Treat "proc-macro" crate type the same as proc-macro = true

src/cargo/util/toml/mod.rs

@@ -1474,7 +1474,14 @@ impl TomlTarget {
     }
 
     fn proc_macro(&self) -> Option<bool> {
-        self.proc_macro.or(self.proc_macro2)
+        self.proc_macro.or(self.proc_macro2).or_else(|| {
+            if let Some(types) = self.crate_types() {
+                if types.contains(&"proc-macro".to_string()) {
+                    return Some(true);
+                }
+            }
+            None
+        })
     }
 
     fn crate_types(&self) -> Option<&Vec<String>> {

src/cargo/util/toml/targets.rs

@@ -206,6 +206,23 @@ fn clean_lib(
     // A plugin requires exporting plugin_registrar so a crate cannot be
     // both at once.
     let crate_types = match (lib.crate_types(), lib.plugin, lib.proc_macro()) {
+        (Some(kinds), _, _) if kinds.contains(&"proc-macro".to_string()) => {
+            if let Some(true) = lib.plugin {
+                // This is a warning to retain backwards compatibility.
+                warnings.push(format!(
+                    "proc-macro library `{}` should not specify `plugin = true`",
+                    lib.name()
+                ));
+            }
+            warnings.push(format!(
+                "library `{}` should only specify `proc-macro = true` instead of setting `crate-type`",
+                lib.name()
+            ));
+            if kinds.len() > 1 {
+                bail!("cannot mix `proc-macro` crate type with others");
+            }
+            vec![LibKind::ProcMacro]
+        }
         (_, Some(true), Some(true)) => bail!("lib.plugin and lib.proc-macro cannot both be true"),
         (Some(kinds), _, _) => kinds.iter().map(|s| s.into()).collect(),
         (None, Some(true), _) => vec![LibKind::Dylib],

tests/testsuite/proc_macro.rs

@@ -279,3 +279,145 @@ fn a() {
         .with_stdout_contains_n("test [..] ... ok", 2)
         .run();
 }
+
+#[test]
+fn proc_macro_crate_type() {
+    // Verify that `crate-type = ["proc-macro"]` is the same as `proc-macro = true`
+    // and that everything, including rustdoc, works correctly.
+    let foo = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            version = "0.1.0"
+            [dependencies]
+            pm = { path = "pm" }
+        "#,
+        )
+        .file(
+            "src/lib.rs",
+            r#"
+            //! ```
+            //! use foo::THING;
+            //! assert_eq!(THING, 123);
+            //! ```
+            #[macro_use]
+            extern crate pm;
+            #[derive(MkItem)]
+            pub struct S;
+            #[cfg(test)]
+            mod tests {
+                use super::THING;
+                #[test]
+                fn it_works() {
+                    assert_eq!(THING, 123);
+                }
+            }
+        "#,
+        )
+        .file(
+            "pm/Cargo.toml",
+            r#"
+            [package]
+            name = "pm"
+            version = "0.1.0"
+            [lib]
+            crate-type = ["proc-macro"]
+        "#,
+        )
+        .file(
+            "pm/src/lib.rs",
+            r#"
+            extern crate proc_macro;
+            use proc_macro::TokenStream;
+
+            #[proc_macro_derive(MkItem)]
+            pub fn mk_item(_input: TokenStream) -> TokenStream {
+                "pub const THING: i32 = 123;".parse().unwrap()
+            }
+        "#,
+        )
+        .build();
+
+    foo.cargo("test")
+        .with_stdout_contains("test tests::it_works ... ok")
+        .with_stdout_contains_n("test [..] ... ok", 2)
+        .run();
+}
+
+#[test]
+fn proc_macro_crate_type_warning() {
+    let foo = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            version = "0.1.0"
+            [lib]
+            crate-type = ["proc-macro"]
+        "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    foo.cargo("build")
+        .with_stderr_contains(
+            "[WARNING] library `foo` should only specify `proc-macro = true` instead of setting `crate-type`")
+        .run();
+}
+
+#[test]
+fn proc_macro_crate_type_warning_plugin() {
+    let foo = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            version = "0.1.0"
+            [lib]
+            crate-type = ["proc-macro"]
+            plugin = true
+        "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    foo.cargo("build")
+        .with_stderr_contains(
+            "[WARNING] proc-macro library `foo` should not specify `plugin = true`")
+        .with_stderr_contains(
+            "[WARNING] library `foo` should only specify `proc-macro = true` instead of setting `crate-type`")
+        .run();
+}
+
+#[test]
+fn proc_macro_crate_type_multiple() {
+    let foo = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            version = "0.1.0"
+            [lib]
+            crate-type = ["proc-macro", "rlib"]
+        "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    foo.cargo("build")
+        .with_stderr(
+            "\
+[ERROR] failed to parse manifest at `[..]/foo/Cargo.toml`
+
+Caused by:
+  cannot mix `proc-macro` crate type with others
+",
+        )
+        .with_status(101)
+        .run();
+}