fix: manual_unwrap_or_default suggests falsely when condition type is uncertain (rust-lang#13889)

fixes rust-lang#12670

Continuation of rust-lang#12688. r? @Jarcho if you don't mind?

changelog: [`manual_unwrap_or_default`] fix wrong suggestions when
condition type is uncertain

Author: llogiq

clippy_lints/src/manual_unwrap_or_default.rs

@@ -9,8 +9,8 @@ use rustc_span::sym;
 use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::higher::IfLetOrMatch;
 use clippy_utils::sugg::Sugg;
-use clippy_utils::ty::implements_trait;
-use clippy_utils::{is_default_equivalent, is_in_const_context, peel_blocks, span_contains_comment};
+use clippy_utils::ty::{expr_type_is_certain, implements_trait};
+use clippy_utils::{is_default_equivalent, is_in_const_context, path_res, peel_blocks, span_contains_comment};
 
 declare_clippy_lint! {
     /// ### What it does
@@ -158,6 +158,36 @@ fn handle<'tcx>(cx: &LateContext<'tcx>, if_let_or_match: IfLetOrMatch<'tcx>, exp
         } else {
             Applicability::MachineApplicable
         };
+
+        // We now check if the condition is a None variant, in which case we need to specify the type
+        if path_res(cx, condition)
+            .opt_def_id()
+            .is_some_and(|id| Some(cx.tcx.parent(id)) == cx.tcx.lang_items().option_none_variant())
+        {
+            return span_lint_and_sugg(
+                cx,
+                MANUAL_UNWRAP_OR_DEFAULT,
+                expr.span,
+                format!("{expr_name} can be simplified with `.unwrap_or_default()`"),
+                "replace it with",
+                format!("{receiver}::<{expr_type}>.unwrap_or_default()"),
+                applicability,
+            );
+        }
+
+        // We check if the expression type is still uncertain, in which case we ask the user to specify it
+        if !expr_type_is_certain(cx, condition) {
+            return span_lint_and_sugg(
+                cx,
+                MANUAL_UNWRAP_OR_DEFAULT,
+                expr.span,
+                format!("{expr_name} can be simplified with `.unwrap_or_default()`"),
+                format!("ascribe the type {expr_type} and replace your expression with"),
+                format!("{receiver}.unwrap_or_default()"),
+                Applicability::Unspecified,
+            );
+        }
+
         span_lint_and_sugg(
             cx,
             MANUAL_UNWRAP_OR_DEFAULT,

tests/ui/manual_unwrap_or_default_unfixable.rs

@@ -0,0 +1,15 @@
+//@no-rustfix
+fn issue_12670() {
+    // no auto: type not found
+    #[allow(clippy::match_result_ok)]
+    let _ = if let Some(x) = "1".parse().ok() {
+        x
+    } else {
+        i32::default()
+    };
+    let _ = if let Some(x) = None { x } else { i32::default() };
+    // auto fix with unwrap_or_default
+    let a: Option<i32> = None;
+    let _ = if let Some(x) = a { x } else { i32::default() };
+    let _ = if let Some(x) = Some(99) { x } else { i32::default() };
+}

tests/ui/manual_unwrap_or_default_unfixable.stderr

@@ -0,0 +1,34 @@
+error: if let can be simplified with `.unwrap_or_default()`
+  --> tests/ui/manual_unwrap_or_default_unfixable.rs:5:13
+   |
+LL |       let _ = if let Some(x) = "1".parse().ok() {
+   |  _____________^
+LL | |         x
+LL | |     } else {
+LL | |         i32::default()
+LL | |     };
+   | |_____^ help: ascribe the type i32 and replace your expression with: `"1".parse().ok().unwrap_or_default()`
+   |
+   = note: `-D clippy::manual-unwrap-or-default` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::manual_unwrap_or_default)]`
+
+error: if let can be simplified with `.unwrap_or_default()`
+  --> tests/ui/manual_unwrap_or_default_unfixable.rs:10:13
+   |
+LL |     let _ = if let Some(x) = None { x } else { i32::default() };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace it with: `None::<i32>.unwrap_or_default()`
+
+error: if let can be simplified with `.unwrap_or_default()`
+  --> tests/ui/manual_unwrap_or_default_unfixable.rs:13:13
+   |
+LL |     let _ = if let Some(x) = a { x } else { i32::default() };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace it with: `a.unwrap_or_default()`
+
+error: if let can be simplified with `.unwrap_or_default()`
+  --> tests/ui/manual_unwrap_or_default_unfixable.rs:14:13
+   |
+LL |     let _ = if let Some(x) = Some(99) { x } else { i32::default() };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace it with: `Some(99).unwrap_or_default()`
+
+error: aborting due to 4 previous errors
+