Auto merge of #51335 - mark-i-m:allocator, r=oli-obk

Prohibit `global_allocator` in submodules

Background: #44113 is caused by weird interactions with hygiene. Hygiene is hard. After a lot of playing around, we decided that the best path forward would be to prohibit `global_allocator`s from being in submodules for now. When somebody gets it working, we can re-enable it.

This PR contains the following
- Some hygiene "fixes" -- things I suspect are the correct thing to do that will make life easier in the future. This includes using call_site hygiene for the generated module and passing the correct crate name to the expansion config.
- Comments and minor formatting fixes
- Some debugging code
- Code to prohibit `global_allocator` in submodules
- Test checking that the proper error occurs.

cc #44113 #49320 #51241

r? @alexcrichton

Author: bors

src/Cargo.lock

@@ -2035,6 +2035,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 name = "rustc_allocator"
 version = "0.0.0"
 dependencies = [
+ "log 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc 0.0.0",
  "rustc_errors 0.0.0",
  "rustc_target 0.0.0",

src/librustc_allocator/Cargo.toml

@@ -14,3 +14,4 @@ rustc_errors = { path = "../librustc_errors" }
 rustc_target = { path = "../librustc_target" }
 syntax = { path = "../libsyntax" }
 syntax_pos = { path = "../libsyntax_pos" }
+log = "0.4"

src/librustc_allocator/expand.rs

@@ -10,37 +10,45 @@
 
 use rustc::middle::allocator::AllocatorKind;
 use rustc_errors;
-use syntax::ast::{Attribute, Crate, LitKind, StrStyle};
-use syntax::ast::{Arg, FnHeader, Generics, Mac, Mutability, Ty, Unsafety};
-use syntax::ast::{self, Expr, Ident, Item, ItemKind, TyKind, VisibilityKind};
-use syntax::attr;
-use syntax::codemap::respan;
-use syntax::codemap::{ExpnInfo, MacroAttribute};
-use syntax::ext::base::ExtCtxt;
-use syntax::ext::base::Resolver;
-use syntax::ext::build::AstBuilder;
-use syntax::ext::expand::ExpansionConfig;
-use syntax::ext::hygiene::{self, Mark, SyntaxContext};
-use syntax::fold::{self, Folder};
-use syntax::parse::ParseSess;
-use syntax::ptr::P;
-use syntax::symbol::Symbol;
-use syntax::util::small_vector::SmallVector;
-use syntax_pos::{Span, DUMMY_SP};
+use syntax::{
+    ast::{
+        self, Arg, Attribute, Crate, Expr, FnHeader, Generics, Ident, Item, ItemKind,
+        LitKind, Mac, Mod, Mutability, StrStyle, Ty, TyKind, Unsafety, VisibilityKind,
+    },
+    attr,
+    codemap::{
+        respan, ExpnInfo, MacroAttribute,
+    },
+    ext::{
+        base::{ExtCtxt, Resolver},
+        build::AstBuilder,
+        expand::ExpansionConfig,
+        hygiene::{self, Mark, SyntaxContext},
+    },
+    fold::{self, Folder},
+    parse::ParseSess,
+    ptr::P,
+    symbol::Symbol,
+    util::small_vector::SmallVector,
+};
+use syntax_pos::Span;
 
 use {AllocatorMethod, AllocatorTy, ALLOCATOR_METHODS};
 
 pub fn modify(
     sess: &ParseSess,
     resolver: &mut Resolver,
     krate: Crate,
+    crate_name: String,
     handler: &rustc_errors::Handler,
 ) -> ast::Crate {
     ExpandAllocatorDirectives {
         handler,
         sess,
         resolver,
         found: false,
+        crate_name: Some(crate_name),
+        in_submod: -1, // -1 to account for the "root" module
     }.fold_crate(krate)
 }
 
@@ -49,10 +57,17 @@ struct ExpandAllocatorDirectives<'a> {
     handler: &'a rustc_errors::Handler,
     sess: &'a ParseSess,
     resolver: &'a mut Resolver,
+    crate_name: Option<String>,
+
+    // For now, we disallow `global_allocator` in submodules because hygiene is hard. Keep track of
+    // whether we are in a submodule or not. If `in_submod > 0` we are in a submodule.
+    in_submod: isize,
 }
 
 impl<'a> Folder for ExpandAllocatorDirectives<'a> {
     fn fold_item(&mut self, item: P<Item>) -> SmallVector<P<Item>> {
+        debug!("in submodule {}", self.in_submod);
+
         let name = if attr::contains_name(&item.attrs, "global_allocator") {
             "global_allocator"
         } else {
@@ -67,57 +82,93 @@ impl<'a> Folder for ExpandAllocatorDirectives<'a> {
             }
         }
 
+        if self.in_submod > 0 {
+            self.handler
+                .span_err(item.span, "`global_allocator` cannot be used in submodules");
+            return SmallVector::one(item);
+        }
+
         if self.found {
-            self.handler.span_err(
-                item.span,
-                "cannot define more than one \
-                 #[global_allocator]",
-            );
+            self.handler
+                .span_err(item.span, "cannot define more than one #[global_allocator]");
             return SmallVector::one(item);
         }
         self.found = true;
 
