Address review comments

src/libsyntax/ast.rs

@@ -105,17 +105,6 @@ pub fn from_ident(ident: Ident) -> Path {
         }
     }
 
-    // Make a "crate root" segment for this path unless it already has it
-    // or starts with something like `self`/`super`/`$crate`/etc.
-    pub fn make_root(&self) -> Option<PathSegment> {
-        if let Some(ident) = self.segments.get(0).map(|seg| seg.ident) {
-            if ident.is_path_segment_keyword() {
-                return None;
-            }
-        }
-        Some(PathSegment::crate_root(self.span.shrink_to_lo()))
-    }
-
     pub fn is_global(&self) -> bool {
         !self.segments.is_empty() && self.segments[0].ident.name == keywords::PathRoot.name()
     }
@@ -144,7 +133,7 @@ impl PathSegment {
     pub fn from_ident(ident: Ident) -> Self {
         PathSegment { ident, id: DUMMY_NODE_ID, args: None }
     }
-    pub fn crate_root(span: Span) -> Self {
+    pub fn path_root(span: Span) -> Self {
         PathSegment::from_ident(Ident::new(keywords::PathRoot.name(), span))
     }
 }

src/libsyntax/ext/build.rs

@@ -318,9 +318,13 @@ fn path_all(&self,
                 args: Vec<ast::GenericArg>,
                 bindings: Vec<ast::TypeBinding> )
                 -> ast::Path {
+        assert!(!idents.is_empty());
+        let add_root = global && !idents[0].is_path_segment_keyword();
+        let mut segments = Vec::with_capacity(idents.len() + add_root as usize);
+        if add_root {
+            segments.push(ast::PathSegment::path_root(span));
+        }
         let last_ident = idents.pop().unwrap();
-        let mut segments: Vec<ast::PathSegment> = vec![];
-
         segments.extend(idents.into_iter().map(|ident| {
             ast::PathSegment::from_ident(ident.with_span_pos(span))
         }));
@@ -334,13 +338,7 @@ fn path_all(&self,
             id: ast::DUMMY_NODE_ID,
             args,
         });
-        let mut path = ast::Path { span, segments };
-        if global {
-            if let Some(seg) = path.make_root() {
-                path.segments.insert(0, seg);
-            }
-        }
-        path
+        ast::Path { span, segments }
     }
 
     /// Constructs a qualified path.

src/libsyntax/parse/parser.rs

@@ -2082,7 +2082,7 @@ pub fn parse_path(&mut self, style: PathStyle) -> PResult<'a, ast::Path> {
         let mut segments = Vec::new();
         let mod_sep_ctxt = self.span.ctxt();
         if self.eat(&token::ModSep) {
-            segments.push(PathSegment::crate_root(lo.shrink_to_lo().with_ctxt(mod_sep_ctxt)));
+            segments.push(PathSegment::path_root(lo.shrink_to_lo().with_ctxt(mod_sep_ctxt)));
         }
         self.parse_path_segments(&mut segments, style, enable_warning)?;
 
@@ -7685,7 +7685,7 @@ fn parse_use_tree(&mut self) -> PResult<'a, UseTree> {
             let mod_sep_ctxt = self.span.ctxt();
             if self.eat(&token::ModSep) {
                 prefix.segments.push(
-                    PathSegment::crate_root(lo.shrink_to_lo().with_ctxt(mod_sep_ctxt))
+                    PathSegment::path_root(lo.shrink_to_lo().with_ctxt(mod_sep_ctxt))
                 );
             }
 

src/libsyntax_pos/symbol.rs

@@ -353,7 +353,7 @@ pub fn fresh() -> Self {
     (2,  DollarCrate,        "$crate")
     (3,  Underscore,         "_")
 
-    // Keywords used in the language.
+    // Keywords that are used in stable Rust.
     (4,  As,                 "as")
     (5,  Box,                "box")
     (6,  Break,              "break")
@@ -391,7 +391,7 @@ pub fn fresh() -> Self {
     (38, Where,              "where")
     (39, While,              "while")
 
-    // Keywords reserved for future use.
+    // Keywords that are used in unstable Rust or reserved for future use.
     (40, Abstract,           "abstract")
     (41, Become,             "become")
     (42, Do,                 "do")
@@ -404,10 +404,10 @@ pub fn fresh() -> Self {
     (49, Virtual,            "virtual")
     (50, Yield,              "yield")
 
-    // Edition-specific keywords used in the language.
+    // Edition-specific keywords that are used in stable Rust.
     (51, Dyn,                "dyn") // >= 2018 Edition only
 
-    // Edition-specific keywords reserved for future use.
+    // Edition-specific keywords that are used in unstable Rust or reserved for future use.
     (52, Async,              "async") // >= 2018 Edition only
     (53, Try,                "try") // >= 2018 Edition only