Add redundant type annotations lint

CHANGELOG.md

@@ -4853,6 +4853,7 @@ Released 2018-09-13
 [`redundant_pub_crate`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_pub_crate
 [`redundant_slicing`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_slicing
 [`redundant_static_lifetimes`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_static_lifetimes
+[`redundant_type_annotations`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_type_annotations
 [`ref_binding_to_reference`]: https://rust-lang.github.io/rust-clippy/master/index.html#ref_binding_to_reference
 [`ref_in_deref`]: https://rust-lang.github.io/rust-clippy/master/index.html#ref_in_deref
 [`ref_option_ref`]: https://rust-lang.github.io/rust-clippy/master/index.html#ref_option_ref

clippy_lints/src/declared_lints.rs

@@ -532,6 +532,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::redundant_slicing::DEREF_BY_SLICING_INFO,
     crate::redundant_slicing::REDUNDANT_SLICING_INFO,
     crate::redundant_static_lifetimes::REDUNDANT_STATIC_LIFETIMES_INFO,
+    crate::redundant_type_annotations::REDUNDANT_TYPE_ANNOTATIONS_INFO,
     crate::ref_option_ref::REF_OPTION_REF_INFO,
     crate::reference::DEREF_ADDROF_INFO,
     crate::regex::INVALID_REGEX_INFO,

clippy_lints/src/lib.rs

@@ -262,6 +262,7 @@ mod redundant_field_names;
 mod redundant_pub_crate;
 mod redundant_slicing;
 mod redundant_static_lifetimes;
+mod redundant_type_annotations;
 mod ref_option_ref;
 mod reference;
 mod regex;
@@ -940,6 +941,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|_| Box::new(allow_attributes::AllowAttribute));
     store.register_late_pass(move |_| Box::new(manual_main_separator_str::ManualMainSeparatorStr::new(msrv())));
     store.register_late_pass(|_| Box::new(unnecessary_struct_initialization::UnnecessaryStruct));
+    store.register_late_pass(|_| Box::new(redundant_type_annotations::RedundantTypeAnnotations));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/redundant_type_annotations.rs

@@ -0,0 +1,52 @@
+use rustc_hir as hir;
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use clippy_utils::diagnostics::span_lint;
+
+declare_clippy_lint! {
+    /// ### What it does
+    ///
+    /// ### Why is this bad?
+    ///
+    /// ### Example
+    /// ```rust
+    /// // example code where clippy issues a warning
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// // example code which does not raise clippy warning
+    /// ```
+    #[clippy::version = "1.70.0"]
+    pub REDUNDANT_TYPE_ANNOTATIONS,
+    pedantic,
+    "default lint description"
+}
+declare_lint_pass!(RedundantTypeAnnotations => [REDUNDANT_TYPE_ANNOTATIONS]);
+
+impl LateLintPass<'_> for RedundantTypeAnnotations {
+    fn check_local(&mut self, cx: &LateContext<'_>, local: &'_ rustc_hir::Local<'_>) {
+        if_chain!(
+            // type annotation part
+            if let Some(ty) = &local.ty;
+            if let hir::TyKind::Path(ty_path) = &ty.kind;
+            if let hir::QPath::Resolved(_, resolved_path_ty) = ty_path;
+            if let hir::def::Res::Def(_, defid_ty) = resolved_path_ty.res;
+            if let Some(middle_ty) = cx.tcx.type_of(defid_ty).no_bound_vars();
+
+            // initialization part
+            if let Some(init) = local.init;
+            if let hir::ExprKind::Call(init_call, _) = init.kind;
+            if let hir::ExprKind::Path(init_path) = &init_call.kind;
+            if let hir::QPath::Resolved(_, resolved_path) = init_path;
+            if let hir::def::Res::Def(_, defid) = resolved_path.res;
+            if let Some(middle_ty_init) = cx.tcx.type_of(defid).no_bound_vars();
+            if middle_ty_init.is_fn();
+            if let Some(init_return_type) = middle_ty_init.fn_sig(cx.tcx).output().no_bound_vars();
+
+            if middle_ty == init_return_type;
+            then {
+                span_lint(cx, REDUNDANT_TYPE_ANNOTATIONS, local.span, "Redundant type annotation")
+            }
+        );
+    }
+}

tests/ui/redundant_type_annotations.rs

@@ -0,0 +1,28 @@
+#![allow(unused)]
+#![warn(clippy::redundant_type_annotations)]
+
+struct A;
+enum E {
+    One,
+    Two
+}
+
+fn f() -> String {
+    String::new()
+}
+
+fn f_struct() -> A {
+    A
+}
+
+fn f_enum() -> E {
+    E::One
+}
+
+fn main() {
+    let a: String = f();
+    let a: A = f_struct();
+    let a: E = f_enum();
+
+    let b = f();
+}

tests/ui/redundant_type_annotations.stderr

@@ -0,0 +1,22 @@
+error: Redundant type annotation
+  --> $DIR/redundant_type_annotations.rs:23:5
+   |
+LL |     let a: String = f();
+   |     ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::redundant-type-annotations` implied by `-D warnings`
+
+error: Redundant type annotation
+  --> $DIR/redundant_type_annotations.rs:24:5
+   |
+LL |     let a: A = f_struct();
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+error: Redundant type annotation
+  --> $DIR/redundant_type_annotations.rs:25:5
+   |
+LL |     let a: E = f_enum();
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+