rustc: Add a #[wasm_custom_section] attribute

This commit is an implementation of adding custom sections to wasm artifacts in
rustc. The intention here is to expose the ability of the wasm binary format to
contain custom sections with arbitrary user-defined data. Currently neither our
version of LLVM nor LLD supports this so the implementation is currently custom
to rustc itself.

The implementation here is to attach a `#[wasm_custom_section = "foo"]`
attribute to any `const` which has a type like `[u8; N]`. Other types of
constants aren't supported yet but may be added one day! This should hopefully
be enough to get off the ground with *some* custom section support.

The current semantics are that any constant tagged with `#[wasm_custom_section]`
section will be *appended* to the corresponding section in the final output wasm
artifact (and this affects dependencies linked in as well, not just the final
crate). This means that whatever is interpreting the contents must be able to
interpret binary-concatenated sections (or each constant needs to be in its own
custom section).

To test this change the existing `run-make` test suite was moved to a
`run-make-fulldeps` folder and a new `run-make` test suite was added which
applies to all targets by default. This test suite currently only has one test
which only runs for the wasm target (using a node.js script to use `WebAssembly`
in JS to parse the wasm output).

Author: alexcrichton

src/bootstrap/builder.rs

@@ -313,6 +313,7 @@ impl<'a> Builder<'a> {
                 test::RunPassFullDepsPretty, test::RunFailFullDepsPretty,
                 test::Crate, test::CrateLibrustc, test::CrateRustdoc, test::Linkcheck,
                 test::Cargotest, test::Cargo, test::Rls, test::ErrorIndex, test::Distcheck,
+                test::RunMakeFullDeps,
                 test::Nomicon, test::Reference, test::RustdocBook, test::RustByExample,
                 test::TheBook, test::UnstableBook,
                 test::Rustfmt, test::Miri, test::Clippy, test::RustdocJS, test::RustdocTheme,

src/bootstrap/compile.rs

@@ -915,7 +915,7 @@ impl Step for Assemble {
             }
         }
 
-        let lld_install = if build.config.lld_enabled && target_compiler.stage > 0 {
+        let lld_install = if build.config.lld_enabled {
             Some(builder.ensure(native::Lld {
                 target: target_compiler.host,
             }))

src/bootstrap/test.rs

@@ -759,12 +759,18 @@ test!(RunFailFullDepsPretty {
     host: true
 });
 
-host_test!(RunMake {
+default_test!(RunMake {
     path: "src/test/run-make",
     mode: "run-make",
     suite: "run-make"
 });
 
+host_test!(RunMakeFullDeps {
+    path: "src/test/run-make-fulldeps",
+    mode: "run-make",
+    suite: "run-make-fulldeps"
+});
+
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
 struct Compiletest {
     compiler: Compiler,
@@ -827,8 +833,7 @@ impl Step for Compiletest {
             // FIXME: Does pretty need librustc compiled? Note that there are
             // fulldeps test suites with mode = pretty as well.
             mode == "pretty" ||
-            mode == "rustdoc" ||
-            mode == "run-make" {
+            mode == "rustdoc" {
             builder.ensure(compile::Rustc { compiler, target });
         }
 
@@ -849,7 +854,7 @@ impl Step for Compiletest {
         cmd.arg("--rustc-path").arg(builder.rustc(compiler));
 
         // Avoid depending on rustdoc when we don't need it.
-        if mode == "rustdoc" || mode == "run-make" {
+        if mode == "rustdoc" || (mode == "run-make" && suite.ends_with("fulldeps")) {
             cmd.arg("--rustdoc-path").arg(builder.rustdoc(compiler.host));
         }
 
@@ -931,7 +936,7 @@ impl Step for Compiletest {
 
             // Only pass correct values for these flags for the `run-make` suite as it
             // requires that a C++ compiler was configured which isn't always the case.
-            if suite == "run-make" {
+            if suite == "run-make-fulldeps" {
                 let llvm_components = output(Command::new(&llvm_config).arg("--components"));
                 let llvm_cxxflags = output(Command::new(&llvm_config).arg("--cxxflags"));
                 cmd.arg("--cc").arg(build.cc(target))
@@ -944,12 +949,12 @@ impl Step for Compiletest {
                 }
             }
         }
-        if suite == "run-make" && !build.config.llvm_enabled {
+        if suite == "run-make-fulldeps" && !build.config.llvm_enabled {
             println!("Ignoring run-make test suite as they generally don't work without LLVM");
             return;
         }
 
