Fix ICE: attempted to remap an already remapped filename

This commit fixes an internal compiler error (ICE) that occurs when
rustdoc attempts to process macros with a remapped filename. The issue
arose during macro expansion when the `--remap-path-prefix` option was
used.

Instead of passing remapped filenames through, which would trigger the
"attempted to remap an already remapped filename" panic, we now
extract the original local path from remapped filenames before
processing them.

A test case has been added to verify this behavior.

Fixes #138520

Signed-off-by: Charalampos Mitrodimas <[email protected]>

Author: charmitro

src/librustdoc/clean/render_macro_matchers.rs

@@ -4,8 +4,8 @@ use rustc_ast_pretty::pprust::PrintState;
 use rustc_ast_pretty::pprust::state::State as Printer;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::parse::ParseSess;
-use rustc_span::Span;
 use rustc_span::symbol::{Ident, Symbol, kw};
+use rustc_span::{FileName, RealFileName, Span};
 
 /// Render a macro matcher in a format suitable for displaying to the user
 /// as part of an item declaration.
@@ -63,7 +63,21 @@ fn snippet_equal_to_token(tcx: TyCtxt<'_>, matcher: &TokenTree) -> Option<String
 
     // Create a Parser.
     let psess = ParseSess::new(rustc_driver::DEFAULT_LOCALE_RESOURCES.to_vec());
-    let file_name = source_map.span_to_filename(span);
+
+    // Get the original filename and ensure we're using a fresh, unmapped filename
+    // to avoid trying to remap an already remapped filename
+    let file_name = match source_map.span_to_filename(span) {
+        FileName::Real(real_filename) => {
+            if let Some(local_path) = real_filename.clone().into_local_path() {
+                FileName::Real(RealFileName::LocalPath(local_path))
+            } else {
+                // Fallback to original if no local path is available
+                FileName::Real(real_filename.clone())
+            }
+        }
+        other => other,
+    };
+
     let mut parser =
         match rustc_parse::new_parser_from_source_str(&psess, file_name, snippet.clone()) {
             Ok(parser) => parser,

tests/rustdoc-ui/remap-path-prefix-macro.rs

@@ -0,0 +1,9 @@
+// Regression test for "attempted to remap an already remapped filename" ICE in rustdoc
+// when using --remap-path-prefix with macro rendering.
+// <https://github.com/rust-lang/rust/issues/138520>
+
+//@ compile-flags:-Z unstable-options --remap-path-prefix={{src-base}}=remapped_path
+//@ rustc-env:RUST_BACKTRACE=0
+//@ build-pass
+
+macro_rules! f(() => {});