Fix ICE: attempted to remap an already remapped filename

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, Span};
 
 /// Render a macro matcher in a format suitable for displaying to the user
 /// as part of an item declaration.
@@ -63,7 +63,7 @@ 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);
+    let file_name = FileName::macro_expansion_source_code(&snippet);
     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(() => {});