Heuristically undo path prefix mappings.

Because the compiler produces better diagnostics if it can find the
source of (potentially remapped) dependencies.

compiler/rustc_span/src/source_map.rs

@@ -17,7 +17,7 @@
 use rustc_data_structures::sync::{AtomicU32, Lrc, MappedReadGuard, ReadGuard, RwLock};
 use std::cmp;
 use std::hash::Hash;
-use std::path::{Path, PathBuf};
+use std::path::{self, Path, PathBuf};
 use std::sync::atomic::Ordering;
 
 use std::fs;
@@ -1071,12 +1071,24 @@ pub fn count_lines(&self) -> usize {
 
     pub fn ensure_source_file_source_present(&self, source_file: Lrc<SourceFile>) -> bool {
         source_file.add_external_src(|| {
-            match source_file.name {
-                FileName::Real(ref name) if let Some(local_path) = name.local_path() => {
-                    self.file_loader.read_file(local_path).ok()
+            let FileName::Real(ref name) = source_file.name else {
+                return None;
+            };
+
+            let local_path: Cow<'_, Path> = match name {
+                RealFileName::LocalPath(local_path) => local_path.into(),
+                RealFileName::Remapped { local_path: Some(local_path), .. } => local_path.into(),
+                RealFileName::Remapped { local_path: None, virtual_name } => {
+                    // The compiler produces better error messages if the sources of dependencies
+                    // are available. Attempt to undo any path mapping so we can find remapped
+                    // dependencies.
+                    // We can only use the heuristic because `add_external_src` checks the file
+                    // content hash.
+                    self.path_mapping.reverse_map_prefix_heuristically(virtual_name)?.into()
                 }
-                _ => None,
-            }
+            };
+
+            self.file_loader.read_file(&local_path).ok()
         })
     }
 
@@ -1277,4 +1289,43 @@ pub fn to_embeddable_absolute_path(
             }
         }
     }
+
+    /// Attempts to (heuristically) reverse a prefix mapping.
+    ///
+    /// Returns [`Some`] if there is exactly one mapping where the "to" part is
+    /// a prefix of `path` and has at least one non-empty
+    /// [`Normal`](path::Component::Normal) component. The component
+    /// restriction exists to avoid reverse mapping overly generic paths like
+    /// `/` or `.`).
+    ///
+    /// This is a heuristic and not guaranteed to return the actual original
+    /// path! Do not rely on the result unless you have other means to verify
+    /// that the mapping is correct (e.g. by checking the file content hash).
+    #[instrument(level = "debug", skip(self), ret)]
+    fn reverse_map_prefix_heuristically(&self, path: &Path) -> Option<PathBuf> {
+        let mut found = None;
+
+        for (from, to) in self.mapping.iter() {
+            let has_normal_component = to.components().any(|c| match c {
+                path::Component::Normal(s) => !s.is_empty(),
+                _ => false,
+            });
+
+            if !has_normal_component {
+                continue;
+            }
+
+            let Ok(rest) = path.strip_prefix(to) else {
+                continue;
+            };
+
+            if found.is_some() {
+                return None;
+            }
+
+            found = Some(from.join(rest));
+        }
+
+        found
+    }
 }

compiler/rustc_span/src/source_map/tests.rs

@@ -344,6 +344,10 @@ fn map_path_prefix(mapping: &FilePathMapping, p: &str) -> String {
     mapping.map_prefix(path(p)).0.to_string_lossy().to_string()
 }
 
+fn reverse_map_prefix(mapping: &FilePathMapping, p: &str) -> Option<String> {
+    mapping.reverse_map_prefix_heuristically(&path(p)).map(|q| q.to_string_lossy().to_string())
+}
+
 #[test]
 fn path_prefix_remapping() {
     // Relative to relative
@@ -480,6 +484,45 @@ fn path_prefix_remapping_expand_to_absolute() {
     );
 }
 
