_string;
+        pub const parse_target_api_feature: &str = parse_opt_comma_list;
     }
 
     #[allow(dead_code)]
@@ -673,6 +674,17 @@ macro_rules! options {
                 None => false,
             }
         }
+
+        fn parse_target_api_feature(slot: &mut Option<Vec<String>>, v: Option<&str>) -> bool {
+            match (slot, v) {
+                (Some(vec), Some(s)) => { vec.extend(s.split(',').map(|s| s.to_string())); true },
+                (slot @ None, Some(s)) => {
+                    *slot = Some(s.split(',').map(|s| s.to_string()).collect());
+                    true
+                },
+                _ => false,
+            }
+        }
     }
 ) }
 
@@ -768,8 +780,9 @@ options! {CodegenOptions, CodegenSetter, basic_codegen_options,
         "save all temporary output files during compilation (default: no)"),
     soft_float: bool = (false, parse_bool, [TRACKED],
         "use soft float ABI (*eabihf targets only) (default: no)"),
-    target_api_features: Option<Vec<String>> = (None, parse_opt_list, [TRACKED],
-        "a list of target APIs that are expected to be be available on the target (default: use target default)"),
+    target_api_feature: Option<Vec<String>> = (None, parse_target_api_feature, [TRACKED],
+        "a comma-separated list of target APIs that are expected to be be available on the target
+        (default: use target default) (can be used multiple times)"),
     target_cpu: Option<String> = (None, parse_opt_string, [TRACKED],
         "select target processor (`rustc --print target-cpus` for details)"),
     target_feature: String = (String::new(), parse_target_feature, [TRACKED],