Auto merge of #56647 - petrochenkov:dcrate2, r=alexcrichton

Rework treatment of `$crate` in procedural macros

Important clarification: `$crate` below means "processed `$crate`" or "output `$crate`". In the input of a decl macro `$crate` is just two separate tokens, but in the *output of a decl macro* `$crate` is a single keyword identifier (https://github.com/rust-lang/rust/issues/55640#issuecomment-435692791).

First of all, this PR removes the `eliminate_crate_var` hack.
`$crate::foo` is no longer replaced with `::foo` or `::crate_name::foo` in the input of derive proc macros, it's passed to the macro instead with its precise span and hygiene data, and can be treated as any other path segment keyword (like `crate` or `self`) after that. (Note: `eliminate_crate_var` was never used for non-derive proc macros.)

This creates an annoying problem - derive macros still may stringify their input before processing and expect `$crate` survive that stringification and refer to the same crate (the Rust 1.15-1.29 way of doing things).
Moreover, the input of proc macro attributes and derives (but not fn-like proc macros) also effectively survives stringification before being passed to the macro (also for legacy implementation reasons).

So we kind of resurrect the `eliminate_crate_var` hack in reduced form, but apply it only to AST pretty-printing.
If an AST fragment is pretty-printed, the resulting *text* will have `$crate` replaced with `crate` or `::crate_name`. This should be enough to keep all the legacy cases working.

Closes https://github.com/rust-lang/rust/issues/55640
Closes https://github.com/rust-lang/rust/issues/56622
r? @ghost

5 files changed, 31 insertions, 30 deletions

diff --git a/src/libsyntax/ext/base.rs b/src/libsyntax/ext/base.rs
index 310cf27689b..b807a65f6ae 100644
--- a/src/libsyntax/ext/base.rs
+++ b/src/libsyntax/ext/base.rs
@@ -732,7 +732,6 @@ pub type NamedSyntaxExtension = (Name, SyntaxExtension);
 pub trait Resolver {
     fn next_node_id(&mut self) -> ast::NodeId;
     fn get_module_scope(&mut self, id: ast::NodeId) -> Mark;
-    fn eliminate_crate_var(&mut self, item: P<ast::Item>) -> P<ast::Item>;
 
     fn visit_ast_fragment_with_placeholders(&mut self, mark: Mark, fragment: &AstFragment,
                                             derives: &[Mark]);
@@ -766,7 +765,6 @@ pub struct DummyResolver;
 impl Resolver for DummyResolver {
     fn next_node_id(&mut self) -> ast::NodeId { ast::DUMMY_NODE_ID }
     fn get_module_scope(&mut self, _id: ast::NodeId) -> Mark { Mark::root() }
-    fn eliminate_crate_var(&mut self, item: P<ast::Item>) -> P<ast::Item> { item }
 
     fn visit_ast_fragment_with_placeholders(&mut self, _invoc: Mark, _fragment: &AstFragment,
                                             _derives: &[Mark]) {}
diff --git a/src/libsyntax/ext/expand.rs b/src/libsyntax/ext/expand.rs
index 03c7aa96824..57ccc3e9817 100644
--- a/src/libsyntax/ext/expand.rs
+++ b/src/libsyntax/ext/expand.rs
@@ -203,10 +203,7 @@ fn macro_bang_format(path: &ast::Path) -> ExpnFormat {
         if i != 0 {
             path_str.push_str("::");
         }
-
-        if segment.ident.name != keywords::PathRoot.name() &&
-            segment.ident.name != keywords::DollarCrate.name()
-        {
+        if segment.ident.name != keywords::PathRoot.name() {
             path_str.push_str(&segment.ident.as_str())
         }
     }
diff --git a/src/libsyntax/parse/token.rs b/src/libsyntax/parse/token.rs
index ed746657459..badcc4ed876 100644
--- a/src/libsyntax/parse/token.rs
+++ b/src/libsyntax/parse/token.rs
@@ -633,7 +633,9 @@ impl Token {
             (&Shebang(a), &Shebang(b)) => a == b,
 
             (&Lifetime(a), &Lifetime(b)) => a.name == b.name,
-            (&Ident(a, b), &Ident(c, d)) => a.name == c.name && b == d,
+            (&Ident(a, b), &Ident(c, d)) => b == d && (a.name == c.name ||
+                                                       a.name == keywords::DollarCrate.name() ||
+                                                       c.name == keywords::DollarCrate.name()),
 
