Improve the readability of diagnostics that involve unresolved type variables

Diagnostics such as the following

```
mismatched types: expected `core::result::Result<uint,()>`, found `core::option::Option<<generic #1>>`
<anon>:6     let a: Result<uint, ()> = None;
                                       ^~~~
mismatched types: expected `&mut <generic #2>`, found `uint`
<anon>:7     f(42u);
               ^~~
```

tend to be fairly unappealing to new users. While specific type var IDs are valuable in
diagnostics that deal with more than one such variable, in practice many messages
only mention one. In those cases, leaving out the specific number makes the messages
slightly less terrifying.

In addition, type variables have been changed to use the type hole syntax `_` in diagnostics.
With a variable ID, they're printed as `_#id` (e.g. `_#1`). In cases where the ID is left out,
it's simply `_`. Integer and float variables have an additional suffix after the number, e.g.
`_#1i` or `_#3f`.

Author: Jakub Bukaj

src/librustc/middle/ty.rs

@@ -1032,10 +1032,8 @@ pub enum type_err {
     terr_ref_mutability,
     terr_vec_mutability,
     terr_tuple_size(expected_found<uint>),
+    terr_fixed_array_size(expected_found<uint>),
     terr_ty_param_size(expected_found<uint>),
-    terr_record_size(expected_found<uint>),
-    terr_record_mutability,
-    terr_record_fields(expected_found<Ident>),
     terr_arg_count,
     terr_regions_does_not_outlive(Region, Region),
     terr_regions_not_same(Region, Region),
@@ -3790,8 +3788,8 @@ pub fn ty_sort_string(cx: &ctxt, t: t) -> String {
 
         ty_enum(id, _) => format!("enum {}", item_path_str(cx, id)),
         ty_uniq(_) => "box".to_string(),
-        ty_vec(_, Some(_)) => "array".to_string(),
-        ty_vec(_, None) => "unsized array".to_string(),
+        ty_vec(_, Some(n)) => format!("array of {} elements", n),
+        ty_vec(_, None) => "slice".to_string(),
         ty_ptr(_) => "*-ptr".to_string(),
         ty_rptr(_, _) => "&-ptr".to_string(),
         ty_bare_fn(_) => "extern fn".to_string(),
@@ -3874,27 +3872,18 @@ pub fn type_err_to_str(cx: &ctxt, err: &type_err) -> String {
                     values.expected,
                     values.found)
         }
-        terr_tuple_size(values) => {
-            format!("expected a tuple with {} elements, \
+        terr_fixed_array_size(values) => {
+            format!("expected an array with a fixed size of {} elements, \
                      found one with {} elements",
                     values.expected,
                     values.found)
         }
-        terr_record_size(values) => {
-            format!("expected a record with {} fields, \
-                     found one with {} fields",
+        terr_tuple_size(values) => {
+            format!("expected a tuple with {} elements, \
+                     found one with {} elements",
                     values.expected,
                     values.found)
         }
-        terr_record_mutability => {
-            "record elements differ in mutability".to_string()
-        }
-        terr_record_fields(values) => {
-            format!("expected a record with field `{}`, found one \
-                     with field `{}`",
-                    token::get_ident(values.expected),
-                    token::get_ident(values.found))
-        }
         terr_arg_count => {
             "incorrect number of function parameters".to_string()
         }

src/librustc/middle/typeck/infer/combine.rs

@@ -515,6 +515,16 @@ pub fn super_tys<'tcx, C: Combine<'tcx>>(this: &C, a: ty::t, b: ty::t) -> cres<t
             Ok(ty::mk_rptr(tcx, r, mt))
       }
 