+#[test]
+fn path_prefix_remapping_reverse() {
+    // Ignores options without alphanumeric chars.
+    {
+        let mapping =
+            &FilePathMapping::new(vec![(path("abc"), path("/")), (path("def"), path("."))]);
+
+        assert_eq!(reverse_map_prefix(mapping, "/hello.rs"), None);
+        assert_eq!(reverse_map_prefix(mapping, "./hello.rs"), None);
+    }
+
+    // Returns `None` if multiple options match.
+    {
+        let mapping = &FilePathMapping::new(vec![
+            (path("abc"), path("/redacted")),
+            (path("def"), path("/redacted")),
+        ]);
+
+        assert_eq!(reverse_map_prefix(mapping, "/redacted/hello.rs"), None);
+    }
+
+    // Distinct reverse mappings.
+    {
+        let mapping = &FilePathMapping::new(vec![
+            (path("abc"), path("/redacted")),
+            (path("def/ghi"), path("/fake/dir")),
+        ]);
+
+        assert_eq!(
+            reverse_map_prefix(mapping, "/redacted/path/hello.rs"),
+            Some(path_str("abc/path/hello.rs"))
+        );
+        assert_eq!(
+            reverse_map_prefix(mapping, "/fake/dir/hello.rs"),
+            Some(path_str("def/ghi/hello.rs"))
+        );
+    }
+}
+
 #[test]
 fn test_next_point() {
     let sm = SourceMap::new(FilePathMapping::empty());

tests/ui/errors/auxiliary/remapped_dep.rs

@@ -0,0 +1,3 @@
+// compile-flags: --remap-path-prefix={{src-base}}/errors/auxiliary=remapped-aux
+
+pub struct SomeStruct {} // This line should be show as part of the error.

tests/ui/errors/remap-path-prefix-reverse.local-self.stderr

@@ -0,0 +1,14 @@
+error[E0423]: expected value, found struct `remapped_dep::SomeStruct`
+  --> $DIR/remap-path-prefix-reverse.rs:22:13
+   |
+LL |     let _ = remapped_dep::SomeStruct;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^ help: use struct literal syntax instead: `remapped_dep::SomeStruct {}`
+   |
+  ::: remapped-aux/remapped_dep.rs:3:1
+   |
+LL | pub struct SomeStruct {} // This line should be show as part of the error.
+   | --------------------- `remapped_dep::SomeStruct` defined here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0423`.

tests/ui/errors/remap-path-prefix-reverse.remapped-self.stderr

@@ -0,0 +1,14 @@
+error[E0423]: expected value, found struct `remapped_dep::SomeStruct`
+  --> remapped/errors/remap-path-prefix-reverse.rs:22:13
+   |
+LL |     let _ = remapped_dep::SomeStruct;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^ help: use struct literal syntax instead: `remapped_dep::SomeStruct {}`
+   |
+  ::: remapped-aux/remapped_dep.rs:3:1
+   |
+LL | pub struct SomeStruct {} // This line should be show as part of the error.
+   | --------------------- `remapped_dep::SomeStruct` defined here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0423`.

tests/ui/errors/remap-path-prefix-reverse.rs

@@ -0,0 +1,23 @@
+// aux-build:remapped_dep.rs
+// compile-flags: --remap-path-prefix={{src-base}}/errors/auxiliary=remapped-aux
+
+// The remapped paths are not normalized by compiletest.
+// normalize-stderr-test: "\\(errors)" -> "/$1"
+
+// revisions: local-self remapped-self
+// [remapped-self]compile-flags: --remap-path-prefix={{src-base}}=remapped
+
+// The paths from `remapped-self` aren't recognized by compiletest, so we
+// cannot use line-specific patterns for the actual error.
+// error-pattern: E0423
+
+// Verify that the expected source code is shown.
+// error-pattern: pub struct SomeStruct {} // This line should be show
+
+extern crate remapped_dep;
+
+fn main() {
+    // The actual error is irrelevant. The important part it that is should show
+    // a snippet of the dependency's source.
+    let _ = remapped_dep::SomeStruct;
+}

tests/ui/errors/remap-path-prefix.rs

@@ -1,5 +1,12 @@
 // compile-flags: --remap-path-prefix={{src-base}}=remapped
 
+// The remapped paths are not normalized by compiletest.
+// normalize-stderr-test: "\\(errors)" -> "/$1"
+
+// The remapped paths aren't recognized by compiletest, so we
+// cannot use line-specific patterns.
+// error-pattern: E0425
+
 fn main() {
     // We cannot actually put an ERROR marker here because
     // the file name in the error message is not what the

tests/ui/errors/remap-path-prefix.stderr

@@ -1,5 +1,5 @@
 error[E0425]: cannot find value `ferris` in this scope
-  --> remapped/remap-path-prefix.rs:8:5
+  --> remapped/errors/remap-path-prefix.rs:15:5
    |
 LL |     ferris
    |     ^^^^^^ not found in this scope