New upstream version 1.52.1+dfsg1

[rustc.git] / src / tools / clippy / clippy_lints / src / methods / map_flatten.rs

diff --git a/src/tools/clippy/clippy_lints/src/methods/map_flatten.rs b/src/tools/clippy/clippy_lints/src/methods/map_flatten.rs
new file mode 100644 (file)
index 0000000..14a14e4
--- /dev/null
+++ b/src/tools/clippy/clippy_lints/src/methods/map_flatten.rs
@@ -0,0 +1,61 @@
+use crate::utils::{is_type_diagnostic_item, match_trait_method, paths, snippet, span_lint_and_sugg};
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_middle::ty;
+use rustc_span::symbol::sym;
+
+use super::MAP_FLATTEN;
+
+/// lint use of `map().flatten()` for `Iterators` and 'Options'
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>, map_args: &'tcx [hir::Expr<'_>]) {
+    // lint if caller of `.map().flatten()` is an Iterator
+    if match_trait_method(cx, expr, &paths::ITERATOR) {
+        let map_closure_ty = cx.typeck_results().expr_ty(&map_args[1]);
+        let is_map_to_option = match map_closure_ty.kind() {
+            ty::Closure(_, _) | ty::FnDef(_, _) | ty::FnPtr(_) => {
+                let map_closure_sig = match map_closure_ty.kind() {
+                    ty::Closure(_, substs) => substs.as_closure().sig(),
+                    _ => map_closure_ty.fn_sig(cx.tcx),
+                };
+                let map_closure_return_ty = cx.tcx.erase_late_bound_regions(map_closure_sig.output());
+                is_type_diagnostic_item(cx, map_closure_return_ty, sym::option_type)
+            },
+            _ => false,
+        };
+
+        let method_to_use = if is_map_to_option {
+            // `(...).map(...)` has type `impl Iterator<Item=Option<...>>
+            "filter_map"
+        } else {
+            // `(...).map(...)` has type `impl Iterator<Item=impl Iterator<...>>
+            "flat_map"
+        };
+        let func_snippet = snippet(cx, map_args[1].span, "..");
+        let hint = format!(".{0}({1})", method_to_use, func_snippet);
+        span_lint_and_sugg(
+            cx,
+            MAP_FLATTEN,
+            expr.span.with_lo(map_args[0].span.hi()),
+            "called `map(..).flatten()` on an `Iterator`",
+            &format!("try using `{}` instead", method_to_use),
+            hint,
+            Applicability::MachineApplicable,
+        );
+    }
+
+    // lint if caller of `.map().flatten()` is an Option
+    if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(&map_args[0]), sym::option_type) {
+        let func_snippet = snippet(cx, map_args[1].span, "..");
+        let hint = format!(".and_then({})", func_snippet);
+        span_lint_and_sugg(
+            cx,
+            MAP_FLATTEN,
+            expr.span.with_lo(map_args[0].span.hi()),
+            "called `map(..).flatten()` on an `Option`",
+            "try using `and_then` instead",
+            hint,
+            Applicability::MachineApplicable,
+        );
+    }
+}