Rollup merge of #48759 - QuietMisdreavus:simd-feature-docs, r=GuillaumeGomez

rustdoc: expose #[target_feature] attributes as doc(cfg) flags

This change exposes `#[target_feature(enable = "feat")]` attributes on an item as if they were also `#[doc(cfg(target_feature = "feat"))]` attributes. This gives them a banner on their documentation listing which feature is required to use the item. It also modifies the rendering code for doc(cfg) tags to handle `target_feature` tags. I made it print just the feature name on "short" printings (as in the function listing on a module page), and use "target feature `feat`" in the full banner on the item page itself.

This way, the function listing in `std::arch` shows which feature is required for each function:

![image](https://user-images.githubusercontent.com/5217170/37003222-f41b9d66-2091-11e8-9656-8719e5b34832.png)

![image](https://user-images.githubusercontent.com/5217170/37003234-feb1a7a2-2091-11e8-94de-6d1d76a2d3ee.png)

src/librustdoc/clean/cfg.rs

@@ -138,7 +138,7 @@ impl Cfg {
 
     /// Renders the configuration for human display, as a short HTML description.
     pub(crate) fn render_short_html(&self) -> String {
-        let mut msg = Html(self).to_string();
+        let mut msg = ShortHtml(self).to_string();
         if self.should_capitalize_first_letter() {
             if let Some(i) = msg.find(|c: char| c.is_ascii_alphanumeric()) {
                 msg[i .. i+1].make_ascii_uppercase();
@@ -149,7 +149,13 @@ impl Cfg {
 
     /// Renders the configuration for long display, as a long HTML description.
     pub(crate) fn render_long_html(&self) -> String {
-        let mut msg = format!("This is supported on <strong>{}</strong>", Html(self));
+        let on = if self.should_use_with_in_description() {
+            "with"
+        } else {
+            "on"
+        };
+
+        let mut msg = format!("This is supported {} <strong>{}</strong>", on, Html(self));
         if self.should_append_only_to_description() {
             msg.push_str(" only");
         }
@@ -180,6 +186,13 @@ impl Cfg {
             }
         }
     }
+
+    fn should_use_with_in_description(&self) -> bool {
+        match *self {
+            Cfg::Cfg(ref name, _) if name == &"target_feature" => true,
+            _ => false,
+        }
+    }
 }
 
 impl ops::Not for Cfg {
@@ -376,6 +389,8 @@ impl<'a> fmt::Display for Html<'a> {
                     },
                     ("target_endian", Some(endian)) => return write!(fmt, "{}-endian", endian),
                     ("target_pointer_width", Some(bits)) => return write!(fmt, "{}-bit", bits),
+                    ("target_feature", Some(feat)) =>
+                        return write!(fmt, "target feature <code>{}</code>", feat),
                     _ => "",
                 };
                 if !human_readable.is_empty() {
@@ -390,6 +405,19 @@ impl<'a> fmt::Display for Html<'a> {
     }
 }
 
+struct ShortHtml<'a>(&'a Cfg);
+
+impl<'a> fmt::Display for ShortHtml<'a> {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        match *self.0 {
+            Cfg::Cfg(ref name, Some(ref vendor)) if name == &"target_feature" => {
+                write!(fmt, "<code>{}</code>", vendor)
+            },
+            ref cfg => write!(fmt, "{}", Html(cfg)),
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::Cfg;
@@ -824,6 +852,10 @@ mod test {
                 ).render_short_html(),
                 "(Debug-assertions enabled or Windows) and Unix"
             );
+            assert_eq!(
+                name_value_cfg("target_feature", "sse2").render_short_html(),
+                "<code>sse2</code>"
+            );
         })
     }
 
@@ -898,6 +930,10 @@ mod test {
                 "This is supported on <strong>(debug-assertions enabled or Windows) and Unix\
                 </strong> only."
             );
+            assert_eq!(
+                name_value_cfg("target_feature", "sse2").render_long_html(),
+                "This is supported with <strong>target feature <code>sse2</code></strong> only."
+            );
         })
     }
 }

src/librustdoc/clean/mod.rs

@@ -828,6 +828,19 @@ impl Attributes {
             })
         }).collect();
 
+        // treat #[target_feature(enable = "feat")] attributes as if they were
+        // #[doc(cfg(target_feature = "feat"))] attributes as well
+        for attr in attrs.lists("target_feature") {
+            if attr.check_name("enable") {
+                if let Some(feat) = attr.value_str() {
+                    let meta = attr::mk_name_value_item_str("target_feature".into(), feat);
+                    if let Ok(feat_cfg) = Cfg::parse(&meta) {
+                        cfg &= feat_cfg;
+                    }
+                }
+            }
+        }
+
         Attributes {
             doc_strings,
             other_attrs,

src/test/rustdoc/doc-cfg.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(doc_cfg)]
+#![feature(target_feature, cfg_target_feature)]
 
 // @has doc_cfg/struct.Portable.html
 // @!has - '//*[@id="main"]/*[@class="stability"]/*[@class="stab portability"]' ''
@@ -45,3 +46,26 @@ pub mod unix_only {
         fn unix_and_arm_only_function() {}
     }
 }
+
+// tagging a function with `#[target_feature]` creates a doc(cfg(target_feature)) node for that
+// item as well
+
+// the portability header is different on the module view versus the full view
+// @has doc_cfg/index.html
+// @matches - '//*[@class=" module-item"]//*[@class="stab portability"]' '\Aavx\Z'
+
+// @has doc_cfg/fn.uses_target_feature.html
+// @has - '//*[@id="main"]/*[@class="stability"]/*[@class="stab portability"]' \
+//        'This is supported with target feature avx only.'
+#[target_feature(enable = "avx")]
+pub unsafe fn uses_target_feature() {
+    content::should::be::irrelevant();
+}
+
+// @has doc_cfg/fn.uses_cfg_target_feature.html
+// @has - '//*[@id="main"]/*[@class="stability"]/*[@class="stab portability"]' \
+//        'This is supported with target feature avx only.'
+#[doc(cfg(target_feature = "avx"))]
+pub fn uses_cfg_target_feature() {
+    uses_target_feature();
+}