Rollup merge of #148001 - Muscraft:fix-diff-symbol-unchanged-lines, r=fmease

fix: Don't add diff symbol to unchanged lines

When rendering a "multi-line" suggestion with the [`Diff`](https://github.com/rust-lang/rust/blob/dc1feabef242259d61bd930713de3250577c1c71/compiler/rustc_errors/src/emitter.rs#L3078) format, `rustc` uses a [diff symbol](https://github.com/rust-lang/rust/blob/dc1feabef242259d61bd930713de3250577c1c71/compiler/rustc_errors/src/emitter.rs#L3017-L3022) for
[any line that has a highlight part](https://github.com/rust-lang/rust/blob/dc1feabef242259d61bd930713de3250577c1c71/compiler/rustc_errors/src/emitter.rs#L2705-L2713). This includes highlight parts that are highlighting nothing, i.e., a span of `0..0`. This leads `rustc` to add a diff symbol unnecessarily to lines that have no changes and are not highlighted. This PR makes it so that `rustc` will not add a diff symbol to lines that contain no changes/highlights.

Note: This PR is part of my ongoing effort to have `rustc` use `annotate-snippets` for rendering. This change will make it so that `rustc` and `annotate-snippets` will match in this case.

Author: Zalathar

tests/ui/derivable_impls.stderr

@@ -14,7 +14,7 @@ LL | | }
 help: replace the manual implementation with a derive attribute
    |
 LL + #[derive(Default)]
-LL ~ struct FooDefault<'a> {
+LL | struct FooDefault<'a> {
    |
 
 error: this `impl` can be derived
@@ -31,7 +31,7 @@ LL | | }
 help: replace the manual implementation with a derive attribute
    |
 LL + #[derive(Default)]
-LL ~ struct TupleDefault(bool, i32, u64);
+LL | struct TupleDefault(bool, i32, u64);
    |
 
 error: this `impl` can be derived
@@ -48,7 +48,7 @@ LL | | }
 help: replace the manual implementation with a derive attribute
    |
 LL + #[derive(Default)]
-LL ~ struct StrDefault<'a>(&'a str);
+LL | struct StrDefault<'a>(&'a str);
    |
 
 error: this `impl` can be derived
@@ -65,7 +65,7 @@ LL | | }
 help: replace the manual implementation with a derive attribute
    |
 LL + #[derive(Default)]
-LL ~ struct Y(u32);
+LL | struct Y(u32);
    |
 
 error: this `impl` can be derived
@@ -82,7 +82,7 @@ LL | | }
 help: replace the manual implementation with a derive attribute
    |
 LL + #[derive(Default)]
-LL ~ struct WithoutSelfCurly {
+LL | struct WithoutSelfCurly {
    |
 
 error: this `impl` can be derived
@@ -99,7 +99,7 @@ LL | | }
 help: replace the manual implementation with a derive attribute
    |
 LL + #[derive(Default)]
-LL ~ struct WithoutSelfParan(bool);
+LL | struct WithoutSelfParan(bool);
    |
 
 error: this `impl` can be derived
@@ -115,7 +115,7 @@ LL | | }
 help: replace the manual implementation with a derive attribute
    |
 LL + #[derive(Default)]
-LL ~ pub struct DirectDefaultDefaultCall {
+LL | pub struct DirectDefaultDefaultCall {
    |
 
 error: this `impl` can be derived
@@ -131,7 +131,7 @@ LL | | }
 help: replace the manual implementation with a derive attribute
    |
 LL + #[derive(Default)]
-LL ~ pub struct EquivalentToDefaultDefaultCallVec {
+LL | pub struct EquivalentToDefaultDefaultCallVec {
    |
 
 error: this `impl` can be derived
@@ -147,7 +147,7 @@ LL | | }
 help: replace the manual implementation with a derive attribute
    |
 LL + #[derive(Default)]
-LL ~ pub struct EquivalentToDefaultDefaultCallLocal {
+LL | pub struct EquivalentToDefaultDefaultCallLocal {
    |
 
 error: this `impl` can be derived
@@ -164,7 +164,7 @@ LL | | }
 help: replace the manual implementation with a derive attribute
    |
 LL + #[derive(Default)]
-LL ~ pub struct RepeatDefault1 {
+LL | pub struct RepeatDefault1 {
    |
 
 error: this `impl` can be derived
@@ -181,7 +181,7 @@ LL | | }
 help: replace the manual implementation with a derive attribute and mark the default variant
    |
 LL + #[derive(Default)]
-LL ~ pub enum SimpleEnum {
+LL | pub enum SimpleEnum {
 LL |     Foo,
 LL ~     #[default]
 LL ~     Bar,