Adjust warning for rustdoc filename collision.

Author: ehuss

src/cargo/core/compiler/context/mod.rs

@@ -412,9 +412,13 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
             "Consider changing their names to be unique or compiling them separately.\n\
              This may become a hard error in the future; see \
              <https://github.com/rust-lang/cargo/issues/6313>.";
+        let rustdoc_suggestion =
+            "This is a known bug where multiple crates with the same name use\n\
+             the same path; see <https://github.com/rust-lang/cargo/issues/6313>.";
         let report_collision = |unit: &Unit<'_>,
                                 other_unit: &Unit<'_>,
-                                path: &PathBuf|
+                                path: &PathBuf,
+                                suggestion: &str|
          -> CargoResult<()> {
             if unit.target.name() == other_unit.target.name() {
                 self.bcx.config.shell().warn(format!(
@@ -443,6 +447,7 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
                     unit, other_unit))
             }
         };
+
         let mut keys = self
             .unit_dependencies
             .keys()
@@ -453,11 +458,17 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
         for unit in keys {
             for output in self.outputs(unit)?.iter() {
                 if let Some(other_unit) = output_collisions.insert(output.path.clone(), unit) {
-                    report_collision(unit, other_unit, &output.path)?;
+                    if unit.mode.is_doc() {
+                        // See https://github.com/rust-lang/rust/issues/56169
+                        // and https://github.com/rust-lang/rust/issues/61378
+                        report_collision(unit, other_unit, &output.path, rustdoc_suggestion)?;
+                    } else {
+                        report_collision(unit, other_unit, &output.path, suggestion)?;
+                    }
                 }
                 if let Some(hardlink) = output.hardlink.as_ref() {
                     if let Some(other_unit) = output_collisions.insert(hardlink.clone(), unit) {
-                        report_collision(unit, other_unit, hardlink)?;
+                        report_collision(unit, other_unit, hardlink, suggestion)?;
                     }
                 }
                 if let Some(ref export_path) = output.export_path {

tests/testsuite/collisions.rs

@@ -141,8 +141,8 @@ The lib target `foo` in package `foo2 v0.1.0 ([..]/foo/foo2)` has the same outpu
 filename as the lib target `foo` in package `foo v0.1.0 ([..]/foo)`.
 Colliding filename is: [..]/foo/target/doc/foo/index.html
 The targets should have unique names.
-Consider changing their names to be unique or compiling them separately.
-This may become a hard error in the future; see <https://github.com/rust-lang/cargo/issues/6313>.
+This is a known bug where multiple crates with the same name use
+the same path; see <https://github.com/rust-lang/cargo/issues/6313>.
 ",
         )
         .run();