[Issue 18079] New: rdmd does not discover all dependencies

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18079

          Issue ID: 18079
           Summary: rdmd does not discover all dependencies
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: tools
          Assignee: nobody at puremagic.com
          Reporter: johnnymarler at gmail.com

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).

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