+      (&ty::ty_vec(a_t, Some(sz_a)), &ty::ty_vec(b_t, Some(sz_b))) => {
+        this.tys(a_t, b_t).and_then(|t| {
+            if sz_a == sz_b {
+                Ok(ty::mk_vec(tcx, t, Some(sz_a)))
+            } else {
+                Err(ty::terr_fixed_array_size(expected_found(this, sz_a, sz_b)))
+            }
+        })
+      }
+
       (&ty::ty_vec(a_t, sz_a), &ty::ty_vec(b_t, sz_b)) => {
         this.tys(a_t, b_t).and_then(|t| {
             if sz_a == sz_b {

src/librustc/middle/typeck/infer/error_reporting.rs

@@ -62,6 +62,7 @@ time of error detection.
 use std::collections::HashSet;
 use middle::def;
 use middle::subst;
+use middle::ty_fold::{mod, TypeFoldable};
 use middle::ty;
 use middle::ty::{Region, ReFree};
 use middle::typeck::infer;
@@ -111,7 +112,7 @@ pub trait ErrorReporting {
 
     fn values_str(&self, values: &ValuePairs) -> Option<String>;
 
-    fn expected_found_str<T:UserString+Resolvable>(
+    fn expected_found_str<T: UserString + Resolvable>(
         &self,
         exp_found: &ty::expected_found<T>)
         -> Option<String>;
@@ -396,16 +397,12 @@ impl<'a, 'tcx> ErrorReporting for InferCtxt<'a, 'tcx> {
          * or None if this is a derived error.
          */
         match *values {
-            infer::Types(ref exp_found) => {
-                self.expected_found_str(exp_found)
-            }
-            infer::TraitRefs(ref exp_found) => {
-                self.expected_found_str(exp_found)
-            }
+            infer::Types(ref exp_found) => self.expected_found_str(exp_found),
+            infer::TraitRefs(ref exp_found) => self.expected_found_str(exp_found)
         }
     }
 
-    fn expected_found_str<T:UserString+Resolvable>(
+    fn expected_found_str<T: UserString + Resolvable>(
         &self,
         exp_found: &ty::expected_found<T>)
         -> Option<String>
@@ -420,9 +417,14 @@ impl<'a, 'tcx> ErrorReporting for InferCtxt<'a, 'tcx> {
             return None;
         }
 
+        // Only include variable IDs in the diagnostics if there are at least two
+        // present across both types/traits.
+        let should_print_var_ids = expected.remaining_type_variables(self.tcx)
+            .union(&found.remaining_type_variables(self.tcx)).count() > 1;
+
         Some(format!("expected `{}`, found `{}`",
-                     expected.user_string(self.tcx),
-                     found.user_string(self.tcx)))
+                     expected.user_string_with_var_ids(self.tcx, should_print_var_ids),
+                     found.user_string_with_var_ids(self.tcx, should_print_var_ids)))
     }
 
     fn report_param_bound_failure(&self,
@@ -1654,6 +1656,7 @@ impl<'a, 'tcx> ErrorReportingHelpers for InferCtxt<'a, 'tcx> {
 pub trait Resolvable {
     fn resolve(&self, infcx: &InferCtxt) -> Self;
     fn contains_error(&self) -> bool;
+    fn remaining_type_variables(&self, tcx: &ty::ctxt) -> HashSet<ty::InferTy>;
 }
 
 impl Resolvable for ty::t {
@@ -1663,6 +1666,22 @@ impl Resolvable for ty::t {
     fn contains_error(&self) -> bool {
         ty::type_is_error(*self)
     }
+    fn remaining_type_variables(&self, tcx: &ty::ctxt) -> HashSet<ty::InferTy> {
+        let mut vars = HashSet::new();
+        {
+            let mut folder = ty_fold::BottomUpFolder {
+                tcx: tcx,
+                fldop: |t| {
+                    if let ty::ty_infer(var) = ty::get(t).sty {
+                        vars.insert(var);
+                    }
+                    t
+                }
+            };
+            self.fold_with(&mut folder);
+        }
+        vars
+    }
 }
 
