2 files changed, 41 insertions, 4 deletions

diff --git a/crates/ra_hir_ty/src/display.rs b/crates/ra_hir_ty/src/display.rs
index 9bb3ece6c..dcca1bace 100644
--- a/crates/ra_hir_ty/src/display.rs
+++ b/crates/ra_hir_ty/src/display.rs
@@ -10,6 +10,7 @@ pub struct HirFormatter<'a, 'b, DB> {
     buf: String,
     curr_size: usize,
     max_size: Option<usize>,
+    should_display_default_types: bool,
 }
 
 pub trait HirDisplay {
@@ -19,7 +20,7 @@ pub trait HirDisplay {
     where
         Self: Sized,
     {
-        HirDisplayWrapper(db, self, None)
+        HirDisplayWrapper(db, self, None, true)
     }
 
     fn display_truncated<'a, DB>(
@@ -30,7 +31,7 @@ pub trait HirDisplay {
     where
         Self: Sized,
     {
-        HirDisplayWrapper(db, self, max_size)
+        HirDisplayWrapper(db, self, max_size, false)
     }
 }
 
@@ -72,9 +73,13 @@ where
             false
         }
     }
+
+    pub fn should_display_default_types(&self) -> bool {
+        self.should_display_default_types
+    }
 }
 
-pub struct HirDisplayWrapper<'a, DB, T>(&'a DB, &'a T, Option<usize>);
+pub struct HirDisplayWrapper<'a, DB, T>(&'a DB, &'a T, Option<usize>, bool);
 
 impl<'a, DB, T> fmt::Display for HirDisplayWrapper<'a, DB, T>
 where
@@ -88,6 +93,7 @@ where
             buf: String::with_capacity(20),
             curr_size: 0,
             max_size: self.2,
+            should_display_default_types: self.3,
         })
     }
 }
diff --git a/crates/ra_hir_ty/src/lib.rs b/crates/ra_hir_ty/src/lib.rs
index 3ad913e55..7ca9e6b8a 100644
--- a/crates/ra_hir_ty/src/lib.rs
+++ b/crates/ra_hir_ty/src/lib.rs
@@ -906,7 +906,38 @@ impl HirDisplay for ApplicationTy {
                 write!(f, "{}", name)?;
                 if self.parameters.len() > 0 {
                     write!(f, "<")?;
-                    f.write_joined(&*self.parameters.0, ", ")?;
+
+                    let mut non_default_parameters = Vec::with_capacity(self.parameters.len());
+                    let parameters_to_write = if f.should_display_default_types() {
+                        self.parameters.0.as_ref()
+                    } else {
+                        match self
+                            .ctor
+                            .as_generic_def()
+                            .map(|generic_def_id| f.db.generic_defaults(generic_def_id))
+                            .filter(|defaults| !defaults.is_empty())
+                        {
+                            Option::None => self.parameters.0.as_ref(),
+                            Option::Some(default_parameters) => {
+                                for (i, parameter) in self.parameters.into_iter().enumerate() {
+                                    match (parameter, default_parameters.get(i)) {
+                                        (&Ty::Unknown, _) | (_, None) => {
+                                            non_default_parameters.push(parameter.clone())
+                                        }
+                                        (_, Some(default_parameter))
+                                            if parameter != default_parameter =>
+                                        {
+                                            non_default_parameters.push(parameter.clone())
+                                        }
+                                        _ => (),
+                                    }
+                                }
+                                &non_default_parameters
+                            }
+                        }
+                    };
+
+                    f.write_joined(parameters_to_write, ", ")?;
                     write!(f, ">")?;
                 }
             }