Add new lint then_some_unwrap_or

New lint suggests using `if .. else ..` instead of
`.then_some(..).unwrap_or(..)`.

Author: arieluy

CHANGELOG.md

@@ -3925,6 +3925,7 @@ Released 2018-09-13
 [`tabs_in_doc_comments`]: https://rust-lang.github.io/rust-clippy/master/index.html#tabs_in_doc_comments
 [`temporary_assignment`]: https://rust-lang.github.io/rust-clippy/master/index.html#temporary_assignment
 [`temporary_cstring_as_ptr`]: https://rust-lang.github.io/rust-clippy/master/index.html#temporary_cstring_as_ptr
+[`then_some_unwrap_or`]: https://rust-lang.github.io/rust-clippy/master/index.html#then_some_unwrap_or
 [`to_digit_is_some`]: https://rust-lang.github.io/rust-clippy/master/index.html#to_digit_is_some
 [`to_string_in_display`]: https://rust-lang.github.io/rust-clippy/master/index.html#to_string_in_display
 [`to_string_in_format_args`]: https://rust-lang.github.io/rust-clippy/master/index.html#to_string_in_format_args

clippy_lints/src/lib.register_all.rs

@@ -203,6 +203,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(methods::STRING_EXTEND_CHARS),
     LintId::of(methods::SUSPICIOUS_MAP),
     LintId::of(methods::SUSPICIOUS_SPLITN),
+    LintId::of(methods::THEN_SOME_UNWRAP_OR),
     LintId::of(methods::UNINIT_ASSUMED_INIT),
     LintId::of(methods::UNNECESSARY_FILTER_MAP),
     LintId::of(methods::UNNECESSARY_FIND_MAP),

clippy_lints/src/lib.register_lints.rs

@@ -345,6 +345,7 @@ store.register_lints(&[
     methods::STRING_EXTEND_CHARS,
     methods::SUSPICIOUS_MAP,
     methods::SUSPICIOUS_SPLITN,
+    methods::THEN_SOME_UNWRAP_OR,
     methods::UNINIT_ASSUMED_INIT,
     methods::UNNECESSARY_FILTER_MAP,
     methods::UNNECESSARY_FIND_MAP,

clippy_lints/src/lib.register_style.rs

@@ -76,6 +76,7 @@ store.register_group(true, "clippy::style", Some("clippy_style"), vec![
     LintId::of(methods::SHOULD_IMPLEMENT_TRAIT),
     LintId::of(methods::SINGLE_CHAR_ADD_STR),
     LintId::of(methods::STRING_EXTEND_CHARS),
+    LintId::of(methods::THEN_SOME_UNWRAP_OR),
     LintId::of(methods::UNNECESSARY_FOLD),
     LintId::of(methods::UNNECESSARY_LAZY_EVALUATIONS),
     LintId::of(methods::UNWRAP_OR_ELSE_DEFAULT),

clippy_lints/src/methods/mod.rs

@@ -62,6 +62,7 @@ mod str_splitn;
 mod string_extend_chars;
 mod suspicious_map;
 mod suspicious_splitn;
+mod then_some_unwrap_or;
 mod uninit_assumed_init;
 mod unnecessary_filter_map;
 mod unnecessary_fold;
@@ -2259,6 +2260,30 @@ declare_clippy_lint! {
     "replace with no effect"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for usages of `.then_some(..).unwrap_or(..)`
+    ///
+    /// ### Why is this bad?
+    /// This can be written more clearly with `if .. else ..`
+    ///
+    /// ### Example
+    /// ```rust
+    /// let x = true;
+    /// x.then_some("a").unwrap_or("b");
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// let x = true;
+    /// if x { "a" } else { "b" };
+    /// ```
+    #[clippy::version = "1.64.0"]
+    pub THEN_SOME_UNWRAP_OR,
+    style,
+    "use of `.then_some(..).unwrap_or(..)` can be written \
+    more clearly with `if .. else ..`"
+}
+
 pub struct Methods {
     avoid_breaking_exported_api: bool,
     msrv: Option<RustcVersion>,
@@ -2360,6 +2385,7 @@ impl_lint_pass!(Methods => [
     IS_DIGIT_ASCII_RADIX,
     NEEDLESS_OPTION_TAKE,
     NO_EFFECT_REPLACE,
+    THEN_SOME_UNWRAP_OR,
 ]);
 
 /// Extracts a method call name, args, and `Span` of the method name.
@@ -2768,6 +2794,9 @@ impl Methods {
                     Some(("map", [m_recv, m_arg], span)) => {
                         option_map_unwrap_or::check(cx, expr, m_recv, m_arg, recv, u_arg, span);
                     },
+                    Some(("then_some", [t_recv, t_arg], _)) => {
+                        then_some_unwrap_or::check(cx, expr, t_recv, t_arg, u_arg);
+                    },
                     _ => {},
                 },
                 ("unwrap_or_else", [u_arg]) => match method_call(recv) {

clippy_lints/src/methods/then_some_unwrap_or.rs

@@ -0,0 +1,42 @@
+// run-rustfix
+
+use super::THEN_SOME_UNWRAP_OR;
+use clippy_utils::{diagnostics::span_lint_and_sugg, source::snippet_with_applicability};
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx hir::Expr<'_>,
+    then_recv: &'tcx hir::Expr<'_>,
+    then_arg: &'tcx hir::Expr<'_>,
+    unwrap_arg: &'tcx hir::Expr<'_>,
+) {
+    // something.then_some(blah).unwrap_or(blah)
+    // ^^^^^^^^^-then_recv ^^^^-then_arg   ^^^^- unwrap_arg
+    // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^- expr
+
+    let recv_ty = cx.typeck_results().expr_ty(then_recv);
+
+    if recv_ty.is_bool() {
+        let mut applicability = Applicability::MachineApplicable;
+        let sugg = format!(
+            "if {} {{ {} }} else {{ {} }}",
+            snippet_with_applicability(cx, then_recv.span, "..", &mut applicability),
+            snippet_with_applicability(cx, then_arg.span, "..", &mut applicability),
+            snippet_with_applicability(cx, unwrap_arg.span, "..", &mut applicability)
+        );
+
+        span_lint_and_sugg(
+            cx,
+            THEN_SOME_UNWRAP_OR,
+            expr.span,
+            "use of `.then_some(..).unwrap_or(..)` can be written \
+            more clearly with `if .. else ..`",
+            "try",
+            sugg,
+            applicability,
+        );
+    }
+}

tests/ui/then_some_unwrap_or.fixed

@@ -0,0 +1,7 @@
+// run-rustfix
+
+#![warn(clippy::then_some_unwrap_or)]
+
+fn main() {
+    if true { "a" } else { "b" };
+}

tests/ui/then_some_unwrap_or.rs

@@ -0,0 +1,7 @@
+// run-rustfix
+
+#![warn(clippy::then_some_unwrap_or)]
+
+fn main() {
+    true.then_some("a").unwrap_or("b");
+}

tests/ui/then_some_unwrap_or.stderr

@@ -0,0 +1,10 @@
+error: use of `.then_some(..).unwrap_or(..)` can be written more clearly with `if .. else ..`
+  --> $DIR/then_some_unwrap_or.rs:6:5
+   |
+LL |     true.then_some("a").unwrap_or("b");
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `if true { "a" } else { "b" }`
+   |
+   = note: `-D clippy::then-some-unwrap-or` implied by `-D warnings`
+
+error: aborting due to previous error
+