Related to (https://issues.dlang.org/show_bug.cgi?id=8858).  That bug was created because dmd would not discover all the imported modules when compiling a single file.  However, this was closed as a "non issue" because the compiler is not "supposed" to discover all dependencies when compiling a single module.  If an application wants to discover all imported modules, then they can use the "-deps" flag to force the compiler to do so.

rdmd currently uses this "import discovery" to obtain a list of all the modules it needs to compile.  However, rdmd does not use the "-deps" flag when discovering dependencies which means that in some cases it will be missing modules to compile, i.e.

main.d
```
import foo;
void main() { printx(); }
```
foo.d
```
module foo;
void printx() { import std.stdio; import bar : x; writefln("x is %s", x); }
```
bar.d
```
module bar;
int x = 42;
```

> rdmd main.d

this could be fixed by modifying rdmd to use the "-deps" flag, however, this has a significant performance cost.  Another solution proposed is to integrate the "compile imports" feature (https://github.com/dlang/dmd/pull/7099) and modify rdmd to use that feature instead (https://github.com/dlang/tools/pull/271).