Rollup merge of #51952 - petrochenkov:transmark, r=alexcrichton

 hygiene: Decouple transparencies from expansion IDs

And remove fallback to parent modules during resolution of names in scope.

This is a breaking change for users of unstable macros 2.0 (both procedural and declarative), code like this:
```rust
#![feature(decl_macro)]

macro m($S: ident) {
    struct $S;
    mod m {
        type A = $S;
    }
}

fn main() {
    m!(S);
}
```
or equivalent
```rust
#![feature(decl_macro)]

macro m($S: ident) {
    mod m {
        type A = $S;
    }
}

fn main() {
    struct S;
    m!(S);
}
```
stops working due to module boundaries being properly enforced.

For proc macro derives this is still reported as a compatibility warning to give `actix_derive`, `diesel_derives` and `palette_derive` time to fix their issues.

Fixes https://github.com/rust-lang/rust/issues/50504 in accordance with [this comment](https://github.com/rust-lang/rust/issues/50504#issuecomment-399764767).

1 files changed, 6 insertions, 12 deletions

diff --git a/src/libsyntax/ext/base.rs b/src/libsyntax/ext/base.rs
index e2424de4d14..2e9c7d6f96c 100644
--- a/src/libsyntax/ext/base.rs
+++ b/src/libsyntax/ext/base.rs
@@ -17,7 +17,7 @@ use syntax_pos::{Span, MultiSpan, DUMMY_SP};
 use edition::Edition;
 use errors::{DiagnosticBuilder, DiagnosticId};
 use ext::expand::{self, AstFragment, Invocation};
-use ext::hygiene::{self, Mark, SyntaxContext};
+use ext::hygiene::{self, Mark, SyntaxContext, Transparency};
 use fold::{self, Folder};
 use parse::{self, parser, DirectoryOwnership};
 use parse::token;
@@ -673,20 +673,14 @@ impl SyntaxExtension {
         }
     }
 
-    pub fn is_modern(&self) -> bool {
+    pub fn default_transparency(&self) -> Transparency {
         match *self {
-            SyntaxExtension::DeclMacro { .. } |
             SyntaxExtension::ProcMacro { .. } |
             SyntaxExtension::AttrProcMacro(..) |
-            SyntaxExtension::ProcMacroDerive(..) => true,
-            _ => false,
-        }
-    }
-
-    pub fn is_transparent(&self) -> bool {
-        match *self {
-            SyntaxExtension::DeclMacro { is_transparent, .. } => is_transparent,
-            _ => false,
+            SyntaxExtension::ProcMacroDerive(..) |
+            SyntaxExtension::DeclMacro { is_transparent: false, .. } => Transparency::Opaque,
+            SyntaxExtension::DeclMacro { is_transparent: true, .. } => Transparency::Transparent,
+            _ => Transparency::SemiTransparent,
         }
     }