Warning about unsupported LTO crete types

src/cargo/core/compiler/lto.rs

@@ -61,7 +61,7 @@ pub fn generate(bcx: &BuildContext<'_, '_>) -> CargoResult<HashMap<Unit, Lto>> {
                 }
             }
         };
-        calculate(bcx, &mut map, unit, root_lto)?;
+        calculate(bcx, &mut map, unit, root_lto, false)?;
     }
     Ok(map)
 }
@@ -91,6 +91,7 @@ fn calculate(
     map: &mut HashMap<Unit, Lto>,
     unit: &Unit,
     parent_lto: Lto,
+    is_dep: bool,
 ) -> CargoResult<()> {
     let crate_types = match unit.mode {
         // Note: Doctest ignores LTO, but for now we'll compute it as-if it is
@@ -106,7 +107,22 @@ fn calculate(
     };
     // LTO can only be performed if *all* of the crate types support it.
     // For example, a cdylib/rlib combination won't allow LTO.
-    let all_lto_types = crate_types.iter().all(CrateType::can_lto);
+    let not_lto_types = crate_types
+        .iter()
+        .filter(|ct| !ct.can_lto())
+        .map(|ct| ct.as_str())
+        .collect::<Vec<_>>();
+    let all_lto_types = not_lto_types.len() == 0;
+    if !all_lto_types && !is_dep {
+        if let profiles::Lto::Named(_) | profiles::Lto::Bool(_) = unit.profile.lto {
+            bcx.config.shell().warn(format!(
+                "LTO can only be performed if all of the crate types support it, \
+                 package `{}` have some crate types do not support LTO: {}",
+                unit.pkg.name(),
+                not_lto_types.join(","),
+            ))?;
+        }
+    }
     // Compute the LTO based on the profile, and what our parent requires.
     let lto = if unit.target.for_host() {
         // Disable LTO for host builds since we only really want to perform LTO
@@ -186,7 +202,7 @@ fn calculate(
     };
 
     for dep in &bcx.unit_graph[unit] {
-        calculate(bcx, map, &dep.unit, merged_lto)?;
+        calculate(bcx, map, &dep.unit, merged_lto, true)?;
     }
     Ok(())
 }

tests/testsuite/lto.rs

@@ -624,6 +624,43 @@ fn dylib() {
         .run();
 }
 
+#[cargo_test]
+fn lto_with_unsupported_crate_types() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.0.0"
+
+                [lib]
+                crate-type = ['dylib','rlib']
+
+                [profile.release]
+                lto = true
+            "#,
+        )
+        .file(
+            "src/lib.rs",
+            r#"
+                pub fn foo() {
+                    println!("foo");
+                }
+            "#,
+        )
+        .build();
+
+    p.cargo("build --release -v")
+        .with_stderr_contains(
+            "\
+[WARNING] LTO can only be performed if all of the crate types support it, \
+package `foo` have some crate types do not support LTO: dylib,rlib
+",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn test_profile() {
     Package::new("bar", "0.0.1")