+        // Create a fresh Mark for the new macro expansion we are about to do
         let mark = Mark::fresh(Mark::root());
         mark.set_expn_info(ExpnInfo {
-            call_site: DUMMY_SP,
+            call_site: item.span, // use the call site of the static
             def_site: None,
             format: MacroAttribute(Symbol::intern(name)),
             allow_internal_unstable: true,
             allow_internal_unsafe: false,
             edition: hygiene::default_edition(),
         });
+
+        // Tie the span to the macro expansion info we just created
         let span = item.span.with_ctxt(SyntaxContext::empty().apply_mark(mark));
-        let ecfg = ExpansionConfig::default(name.to_string());
+
+        // Create an expansion config
+        let ecfg = ExpansionConfig::default(self.crate_name.take().unwrap());
+
+        // Generate a bunch of new items using the AllocFnFactory
         let mut f = AllocFnFactory {
             span,
             kind: AllocatorKind::Global,
             global: item.ident,
             core: Ident::from_str("core"),
             cx: ExtCtxt::new(self.sess, ecfg, self.resolver),
         };
+
+        // We will generate a new submodule. To `use` the static from that module, we need to get
+        // the `super::...` path.
         let super_path = f.cx.path(f.span, vec![Ident::from_str("super"), f.global]);
+
+        // Generate the items in the submodule
         let mut items = vec![
+            // import `core` to use allocators
             f.cx.item_extern_crate(f.span, f.core),
+            // `use` the `global_allocator` in `super`
             f.cx.item_use_simple(
                 f.span,
                 respan(f.span.shrink_to_lo(), VisibilityKind::Inherited),
                 super_path,
             ),
         ];
-        for method in ALLOCATOR_METHODS {
-            items.push(f.allocator_fn(method));
-        }
+
+        // Add the allocator methods to the submodule
+        items.extend(
+            ALLOCATOR_METHODS
+                .iter()
+                .map(|method| f.allocator_fn(method)),
+        );
+
+        // Generate the submodule itself
         let name = f.kind.fn_name("allocator_abi");
         let allocator_abi = Ident::with_empty_ctxt(Symbol::gensym(&name));
         let module = f.cx.item_mod(span, span, allocator_abi, Vec::new(), items);
         let module = f.cx.monotonic_expander().fold_item(module).pop().unwrap();
 
-        let mut ret = SmallVector::new();
+        // Return the item and new submodule
+        let mut ret = SmallVector::with_capacity(2);
         ret.push(item);
         ret.push(module);
+
         return ret;
     }
 
+    // If we enter a submodule, take note.
+    fn fold_mod(&mut self, m: Mod) -> Mod {
+        debug!("enter submodule");
+        self.in_submod += 1;
+        let ret = fold::noop_fold_mod(m, self);
+        self.in_submod -= 1;
+        debug!("exit submodule");
+        ret
+    }
+
+    // `fold_mac` is disabled by default. Enable it here.
     fn fold_mac(&mut self, mac: Mac) -> Mac {
         fold::noop_fold_mac(mac, self)
     }

src/librustc_allocator/lib.rs

@@ -10,6 +10,7 @@
 
 #![feature(rustc_private)]
 
+#[macro_use] extern crate log;
 extern crate rustc;
 extern crate rustc_errors;
 extern crate rustc_target;

src/librustc_driver/driver.rs

@@ -1051,10 +1051,19 @@ where
         });
     }
 
+    // Expand global allocators, which are treated as an in-tree proc macro
     krate = time(sess, "creating allocators", || {
-        allocator::expand::modify(&sess.parse_sess, &mut resolver, krate, sess.diagnostic())
+        allocator::expand::modify(
+            &sess.parse_sess,
+            &mut resolver,
+            krate,
+            crate_name.to_string(),
+            sess.diagnostic(),
+        )
     });
 
+    // Done with macro expansion!
+
     after_expand(&krate)?;
 
     if sess.opts.debugging_opts.input_stats {

src/test/ui/allocator-submodule.rs

@@ -0,0 +1,40 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Tests that it is possible to create a global allocator in a submodule, rather than in the crate
+// root.
+
+#![feature(alloc, allocator_api, global_allocator)]
+
+extern crate alloc;
+
+use std::{
+    alloc::{GlobalAlloc, Layout},
+    ptr,
+};
+
+struct MyAlloc;
+
+unsafe impl GlobalAlloc for MyAlloc {
+    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+        ptr::null_mut()
+    }
+
+    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {}
+}
+
+mod submod {
+    use super::MyAlloc;
+
+    #[global_allocator]
+    static MY_HEAP: MyAlloc = MyAlloc; //~ ERROR global_allocator
+}
+
+fn main() {}

src/test/ui/allocator-submodule.stderr

@@ -0,0 +1,8 @@
+error: `global_allocator` cannot be used in submodules
+  --> $DIR/allocator-submodule.rs:37:5
+   |
+LL |     static MY_HEAP: MyAlloc = MyAlloc; //~ ERROR global_allocator
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+