rust-lang/rust · critical

missing source file

Error message

missing source file

What it means

Fires during source-map import in the decoder (decoder.rs:1811). When a `Span` references a source file by index, the decoder lazily imports it from `root.source_map`; `.expect("missing source file")` panics when the index is absent from the serialized source-map table. The span's `metadata_index` points past the end of the encoded file list.

Source

Thrown at compiler/rustc_metadata/src/rmeta/decoder.rs:1811

                        *name =
                            rustc_span::FileName::Real(rustc_span::RealFileName::from_virtual_path(
                                &virtual_dir.join(subdir).join(rest),
                            ))
                    }
                }
            };

        let mut import_info = self.source_map_import_info.lock();
        for _ in import_info.len()..=(source_file_index as usize) {
            import_info.push(None);
        }
        import_info[source_file_index as usize]
            .get_or_insert_with(|| {
                let source_file_to_import = self
                    .root
                    .source_map
                    .get(self, source_file_index)
                    .expect("missing source file")
                    .decode((self, tcx));

                // We can't reuse an existing SourceFile, so allocate a new one
                // containing the information we need.
                let original_end_pos = source_file_to_import.end_position();
                let rustc_span::SourceFile {
                    mut name,
                    src_hash,
                    checksum_hash,
                    start_pos: original_start_pos,
                    normalized_source_len,
                    unnormalized_source_len,
                    lines,
                    multibyte_chars,
                    normalized_pos,
                    stable_id,
                    ..
                } = source_file_to_import;

View on GitHub (pinned to 7088e4b63a)

Solutions

1. `cargo clean` to rebuild the source-map table consistently with the current spans.
2. Ensure debuginfo/remap flags (`--remap-path-prefix`, `-Z simulate-remapped-rust-src-base`) are identical across the build graph.
3. For proc-macro crates, rebuild them with the active compiler so their source-map indices line up.
4. Clean-build ICE -> report a span/source-map index mismatch.

# Keep debuginfo/remap flags identical across the graph so span<->source-map
# indices stay consistent.
export RUSTFLAGS="--remap-path-prefix=$PWD/="   # identical for every crate
export RUSTDOCFLAGS="$RUSTFLAGS"
cargo clean && cargo build

cargo build || { cargo clean && cargo build; }

When it happens

Trigger: A span carrying a `metadata_index` that exceeds the encoded `source_map` array (index/count mismatch between span encoding and source-map encoding); reading proc-macro metadata where source files were not serialized but a span still references them.

Common situations: Remapping/debuginfo options changed between builds while reusing `target/`; proc-macro crate decoded with mismatched `-Z simulate-remapped-rust-src-base` settings; rmeta from a different rustc whose source-map layout differs.

Related errors

• no encoded ident for item
• variants are not encoded for an enum
• provided `DefIndex` must refer to a module-like item
• argument names not encoded for a function
• no parent for a constructor

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/fc5b4871d3da5807. Report an issue: GitHub.