Auto merge of rust-lang#12216 - bpandreotti:redundant-type-annotations-fix, r=Jarcho

Fix false positive in `redundant_type_annotations` lint

This PR changes the `redundant_type_annotations` lint to allow slice type annotations (i.e., `&[u8]`) for byte string literals. It will still consider _array_ type annotations (i.e., `&[u8; 4]`) as redundant. The reasoning behind this is that the type of byte string literals is by default a reference to an array, but, by using a type annotation, you can force it to be a slice. For example:
```rust
let a: &[u8; 4] = b"test";
let b: &[u8] = b"test";
```

Now, the type annotation for `a` will still be linted (as it is still redundant), but the type annotation for `b` will not.

Fixes rust-lang#12212.

changelog: [`redundant_type_annotations`]: Fix false positive with byte string literals

Author: bors

clippy_lints/src/redundant_type_annotations.rs

@@ -188,7 +188,6 @@ impl LateLintPass<'_> for RedundantTypeAnnotations {
                     match init_lit.node {
                         // In these cases the annotation is redundant
                         LitKind::Str(..)
-                        | LitKind::ByteStr(..)
                         | LitKind::Byte(..)
                         | LitKind::Char(..)
                         | LitKind::Bool(..)
@@ -202,6 +201,16 @@ impl LateLintPass<'_> for RedundantTypeAnnotations {
                             }
                         },
                         LitKind::Err => (),
+                        LitKind::ByteStr(..) => {
+                            // We only lint if the type annotation is an array type (e.g. &[u8; 4]).
+                            // If instead it is a slice (e.g. &[u8]) it may not be redundant, so we
+                            // don't lint.
+                            if let hir::TyKind::Ref(_, mut_ty) = ty.kind
+                                && matches!(mut_ty.ty.kind, hir::TyKind::Array(..))
+                            {
+                                span_lint(cx, REDUNDANT_TYPE_ANNOTATIONS, local.span, "redundant type annotation");
+                            }
+                        },
                     }
                 },
                 _ => (),

tests/ui/redundant_type_annotations.rs

@@ -196,13 +196,18 @@ fn test_simple_types() {
     let _var: &str = "test";
     //~^ ERROR: redundant type annotation
 
-    let _var: &[u8] = b"test";
+    let _var: &[u8; 4] = b"test";
     //~^ ERROR: redundant type annotation
 
     let _var: bool = false;
     //~^ ERROR: redundant type annotation
 }
 
+fn issue12212() {
+    // This should not be linted
+    let _var: &[u8] = b"test";
+}
+
 fn issue11190() {}
 
 fn main() {}

tests/ui/redundant_type_annotations.stderr

@@ -94,8 +94,8 @@ LL |     let _var: &str = "test";
 error: redundant type annotation
   --> $DIR/redundant_type_annotations.rs:199:5
    |
-LL |     let _var: &[u8] = b"test";
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     let _var: &[u8; 4] = b"test";
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: redundant type annotation
   --> $DIR/redundant_type_annotations.rs:202:5