Add trait diff highlighting logic and use it in E0277

When a trait is not implemented for a type, but there *is* an `impl`
for another type or different trait params, we format the output to
use highlighting in the same way that E0308 does for types.

The logic accounts for 3 cases:
- When both the type and trait in the expected predicate and the candidate are different
- When only the types are different
- When only the trait generic params are different

For each case, we use slightly different formatting and wording.

1 files changed, 15 insertions, 13 deletions

diff --git a/tests/ui/impl-trait/diagnostics/highlight-difference-between-expected-trait-and-found-trait.svg b/tests/ui/impl-trait/diagnostics/highlight-difference-between-expected-trait-and-found-trait.svg
index a18fc11a1e3..277fd1536bc 100644
--- a/tests/ui/impl-trait/diagnostics/highlight-difference-between-expected-trait-and-found-trait.svg
+++ b/tests/ui/impl-trait/diagnostics/highlight-difference-between-expected-trait-and-found-trait.svg
@@ -1,4 +1,4 @@
-<svg width="1104px" height="344px" xmlns="http://www.w3.org/2000/svg">
+<svg width="1104px" height="362px" xmlns="http://www.w3.org/2000/svg">
   <style>
     .fg { fill: #AAAAAA }
     .bg { background: #000000 }
@@ -33,29 +33,31 @@
 </tspan>
     <tspan x="10px" y="118px"><tspan>   </tspan><tspan class="fg-ansi256-012 bold">|</tspan>
 </tspan>
-    <tspan x="10px" y="136px"><tspan>   </tspan><tspan class="fg-ansi256-012 bold">= </tspan><tspan class="bold">help</tspan><tspan>: the trait `Bar&lt;()&gt;` </tspan><tspan class="fg-magenta bold">is</tspan><tspan> implemented for `Struct`</tspan>
+    <tspan x="10px" y="136px"><tspan>   </tspan><tspan class="fg-ansi256-012 bold">= </tspan><tspan class="bold">help</tspan><tspan>: the trait `Bar&lt;</tspan><tspan class="fg-magenta bold">i32</tspan><tspan>&gt;` </tspan><tspan class="fg-magenta bold">is not</tspan><tspan> implemented for `Struct`</tspan>
 </tspan>
-    <tspan x="10px" y="154px"><tspan>   </tspan><tspan class="fg-ansi256-012 bold">= </tspan><tspan class="bold">help</tspan><tspan>: for that trait implementation, expected `</tspan><tspan class="fg-magenta bold">()</tspan><tspan>`, found `</tspan><tspan class="fg-magenta bold">i32</tspan><tspan>`</tspan>
+    <tspan x="10px" y="154px"><tspan>           but trait `Bar&lt;</tspan><tspan class="fg-magenta bold">()</tspan><tspan>&gt;` </tspan><tspan class="fg-magenta bold">is</tspan><tspan> implemented for it</tspan>
 </tspan>
-    <tspan x="10px" y="172px"><tspan class="fg-ansi256-010 bold">note</tspan><tspan>: required for `Struct` to implement `Foo&lt;i32&gt;`</tspan>
+    <tspan x="10px" y="172px"><tspan>   </tspan><tspan class="fg-ansi256-012 bold">= </tspan><tspan class="bold">help</tspan><tspan>: for that trait implementation, expected `</tspan><tspan class="fg-magenta bold">()</tspan><tspan>`, found `</tspan><tspan class="fg-magenta bold">i32</tspan><tspan>`</tspan>
 </tspan>
-    <tspan x="10px" y="190px"><tspan>  </tspan><tspan class="fg-ansi256-012 bold">--&gt; </tspan><tspan>$DIR/highlight-difference-between-expected-trait-and-found-trait.rs:11:12</tspan>
+    <tspan x="10px" y="190px"><tspan class="fg-ansi256-010 bold">note</tspan><tspan>: required for `Struct` to implement `Foo&lt;i32&gt;`</tspan>
 </tspan>
-    <tspan x="10px" y="208px"><tspan>   </tspan><tspan class="fg-ansi256-012 bold">|</tspan>
+    <tspan x="10px" y="208px"><tspan>  </tspan><tspan class="fg-ansi256-012 bold">--&gt; </tspan><tspan>$DIR/highlight-difference-between-expected-trait-and-found-trait.rs:11:12</tspan>
 </tspan>
-    <tspan x="10px" y="226px"><tspan class="fg-ansi256-012 bold">LL</tspan><tspan> </tspan><tspan class="fg-ansi256-012 bold">|</tspan><tspan> impl&lt;T, K&gt; Foo&lt;K&gt; for T where T: Bar&lt;K&gt;</tspan>
+    <tspan x="10px" y="226px"><tspan>   </tspan><tspan class="fg-ansi256-012 bold">|</tspan>
 </tspan>
-    <tspan x="10px" y="244px"><tspan>   </tspan><tspan class="fg-ansi256-012 bold">|</tspan><tspan>            </tspan><tspan class="fg-ansi256-010 bold">^^^^^^</tspan><tspan>     </tspan><tspan class="fg-ansi256-010 bold">^</tspan><tspan>          </tspan><tspan class="fg-ansi256-012 bold">------</tspan><tspan> </tspan><tspan class="fg-ansi256-012 bold">unsatisfied trait bound introduced here</tspan>
+    <tspan x="10px" y="244px"><tspan class="fg-ansi256-012 bold">LL</tspan><tspan> </tspan><tspan class="fg-ansi256-012 bold">|</tspan><tspan> impl&lt;T, K&gt; Foo&lt;K&gt; for T where T: Bar&lt;K&gt;</tspan>
 </tspan>
-    <tspan x="10px" y="262px">
+    <tspan x="10px" y="262px"><tspan>   </tspan><tspan class="fg-ansi256-012 bold">|</tspan><tspan>            </tspan><tspan class="fg-ansi256-010 bold">^^^^^^</tspan><tspan>     </tspan><tspan class="fg-ansi256-010 bold">^</tspan><tspan>          </tspan><tspan class="fg-ansi256-012 bold">------</tspan><tspan> </tspan><tspan class="fg-ansi256-012 bold">unsatisfied trait bound introduced here</tspan>
 </tspan>
-    <tspan x="10px" y="280px"><tspan class="fg-ansi256-009 bold">error</tspan><tspan class="bold">: aborting due to 1 previous error</tspan>
+    <tspan x="10px" y="280px">
 </tspan>
-    <tspan x="10px" y="298px">
+    <tspan x="10px" y="298px"><tspan class="fg-ansi256-009 bold">error</tspan><tspan class="bold">: aborting due to 1 previous error</tspan>
 </tspan>
-    <tspan x="10px" y="316px"><tspan class="bold">For more information about this error, try `rustc --explain E0277`.</tspan>
+    <tspan x="10px" y="316px">
 </tspan>
-    <tspan x="10px" y="334px">
+    <tspan x="10px" y="334px"><tspan class="bold">For more information about this error, try `rustc --explain E0277`.</tspan>
+</tspan>
+    <tspan x="10px" y="352px">
 </tspan>
   </text>