Do not re-export macros that are already at the crate root

[cjgillot]

Follow-up to #88019

Exported macro_rules were unconditionally reexported at the crate root. In the case of macro_rules that were already at the root, both the macro itself and its reexport appear with the same name in the same scope, which confuses the resolver for downstream crates.

This PR skips creating a re-export when the macro was already at the crate root.
r? @petrochenkov

[rust-highfive]

Some changes occurred in src/tools/clippy.

cc @rust-lang/clippy

[petrochenkov]

This is incorrect, all macro_export-ed macro_rules should be "planted" into the root module (this is what define does), without that they will remain non-modularized even if the macro_rules itself is in the root module.

decoder.rs must also filter away macro_rules items in each_child_of_item or at some level above, there's no way around that - macro_rules items should not appear in the list of module childern from name resolution point of view.

[petrochenkov]

Other users of each_child_of_item need to be audited on whether they want to see macro_rules items as children, but fn build_reduced_graph_external is the one use that certainly should filter them away.

[petrochenkov]

Test case for filtering in build_reduced_graph_external:

macro m() {}
macro_rules! m { () => () }

If macro_rules items are not skipped, then this will cause an error during decoding.

If macro_rules items are not skipped, then paths like other_crate::some_mod::some_exported_macro_rules will suddenly start working as well.

[petrochenkov]

This bit of logic is correct - we don't need to represent any macros as reexports, except for the "root reexports" of macro_exported macro_rules.

[rust-log-analyzer]

The job x86_64-gnu-llvm-10 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   Compiling parking_lot v0.11.1
   Compiling cstr v0.2.8
   Compiling rand_core v0.5.1
   Compiling regex-automata v0.1.10
error[E0428]: the name `extern_item` is defined multiple times
  --> /cargo/registry/src/github.com-1ecc6299db9ec823/psm-0.1.11/src/lib.rs:27:1
15 | macro_rules! extern_item {
15 | macro_rules! extern_item {
   | ------------------------ previous definition of the macro `extern_item` here
27 | macro_rules! extern_item {
27 | macro_rules! extern_item {
   | ^^^^^^^^^^^^^^^^^^^^^^^^ `extern_item` redefined here
   |
   = note: `extern_item` must be defined only once in the macro namespace of this crate
   Compiling regex v1.4.6
   Compiling rand_chacha v0.3.0
For more information about this error, try `rustc --explain E0428`.
error: could not compile `stacker` due to previous error

[bors]

☔ The latest upstream changes (presumably #88371) made this pull request unmergeable. Please resolve the merge conflicts.

[petrochenkov]

#88019 has been merged, unblocking.

[JohnCSimon]

Triage: Merge conflicts

[JohnCSimon]

Ping @cjgillot

#88019 has been merged, unblocking.

[petrochenkov]

I'm going to close this in favor of #91795.