Merge #2381

2381: Add proc-macro crate type handling r=JasperDeSutter a=JasperDeSutter

Resolves the libproc_macro crate in crates that are the proc-macro type.
This doesn't seem the ideal implementation though, since the compiler still requires you to write `extern crate proc_macro;` (even in 2018 edition).

Co-authored-by: JasperDeSutter <jasper.desutter@gmail.com>

crates/ra_project_model/src/sysroot.rs

@@ -39,6 +39,10 @@ impl Sysroot {
         self.by_name("std")
     }
 
+    pub fn proc_macro(&self) -> Option<SysrootCrate> {
+        self.by_name("proc_macro")
+    }
+
     pub fn crates<'a>(&'a self) -> impl Iterator<Item = SysrootCrate> + ExactSizeIterator + 'a {
         self.crates.iter().map(|(id, _data)| id)
     }
@@ -74,7 +78,7 @@ impl Sysroot {
             }
         }
         if let Some(alloc) = sysroot.by_name("alloc") {
-            if let Some(core) = sysroot.by_name("core") {
+            if let Some(core) = sysroot.core() {
                 sysroot.crates[alloc].deps.push(core);
             }
         }