Rollup merge of #103283 - nbarrios1337:unsafe-impl-suggestions, r=cjgillot

Add suggestions for unsafe impl error codes

Adds suggestions for users to add `unsafe` to trait impls that should be `unsafe`, and remove `unsafe` from trait impls that do not require `unsafe`

With the folllowing code:

```rust
struct Foo {}

struct Bar {}

trait Safe {}

unsafe trait Unsafe {}

impl Safe for Foo {} // ok

impl Unsafe for Foo {} // E0200

unsafe impl Safe for Bar {} // E0199

unsafe impl Unsafe for Bar {} // ok

// omitted empty main fn
```

The current rustc output is:
```
error[E0199]: implementing the trait `Safe` is not unsafe
  --> e0200.rs:13:1
   |
13 | unsafe impl Safe for Bar {} // E0199
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0200]: the trait `Unsafe` requires an `unsafe impl` declaration
  --> e0200.rs:11:1
   |
11 | impl Unsafe for Foo {} // E0200
   | ^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0199, E0200.
For more information about an error, try `rustc --explain E0199`.
```

With this PR, the future rustc output would be:
```
error[E0199]: implementing the trait `Safe` is not unsafe
  --> ../../temp/e0200.rs:13:1
   |
13 | unsafe impl Safe for Bar {} // E0199
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
help: remove `unsafe` from this trait implementation
   |
13 - unsafe impl Safe for Bar {} // E0199
13 + impl Safe for Bar {} // E0199
   |

error[E0200]: the trait `Unsafe` requires an `unsafe impl` declaration
  --> ../../temp/e0200.rs:11:1
   |
11 | impl Unsafe for Foo {} // E0200
   | ^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: the trait `Unsafe` enforces invariants that the compiler can't check. Review the trait documentation and make sure this implementation upholds those invariants before adding the `unsafe` keyword
help: add `unsafe` to this trait implementation
   |
11 | unsafe impl Unsafe for Foo {} // E0200
   | ++++++

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0199, E0200.
For more information about an error, try `rustc --explain E0199`.
```

``@rustbot`` label +T-compiler +A-diagnostics +A-suggestion-diagnostics

compiler/rustc_hir_analysis/src/coherence/unsafety.rs

@@ -26,6 +26,12 @@ pub(super) fn check_item(tcx: TyCtxt<'_>, def_id: LocalDefId) {
                     "implementing the trait `{}` is not unsafe",
                     trait_ref.print_only_trait_path()
                 )
+                .span_suggestion_verbose(
+                    item.span.with_hi(item.span.lo() + rustc_span::BytePos(7)),
+                    "remove `unsafe` from this trait implementation",
+                    "",
+                    rustc_errors::Applicability::MachineApplicable,
+                )
                 .emit();
             }
 
@@ -37,6 +43,18 @@ pub(super) fn check_item(tcx: TyCtxt<'_>, def_id: LocalDefId) {
                     "the trait `{}` requires an `unsafe impl` declaration",
                     trait_ref.print_only_trait_path()
                 )
+                .note(format!(
+                    "the trait `{}` enforces invariants that the compiler can't check. \
+                    Review the trait documentation and make sure this implementation \
+                    upholds those invariants before adding the `unsafe` keyword",
+                    trait_ref.print_only_trait_path()
+                ))
+                .span_suggestion_verbose(
+                    item.span.shrink_to_lo(),
+                    "add `unsafe` to this trait implementation",
+                    "unsafe ",
+                    rustc_errors::Applicability::MaybeIncorrect,
+                )
                 .emit();
             }
 
@@ -48,6 +66,18 @@ pub(super) fn check_item(tcx: TyCtxt<'_>, def_id: LocalDefId) {
                     "requires an `unsafe impl` declaration due to `#[{}]` attribute",
                     attr_name
                 )
+                .note(format!(
+                    "the trait `{}` enforces invariants that the compiler can't check. \
+                    Review the trait documentation and make sure this implementation \
+                    upholds those invariants before adding the `unsafe` keyword",
+                    trait_ref.print_only_trait_path()
+                ))
+                .span_suggestion_verbose(
+                    item.span.shrink_to_lo(),
+                    "add `unsafe` to this trait implementation",
+                    "unsafe ",
+                    rustc_errors::Applicability::MaybeIncorrect,
+                )
                 .emit();
             }