Make suggestion have a more targetted underline

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -70,7 +70,7 @@ use rustc_middle::ty::{
     subst::{Subst, SubstsRef},
     Region, Ty, TyCtxt, TypeFoldable,
 };
-use rustc_span::{DesugaringKind, Pos, Span};
+use rustc_span::{BytePos, DesugaringKind, Pos, Span};
 use rustc_target::spec::abi;
 use std::{cmp, fmt};
 
@@ -731,16 +731,12 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         return_sp: Span,
         arm_spans: impl Iterator<Item = Span>,
     ) {
-        let snippet = self
+        err.multipart_suggestion(
-            .tcx
-            .sess
-            .source_map()
-            .span_to_snippet(return_sp)
-            .unwrap_or_else(|_| "dyn Trait".to_string());
-        err.span_suggestion_verbose(
-            return_sp,
             "you could change the return type to be a boxed trait object",
-            format!("Box<dyn {}>", &snippet[5..]),
+            vec![
+                (return_sp.with_hi(return_sp.lo() + BytePos(4)), "Box<dyn".to_string()),
+                (return_sp.shrink_to_hi(), ">".to_string()),
+            ],
             Applicability::MaybeIncorrect,
         );
         let sugg = arm_spans

compiler/rustc_typeck/src/check/coercion.rs

@@ -51,7 +51,7 @@ use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::{self, Ty, TypeAndMut};
 use rustc_session::parse::feature_err;
 use rustc_span::symbol::sym;
-use rustc_span::{self, Span};
+use rustc_span::{self, BytePos, Span};
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
 use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode};
@@ -1523,10 +1523,12 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
         };
         if has_impl {
             if is_object_safe {
-                err.span_suggestion_verbose(
+                err.multipart_suggestion(
-                    return_sp,
                     "you could change the return type to be a boxed trait object",
-                    format!("Box<dyn {}>", &snippet[5..]),
+                    vec![
+                        (return_sp.with_hi(return_sp.lo() + BytePos(4)), "Box<dyn".to_string()),
+                        (return_sp.shrink_to_hi(), ">".to_string()),
+                    ],
                     Applicability::MachineApplicable,
                 );
             } else {

src/test/ui/impl-trait/object-unsafe-trait-in-return-position-impl-trait.stderr

@@ -35,7 +35,7 @@ LL |     B
 help: you could change the return type to be a boxed trait object
    |
 LL | fn cat() -> Box<dyn ObjectSafe> {
-   |             ^^^^^^^^^^^^^^^^^^^
+   |             ^^^^^^^           ^
 
 error: aborting due to 2 previous errors
 

src/test/ui/point-to-type-err-cause-on-impl-trait-return.stderr

@@ -17,7 +17,7 @@ LL |     1u32
 help: you could change the return type to be a boxed trait object
    |
 LL | fn foo() -> Box<dyn std::fmt::Display> {
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |             ^^^^^^^                  ^
 
 error[E0308]: mismatched types
   --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:12:16
@@ -38,7 +38,7 @@ LL |         return 1u32;
 help: you could change the return type to be a boxed trait object
    |
 LL | fn bar() -> Box<dyn std::fmt::Display> {
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |             ^^^^^^^                  ^
 
 error[E0308]: mismatched types
   --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:20:9
@@ -59,7 +59,7 @@ LL |         1u32
 help: you could change the return type to be a boxed trait object
    |
 LL | fn baz() -> Box<dyn std::fmt::Display> {
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |             ^^^^^^^                  ^
 
 error[E0308]: `if` and `else` have incompatible types
   --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:28:9
@@ -76,7 +76,7 @@ LL | |     }
 help: you could change the return type to be a boxed trait object
    |
 LL | fn qux() -> Box<dyn std::fmt::Display> {
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |             ^^^^^^^                  ^
 help: if you change the return type to expect trait objects box the returned expressions
    |
 LL |         Box::new(0i32)
@@ -102,7 +102,7 @@ LL |         _ => 1u32,
 help: you could change the return type to be a boxed trait object
    |
 LL | fn bat() -> Box<dyn std::fmt::Display> {
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |             ^^^^^^^                  ^
 
 error[E0308]: mismatched types
   --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:40:5
@@ -124,7 +124,7 @@ LL | |     }
 help: you could change the return type to be a boxed trait object
    |
 LL | fn can() -> Box<dyn std::fmt::Display> {
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |             ^^^^^^^                  ^
 
 error[E0308]: mismatched types
   --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:53:13
@@ -145,7 +145,7 @@ LL |             1u32
 help: you could change the return type to be a boxed trait object
    |
 LL | fn cat() -> Box<dyn std::fmt::Display> {
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |             ^^^^^^^                  ^
 
 error[E0308]: `match` arms have incompatible types
   --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:61:14
@@ -162,7 +162,7 @@ LL | |     }
 help: you could change the return type to be a boxed trait object
    |
 LL | fn dog() -> Box<dyn std::fmt::Display> {
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |             ^^^^^^^                  ^
 help: if you change the return type to expect trait objects box the returned expressions
    |
 LL |         0 => Box::new(0i32),