Remove less relevant info from diagnostic

```
error[E0277]: the trait bound `dep_2_reexport::Type: Trait` is not satisfied because the trait comes from a different crate version
 --> multiple-dep-versions.rs:7:18
  |
7 |     do_something(Type);
  |                  ^^^^ the trait `Trait` is not implemented for `dep_2_reexport::Type`
  |
note: there are multiple different versions of crate `dependency` in the dependency graph
 --> /home/gh-estebank/rust/build/x86_64-unknown-linux-gnu/test/run-make/crate-loading/rmake_out/multiple-dep-versions-1.rs:4:1
  |
3 | pub struct Type(pub i32);
  | --------------- this type implements the required trait
4 | pub trait Trait {
  | ^^^^^^^^^^^^^^^ this is the required trait
  |
 ::: multiple-dep-versions.rs:1:1
  |
1 | extern crate dep_2_reexport;
  | ---------------------------- one version of crate `dependency` is used here, as a dependency of crate `foo`
2 | extern crate dependency;
  | ------------------------ one version of crate `dependency` is used here, as a direct dependency of the current crate
  |
 ::: /home/gh-estebank/rust/build/x86_64-unknown-linux-gnu/test/run-make/crate-loading/rmake_out/multiple-dep-versions-2.rs:3:1
  |
3 | pub struct Type;
  | --------------- this type doesn't implement the required trait
4 | pub trait Trait {
  | --------------- this is the found trait
  = note: two types coming from two different versions of the same crate are different types even if they look the same
  = help: you can use `cargo tree` to explore your dependency tree
```

The approach to accomplish this is a HACK, and we'd want a better way to do this. I believe that moving E0277 to be a structured diagnostic would help in that regard.

Author: estebank

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -1796,6 +1796,24 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 StringPart::highlighted("cargo tree".to_string()),
                 StringPart::normal("` to explore your dependency tree".to_string()),
             ]);
+
+            // FIXME: this is a giant hack for the benefit of this specific diagnostic. Because
+            // we're so nested in method calls before the error gets emitted, bubbling a single bit
+            // flag informing the top level caller to stop adding extra detail to the diagnostic,
+            // would actually be harder to follow. So we do something naughty here: we consume the
+            // diagnostic, emit it and leave in its place a "delayed bug" that will continue being
+            // modified but won't actually be printed to end users. This *is not ideal*, but allows
+            // us to reduce the verbosity of an error that is already quite verbose and increase its
+            // specificity. Below we modify the main message as well, in a way that *could* break if
+            // the implementation of Diagnostics change significantly, but that would be caught with
+            // a make test failure when this diagnostic is tested.
+            err.primary_message(format!(
+                "{} because the trait comes from a different crate version",
+                err.messages[0].0.as_str().unwrap(),
+            ));
+            let diag = err.clone();
+            err.downgrade_to_delayed_bug();
+            self.tcx.dcx().emit_diagnostic(diag);
             return true;
         }
 

tests/run-make/crate-loading/rmake.rs

@@ -18,37 +18,31 @@ fn main() {
         .extern_("dependency", rust_lib_name("dependency"))
         .extern_("dep_2_reexport", rust_lib_name("foo"))
         .run_fail()
-        .assert_stderr_contains(r#"error[E0277]: the trait bound `dep_2_reexport::Type: Trait` is not satisfied
-  --> multiple-dep-versions.rs:7:18
-   |
-7  |     do_something(Type);
-   |     ------------ ^^^^ the trait `Trait` is not implemented for `dep_2_reexport::Type`
-   |     |
-   |     required by a bound introduced by this call
-   |
+        .assert_stderr_contains(r#"error[E0277]: the trait bound `dep_2_reexport::Type: Trait` is not satisfied because the trait comes from a different crate version
+ --> multiple-dep-versions.rs:7:18
+  |
+7 |     do_something(Type);
+  |                  ^^^^ the trait `Trait` is not implemented for `dep_2_reexport::Type`
+  |
 note: there are multiple different versions of crate `dependency` in the dependency graph"#)
         .assert_stderr_contains(r#"
-3  | pub struct Type(pub i32);
-   | --------------- this type implements the required trait
-4  | pub trait Trait {
-   | ^^^^^^^^^^^^^^^ this is the required trait
+3 | pub struct Type(pub i32);
+  | --------------- this type implements the required trait
+4 | pub trait Trait {
+  | ^^^^^^^^^^^^^^^ this is the required trait
 "#)
         .assert_stderr_contains(r#"
-1  | extern crate dep_2_reexport;
-   | ---------------------------- one version of crate `dependency` is used here, as a dependency of crate `foo`
-2  | extern crate dependency;
-   | ------------------------ one version of crate `dependency` is used here, as a direct dependency of the current crate"#)
+1 | extern crate dep_2_reexport;
+  | ---------------------------- one version of crate `dependency` is used here, as a dependency of crate `foo`
+2 | extern crate dependency;
+  | ------------------------ one version of crate `dependency` is used here, as a direct dependency of the current crate"#)
         .assert_stderr_contains(r#"
-3  | pub struct Type;
-   | --------------- this type doesn't implement the required trait
-4  | pub trait Trait {
-   | --------------- this is the found trait
-   = note: two types coming from two different versions of the same crate are different types even if they look the same
-   = help: you can use `cargo tree` to explore your dependency tree"#)
-        .assert_stderr_contains(r#"note: required by a bound in `do_something`"#)
-        .assert_stderr_contains(r#"
-12 | pub fn do_something<X: Trait>(_: X) {}
-   |                        ^^^^^ required by this bound in `do_something`"#)
+3 | pub struct Type;
+  | --------------- this type doesn't implement the required trait
+4 | pub trait Trait {
+  | --------------- this is the found trait
+  = note: two types coming from two different versions of the same crate are different types even if they look the same
+  = help: you can use `cargo tree` to explore your dependency tree"#)
         .assert_stderr_contains(r#"error[E0599]: no method named `foo` found for struct `dep_2_reexport::Type` in the current scope
  --> multiple-dep-versions.rs:8:10
   |