Merge #785

785: rust: track inline module scopes for module file resolution r=CohenArthur a=mathstuf

The set of inline modules is required to find the expected location of a
module file. Track this information with an RAII object
(`InlineModuleStackScope`) and pass it down to any out-of-line modules
so that, when requested, the set of inline modules can be added to the
search path.

Signed-off-by: Ben Boeckel <mathstuf@gmail.com>

---
Note that this does not include a test case because I have no idea how to mark up this for all the warnings that come out (#676):

```diff
diff --git a/gcc/testsuite/rust/compile/missing_middle/sub/mod.rs b/gcc/testsuite/rust/compile/missing_middle/sub/mod.rs
new file mode 100644
index 00000000000..f099d61e04a
--- /dev/null
+++ b/gcc/testsuite/rust/compile/missing_middle/sub/mod.rs
`@@` -0,0 +1,3 `@@`
+pub fn f() -> u32 {
+    1
+}
diff --git a/gcc/testsuite/rust/compile/mod_missing_middle.rs b/gcc/testsuite/rust/compile/mod_missing_middle.rs
new file mode 100644
index 00000000000..d9cdf0a54f1
--- /dev/null
+++ b/gcc/testsuite/rust/compile/mod_missing_middle.rs
`@@` -0,0 +1,11 `@@`
+pub mod missing_middle {
+    pub mod sub;
+}
```

Observed warnings:

```
/home/boeckb/code/depot/group-compilers/gcc/src-gccrs/gcc/testsuite/rust/compile/missing_middle/sub/mod.rs:1:5: warning: unused name 'f'
/home/boeckb/code/depot/group-compilers/gcc/src-gccrs/gcc/testsuite/rust/compile/mod_missing_middle.rs:2:9: warning: unused name 'sub'
/home/boeckb/code/depot/group-compilers/gcc/src-gccrs/gcc/testsuite/rust/compile/missing_middle/sub/mod.rs:1:5: warning: unused name 'sub::f'
/home/boeckb/code/depot/group-compilers/gcc/src-gccrs/gcc/testsuite/rust/compile/mod_missing_middle.rs:2:9: warning: unused name 'missing_middle::sub'
/home/boeckb/code/depot/group-compilers/gcc/src-gccrs/gcc/testsuite/rust/compile/mod_missing_middle.rs:1:5: warning: unused name 'missing_middle'
/home/boeckb/code/depot/group-compilers/gcc/src-gccrs/gcc/testsuite/rust/compile/missing_middle/sub/mod.rs:1:5: warning: unused name 'missing_middle::sub::f'
```

Fixes: #645 

Here is a checklist to help you with your PR.

- \[x] GCC development requires copyright assignment or the Developer's Certificate of Origin sign-off, see https://gcc.gnu.org/contribute.html or https://gcc.gnu.org/dco.html
- \[x] Read contributing guidlines
- \[x] `make check-rust` passes locally
- \[x] Run `clang-format` (leaving to CI)
- \[x] Added any relevant test cases to `gcc/testsuite/rust/` (see above)

Co-authored-by: Ben Boeckel <mathstuf@gmail.com>

1 files changed, 73 insertions, 0 deletions

diff --git a/gcc/rust/parse/rust-parse.cc b/gcc/rust/parse/rust-parse.cc
index fdad97c..e78de51 100644
--- a/gcc/rust/parse/rust-parse.cc
+++ b/gcc/rust/parse/rust-parse.cc
@@ -46,4 +46,77 @@ along with GCC; see the file COPYING3.  If not see
 
 namespace Rust {
 
+std::string
+extract_module_path (const AST::AttrVec &inner_attrs,
+		     const AST::AttrVec &outer_attrs, const std::string &name)
+{
+  AST::Attribute path_attr = AST::Attribute::create_empty ();
+  for (const auto &attr : inner_attrs)
+    {
+      if (attr.get_path ().as_string () == "path")
+	{
+	  path_attr = attr;
+	  break;
+	}
+    }
+
+  // Here, we found a path attribute, but it has no associated string. This is
+  // invalid
+  if (!path_attr.is_empty () && !path_attr.has_attr_input ())
+    {
+      rust_error_at (
+	path_attr.get_locus (),
+	// Split the format string so that -Wformat-diag does not complain...
+	"path attributes must contain a filename: '%s'", "#![path = \"file\"]");
+      return name;
+    }
+
+  for (const auto &attr : outer_attrs)
+    {
+      if (attr.get_path ().as_string () == "path")
+	{
+	  path_attr = attr;
+	  break;
+	}
+    }
+
+  // We didn't find a path attribute. This is not an error, there simply isn't
+  // one present
+  if (path_attr.is_empty ())
+    return name;
+
+  // Here, we found a path attribute, but it has no associated string. This is
+  // invalid
+  if (!path_attr.has_attr_input ())
+    {
+      rust_error_at (
+	path_attr.get_locus (),
+	// Split the format string so that -Wformat-diag does not complain...
+	"path attributes must contain a filename: '%s'", "#[path = \"file\"]");
+      return name;
+    }
+
+  auto path_value = path_attr.get_attr_input ().as_string ();
+
+  // At this point, the 'path' is of the following format: '= "<file.rs>"'
+  // We need to remove the equal sign and only keep the actual filename.
+  // In order to do this, we can simply go through the string until we find
+  // a character that is not an equal sign or whitespace
+  auto filename_begin = path_value.find_first_not_of ("=\t ");
+
+  auto path = path_value.substr (filename_begin);
+
+  // On windows, the path might mix '/' and '\' separators. Replace the
+  // UNIX-like separators by MSDOS separators to make sure the path will resolve
+  // properly.
+  //
+  // Source: rustc compiler
+  // (https://github.com/rust-lang/rust/blob/9863bf51a52b8e61bcad312f81b5193d53099f9f/compiler/rustc_expand/src/module.rs#L174)
+#if defined(HAVE_DOS_BASED_FILE_SYSTEM)
+  path.replace ('/', '\\');
+#endif /* HAVE_DOS_BASED_FILE_SYSTEM */
+
+  return path;
+}
+
 } // namespace Rust