Stdin build silently ignores non-inline mod items

[mystor]

Missing external modules don't report a build error when a crate is built from stdin.

Expected: When parsing an external submodule from stdin, a compiler error (or warning) is emitted noting that external submodules are not supported for stdio crates.

Actual: No diagnostics are emitted and an empty submodule is implicitly inserted.

Generally if a mod statement references a file which does not exist, an E0583 compile error will be emitted. However, when building a crate from stdin, no relative filesystem is present, yet no compile errors are emitted, even with explicit #[path="x"] attributes. Instead they are implicitly substituted with empty inline modules.

# Surprising stdin cases
$ echo 'mod x;' | rustc --crate-type=rlib -  # Builds
$ echo '#[path="/y"] mod x;' | rustc --crate-type=rlib -  # Builds

# Working file case
$ echo 'mod x;' > y.rs
$ rustc --crate-type=rlib y.rs
error[E0583]: file not found for module `x`
 --> y.rs:1:5
[...]

I'm not super familiar with rustc, so this might not be handy, but I'm guessing this is caused by the Parser::recurse_into_file_modules flag being unset when parsing from a string rather than a file (rustc_interface::passes::parse -> syntax::parse::parse_crate_from_source_str -> syntax::parse::new_parser_from_source_str), leading to a dummy module being inserted rather than trying to access the filesystem or emit an error during the parse.

[Enselic]

This was fixed in nightly-2020-03-20, probably by #69838

I don't think this is worth adding a regression test for this TBH, so I think we should simply close this issue. It remains fixed in current stable (1.71).

$ echo 'mod x;' | rustc +nightly-2020-03-19 --crate-type=rlib -
$ echo 'mod x;' | rustc +nightly-2020-03-20 --crate-type=rlib -
error[E0583]: file not found for module `x`
 --> <anon>:1:1
  |
1 | mod x;
  | ^^^^^^
  |
  = help: to create the module `x`, create file "x.rs"

error: aborting due to previous error

For more information about this error, try `rustc --explain E0583`.
$ echo 'mod x;' | rustc +stable --crate-type=rlib -
error[E0583]: file not found for module `x`
 --> <anon>:1:1
  |
1 | mod x;
  | ^^^^^^
  |
  = help: to create the module `x`, create file "x.rs" or "x/mod.rs"

error: aborting due to previous error

For more information about this error, try `rustc --explain E0583`.

[Enselic]

On a second thought the right way to close this issue is probably by writing a regression test after all. Otherwise the bug can re-appear. I was being too lazy.

Edit: Fix: #114093

@rustbot label E-needs-test