fix a bug in compiletest JSON parsing for duplicate errors

In some cases, we give multiple primary spans, in which case we would
report one `//~` annotation per primary span. That was very confusing
because these things are reported to the user as a single error.

UI tests would be better here.

Author: nikomatsakis

src/test/compile-fail/binop-move-semantics.rs

@@ -62,7 +62,6 @@ fn mut_plus_immut() {
     &mut f
     +
     &f;  //~ ERROR: cannot borrow `f` as immutable because it is also borrowed as mutable
-    //~^ cannot borrow `f` as immutable because it is also borrowed as mutable
 }
 
 fn immut_plus_mut() {
@@ -71,7 +70,6 @@ fn immut_plus_mut() {
     &f
     +
     &mut f;  //~ ERROR: cannot borrow `f` as mutable because it is also borrowed as immutable
-    //~^ cannot borrow `f` as mutable because it is also borrowed as immutable
 }
 
 fn main() {}

src/test/compile-fail/borrowck/borrowck-lend-flow-loop.rs

@@ -109,7 +109,6 @@ fn while_aliased_mut_cond(cond: bool, cond2: bool) {
         borrow(&*v); //~ ERROR cannot borrow
         if cond2 {
             x = &mut v; //~ ERROR cannot borrow
-            //~^ ERROR cannot borrow
         }
     }
 }

src/test/compile-fail/borrowck/borrowck-mut-borrow-linear-errors.rs

@@ -19,7 +19,6 @@ fn main() {
         match 1 {
             1 => { addr = &mut x; }
             //~^ ERROR cannot borrow `x` as mutable more than once at a time
-            //~| ERROR cannot borrow `x` as mutable more than once at a time
             2 => { addr = &mut x; }
             //~^ ERROR cannot borrow `x` as mutable more than once at a time
             _ => { addr = &mut x; }

src/test/compile-fail/issue-25579.rs

@@ -17,7 +17,6 @@ fn causes_ice(mut l: &mut Sexpression) {
     loop { match l {
         &mut Sexpression::Num(ref mut n) => {},
         &mut Sexpression::Cons(ref mut expr) => { //~ ERROR cannot borrow `l.0`
-            //~| ERROR cannot borrow `l.0`
             l = &mut **expr; //~ ERROR cannot assign to `l`
         }
     }}

src/test/compile-fail/issue-38412.rs

@@ -11,7 +11,6 @@
 fn main() {
     let Box(a) = loop { };
     //~^ ERROR expected tuple struct/variant, found struct `Box`
-    //~| ERROR expected tuple struct/variant, found struct `Box`
 
     // (The below is a trick to allow compiler to infer a type for
     // variable `a` without attempting to ascribe a type to the

src/test/compile-fail/lint-unused-imports.rs

@@ -21,7 +21,6 @@ use std::fmt::{};
 // Should get errors for both 'Some' and 'None'
 use std::option::Option::{Some, None};
 //~^ ERROR unused imports: `None`, `Some`
-//~| ERROR unused imports: `None`, `Some`
 
 use test::A;       //~ ERROR unused import: `test::A`
 // Be sure that if we just bring some methods into scope that they're also

src/tools/compiletest/src/json.rs

@@ -97,6 +97,7 @@ fn push_expected_errors(expected_errors: &mut Vec<Error>,
     let primary_spans: Vec<_> = spans_in_this_file.iter()
         .cloned()
         .filter(|span| span.is_primary)
+        .take(1) // sometimes we have more than one showing up in the json; pick first
         .collect();
     let primary_spans = if primary_spans.is_empty() {
         // subdiagnostics often don't have a span of their own;