 impl Resolvable for Rc<ty::TraitRef> {
@@ -1672,6 +1691,9 @@ impl Resolvable for Rc<ty::TraitRef> {
     fn contains_error(&self) -> bool {
         ty::trait_ref_contains_error(&**self)
     }
+    fn remaining_type_variables(&self, _: &ty::ctxt) -> HashSet<ty::InferTy> {
+        HashSet::new()
+    }
 }
 
 fn lifetimes_in_scope(tcx: &ty::ctxt,

src/librustc/middle/typeck/infer/mod.rs

@@ -36,7 +36,8 @@ use syntax::ast;
 use syntax::codemap;
 use syntax::codemap::Span;
 use util::common::indent;
-use util::ppaux::{bound_region_to_string, ty_to_string, trait_ref_to_string, Repr};
+use util::ppaux::{bound_region_to_string, ty_to_string};
+use util::ppaux::{trait_ref_to_string, Repr};
 
 use self::coercion::Coerce;
 use self::combine::{Combine, CombineFields};
@@ -900,32 +901,25 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                                                 err: Option<&ty::type_err>) {
         debug!("hi! expected_ty = {}, actual_ty = {}", expected_ty, actual_ty);
 
-        let error_str = err.map_or("".to_string(), |t_err| {
-            format!(" ({})", ty::type_err_to_str(self.tcx, t_err))
-        });
         let resolved_expected = expected_ty.map(|e_ty| {
             self.resolve_type_vars_if_possible(e_ty)
         });
-        if !resolved_expected.map_or(false, |e| { ty::type_is_error(e) }) {
-            match resolved_expected {
-                None => {
-                    self.tcx
-                        .sess
-                        .span_err(sp,
-                                  format!("{}{}",
-                                          mk_msg(None, actual_ty),
-                                          error_str).as_slice())
-                }
-                Some(e) => {
-                    self.tcx.sess.span_err(sp,
-                        format!("{}{}",
-                                mk_msg(Some(self.ty_to_string(e)), actual_ty),
-                                error_str).as_slice());
+
+        match resolved_expected {
+            Some(t) if ty::type_is_error(t) => (),
+            _ => {
+                let error_str = err.map_or("".to_string(), |t_err| {
+                    format!(" ({})", ty::type_err_to_str(self.tcx, t_err))
+                });
+
+                self.tcx.sess.span_err(sp, format!("{}{}",
+                    mk_msg(resolved_expected.map(|t| self.ty_to_string(t)), actual_ty),
+                    error_str).as_slice());
+
+                for err in err.iter() {
+                    ty::note_and_explain_type_err(self.tcx, *err)
                 }
             }
-            for err in err.iter() {
-                ty::note_and_explain_type_err(self.tcx, *err)
-            }
         }
     }
 
@@ -945,25 +939,18 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     }
 
     pub fn report_mismatched_types(&self,
-                                   sp: Span,
-                                   e: ty::t,
-                                   a: ty::t,
+                                   span: Span,
+                                   expected: ty::t,
+                                   actual: ty::t,
                                    err: &ty::type_err) {
-        let resolved_expected =
-            self.resolve_type_vars_if_possible(e);
-        let mk_msg = match ty::get(resolved_expected).sty {
-            // Don't report an error if expected is ty_err
-            ty::ty_err => return,
-            _ => {
-                // if I leave out : String, it infers &str and complains
-                |actual: String| {
-                    format!("mismatched types: expected `{}`, found `{}`",
-                            self.ty_to_string(resolved_expected),
-                            actual)
-                }
-            }
+        let trace = TypeTrace {
+            origin: Misc(span),
+            values: Types(ty::expected_found {
+                expected: expected,
+                found: actual
+            })
         };
-        self.type_error_message(sp, mk_msg, a, Some(err));
+        self.report_and_explain_type_error(trace, err);
     }
 
     pub fn replace_late_bound_regions_with_fresh_regions(&self,