-        if suite != "run-make" {
+        if suite != "run-make-fulldeps" {
             cmd.arg("--cc").arg("")
                .arg("--cxx").arg("")
                .arg("--cflags").arg("")

src/librustc/dep_graph/dep_node.rs

@@ -650,6 +650,8 @@ define_dep_nodes!( <'tcx>
 
     [] GetSymbolExportLevel(DefId),
 
+    [] WasmCustomSections(CrateNum),
+
     [input] Features,
 
     [] ProgramClausesFor(DefId),

src/librustc/hir/check_attr.rs

@@ -25,6 +25,7 @@ enum Target {
     Struct,
     Union,
     Enum,
+    Const,
     Other,
 }
 
@@ -35,6 +36,7 @@ impl Target {
             hir::ItemStruct(..) => Target::Struct,
             hir::ItemUnion(..) => Target::Union,
             hir::ItemEnum(..) => Target::Enum,
+            hir::ItemConst(..) => Target::Const,
             _ => Target::Other,
         }
     }
@@ -60,6 +62,17 @@ impl<'a, 'tcx> CheckAttrVisitor<'a, 'tcx> {
                 if name == "inline" {
                     self.check_inline(attr, item, target)
                 }
+
+                if name == "wasm_custom_section" {
+                    if target != Target::Const {
+                        self.tcx.sess.span_err(attr.span, "only allowed on consts");
+                    }
+
+                    if attr.value_str().is_none() {
+                        self.tcx.sess.span_err(attr.span, "must be of the form \
+                            #[wasm_custom_section = \"foo\"]");
+                    }
+                }
             }
         }
 

src/librustc/middle/dead.rs

@@ -318,6 +318,11 @@ fn has_allow_dead_code_or_lang_attr(tcx: TyCtxt,
         return true;
     }
 
+    // These constants are special for wasm
+    if attr::contains_name(attrs, "wasm_custom_section") {
+        return true;
+    }
+
     tcx.lint_level_at_node(lint::builtin::DEAD_CODE, id).0 == lint::Allow
 }
 

src/librustc/ty/maps/config.rs

@@ -678,6 +678,12 @@ impl<'tcx> QueryDescription<'tcx> for queries::instance_def_size_estimate<'tcx>
     }
 }
 
+impl<'tcx> QueryDescription<'tcx> for queries::wasm_custom_sections<'tcx> {
+    fn describe(_tcx: TyCtxt, _: CrateNum) -> String {
+        format!("custom wasm sections for a crate")
+    }
+}
+
 impl<'tcx> QueryDescription<'tcx> for queries::generics_of<'tcx> {
     #[inline]
     fn cache_on_disk(def_id: Self::Key) -> bool {

src/librustc/ty/maps/mod.rs

@@ -424,6 +424,8 @@ define_maps! { <'tcx>
     [] fn features_query: features_node(CrateNum) -> Lrc<feature_gate::Features>,
 
     [] fn program_clauses_for: ProgramClausesFor(DefId) -> Lrc<Vec<Clause<'tcx>>>,
+
+    [] fn wasm_custom_sections: WasmCustomSections(CrateNum) -> Lrc<Vec<DefId>>,
 }
 
 //////////////////////////////////////////////////////////////////////

src/librustc/ty/maps/plumbing.rs

@@ -940,6 +940,7 @@ pub fn force_from_dep_node<'a, 'gcx, 'lcx>(tcx: TyCtxt<'a, 'gcx, 'lcx>,
         DepKind::Features => { force!(features_query, LOCAL_CRATE); }
 
         DepKind::ProgramClausesFor => { force!(program_clauses_for, def_id!()); }
+        DepKind::WasmCustomSections => { force!(wasm_custom_sections, krate!()); }
     }
 
     true

src/librustc_metadata/cstore_impl.rs

@@ -271,6 +271,8 @@ provide! { <'tcx> tcx, def_id, other, cdata,
 
         Arc::new(cdata.exported_symbols())
     }
+
+    wasm_custom_sections => { Lrc::new(cdata.wasm_custom_sections()) }
 }
 
 pub fn provide<'tcx>(providers: &mut Providers<'tcx>) {