compiler: allow //go:embed in files underscore-importing embed

The embed spec allows for //go:embed to be used in files that
underscore-import package "embed". This is useful for embeds to
[]byte and string vars because the embed.FS type may not be referenced
if those are the only types of embeds in a file. Because the compiler
previously checked whether there were any aliases to the embed
package to decide if //go:embed could be used, it would reject
files with only underscore imports of embed. Instead, record
whether the embed import is encountered at all, similar to what
is done with unsafe, to decide whether //go:embed is allowed.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/297553

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-56cf388da8d04bbd3824c4df34d77a8afa69749b
+2c5188b5ad6143e791f2ba42f02a4ea7887d87b6
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/embed.cc

@@ -663,21 +663,6 @@ Embedcfg_reader::error(const char* msg)
 		this->filename_, msg);
 }
 
-// Return whether the current file imports "embed".
-
-bool
-Gogo::is_embed_imported() const
-{
-  Packages::const_iterator p = this->packages_.find("embed");
-  if (p == this->packages_.end())
-    return false;
-
-  // We track current file imports in the package aliases, where a
-  // typical import will just list the package name in aliases.  So
-  // the package has been imported if there is at least one alias.
-  return !p->second->aliases().empty();
-}
-
 // Implement the sort order for a list of embedded files, as discussed
 // at the docs for embed.FS.
 

gcc/go/gofrontend/gogo.cc

@@ -37,6 +37,7 @@ Gogo::Gogo(Backend* backend, Linemap* linemap, int, int pointer_size)
     imports_(),
     imported_unsafe_(false),
     current_file_imported_unsafe_(false),
+    current_file_imported_embed_(false),
     packages_(),
     init_functions_(),
     var_deps_(),
@@ -469,6 +470,9 @@ Gogo::import_package(const std::string& filename,
       return;
     }
 
+  if (filename == "embed")
+    this->current_file_imported_embed_ = true;
+
   Imports::const_iterator p = this->imports_.find(filename);
   if (p != this->imports_.end())
     {
@@ -2717,6 +2721,7 @@ Gogo::clear_file_scope()
     }
 
   this->current_file_imported_unsafe_ = false;
+  this->current_file_imported_embed_ = false;
 }
 
 // Queue up a type-specific hash function for later writing.  These

gcc/go/gofrontend/gogo.h

@@ -397,10 +397,6 @@ class Gogo
   void
   read_embedcfg(const char* filename);
 
-  // Return whether the current file imports "embed".
-  bool
-  is_embed_imported() const;
-
   // Build an initializer for a variable with a go:embed directive.
   Expression*
   initializer_for_embeds(Type*, const std::vector<std::string>*, Location);
@@ -709,6 +705,11 @@ class Gogo
   current_file_imported_unsafe() const
   { return this->current_file_imported_unsafe_; }
 
+  // Return whether the current file imported the embed package.
+  bool
+  current_file_imported_embed() const
+  { return this->current_file_imported_embed_; }
+
   // Clear out all names in file scope.  This is called when we start
   // parsing a new file.
   void
@@ -1251,6 +1252,8 @@ class Gogo
   bool imported_unsafe_;
   // Whether the magic unsafe package was imported by the current file.
   bool current_file_imported_unsafe_;
+  // Whether the embed package was imported by the current file.
+  bool current_file_imported_embed_;
   // Mapping from package names we have seen to packages.  This does
   // not include the package we are compiling.
   Packages packages_;

gcc/go/gofrontend/parse.cc

@@ -1321,7 +1321,7 @@ Parse::declaration()
       embeds = new(std::vector<std::string>);
       this->lex_->get_and_clear_embeds(embeds);
 
-      if (!this->gogo_->is_embed_imported())
+      if (!this->gogo_->current_file_imported_embed())
 	{
 	  go_error_at(token->location(),
 		      "invalid go:embed: missing import %<embed%>");