             (&Literal(ref a, b), &Literal(ref c, d)) => {
                 b == d && a.probably_equal_for_proc_macro(c)
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 41165c7e36d..5e7707f4e5c 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -16,7 +16,6 @@ use util::parser::{self, AssocOp, Fixity};
 use attr;
 use source_map::{self, SourceMap, Spanned};
 use syntax_pos::{self, BytePos};
-use syntax_pos::hygiene::{Mark, SyntaxContext};
 use parse::token::{self, BinOpToken, Token};
 use parse::lexer::comments;
 use parse::{self, ParseSess};
@@ -724,12 +723,12 @@ pub trait PrintState<'a> {
             if i > 0 {
                 self.writer().word("::")?
             }
-            if segment.ident.name != keywords::PathRoot.name() &&
-               segment.ident.name != keywords::DollarCrate.name()
-            {
-                self.writer().word(segment.ident.as_str().get())?;
-            } else if segment.ident.name == keywords::DollarCrate.name() {
-                self.print_dollar_crate(segment.ident.span.ctxt())?;
+            if segment.ident.name != keywords::PathRoot.name() {
+                if segment.ident.name == keywords::DollarCrate.name() {
+                    self.print_dollar_crate(segment.ident)?;
+                } else {
+                    self.writer().word(segment.ident.as_str().get())?;
+                }
             }
         }
         Ok(())
@@ -843,17 +842,19 @@ pub trait PrintState<'a> {
 
     fn nbsp(&mut self) -> io::Result<()> { self.writer().word(" ") }
 
-    fn print_dollar_crate(&mut self, mut ctxt: SyntaxContext) -> io::Result<()> {
-        if let Some(mark) = ctxt.adjust(Mark::root()) {
-            // Make a best effort to print something that complies
-            if mark.is_builtin() {
-                if let Some(name) = std_inject::injected_crate_name() {
-                    self.writer().word("::")?;
-                    self.writer().word(name)?;
-                }
-            }
+    // AST pretty-printer is used as a fallback for turning AST structures into token streams for
+    // proc macros. Additionally, proc macros may stringify their input and expect it survive the
+    // stringification (especially true for proc macro derives written between Rust 1.15 and 1.30).
+    // So we need to somehow pretty-print `$crate` in paths in a way preserving at least some of
+    // its hygiene data, most importantly name of the crate it refers to.
+    // As a result we print `$crate` as `crate` if it refers to the local crate
+    // and as `::other_crate_name` if it refers to some other crate.
+    fn print_dollar_crate(&mut self, ident: ast::Ident) -> io::Result<()> {
+        let name = ident.span.ctxt().dollar_crate_name();
+        if !ast::Ident::with_empty_ctxt(name).is_path_segment_keyword() {
+            self.writer().word("::")?;
         }
-        Ok(())
+        self.writer().word(name.as_str().get())
     }
 }
 
@@ -2463,14 +2464,15 @@ impl<'a> State<'a> {
                           colons_before_params: bool)
                           -> io::Result<()>
     {
-        if segment.ident.name != keywords::PathRoot.name() &&
-           segment.ident.name != keywords::DollarCrate.name() {
-            self.print_ident(segment.ident)?;
+        if segment.ident.name != keywords::PathRoot.name() {
+            if segment.ident.name == keywords::DollarCrate.name() {
+                self.print_dollar_crate(segment.ident)?;
+            } else {
+                self.print_ident(segment.ident)?;
+            }
             if let Some(ref args) = segment.args {
                 self.print_generic_args(args, colons_before_params)?;
             }
-        } else if segment.ident.name == keywords::DollarCrate.name() {
-            self.print_dollar_crate(segment.ident.span.ctxt())?;
         }
         Ok(())
     }
diff --git a/src/libsyntax/tokenstream.rs b/src/libsyntax/tokenstream.rs
index c11ef33f931..013ecd3d343 100644
--- a/src/libsyntax/tokenstream.rs
+++ b/src/libsyntax/tokenstream.rs
@@ -348,7 +348,9 @@ impl TokenStream {
                 | TokenTree::Token(_, Token::Semi)
                 // The pretty printer collapses whitespace arbitrarily and can
                 // introduce whitespace from `NoDelim`.
-                | TokenTree::Token(_, Token::Whitespace) => false,
+                | TokenTree::Token(_, Token::Whitespace)
+                // The pretty printer can turn `$crate` into `::crate_name`
+                | TokenTree::Token(_, Token::ModSep) => false,
                 _ => true
             }
         }