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>

0 files changed, 0 insertions, 0 deletions