Suggest removing Box::new

Author: camsteffen

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -2955,6 +2955,29 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         )
                     {
                         let deref_kind = if checked_ty.is_box() {
+                            // detect Box::new(..)
+                            if let ExprKind::Call(box_new, [arg]) = expr.kind
+                                && let ExprKind::Path(qpath) = &box_new.kind
+                                && let Res::Def(DefKind::AssocFn, fn_id) =
+                                    self.typeck_results.borrow().qpath_res(qpath, box_new.hir_id)
+                                && let Some(impl_id) = self.tcx.inherent_impl_of_assoc(fn_id)
+                                && self.tcx.type_of(impl_id).skip_binder().is_box()
+                                && self.tcx.item_name(fn_id) == sym::new
+                                && let source_map = self.tcx.sess.source_map()
+                                && let Ok(paren) =
+                                    source_map.span_extend_prev_n_chars(expr.span.shrink_to_hi(), 1)
+                            {
+                                return Some((
+                                    vec![
+                                        (expr.span.until(arg.span), "".to_string()),
+                                        (paren, "".to_string()),
+                                    ],
+                                    "consider removing the Box".to_string(),
+                                    Applicability::MachineApplicable,
+                                    false,
+                                    false,
+                                ));
+                            }
                             "unboxing the value"
                         } else if checked_ty.is_ref() {
                             "dereferencing the borrow"

compiler/rustc_span/src/source_map.rs

@@ -551,7 +551,7 @@ impl SourceMap {
     /// the slice for the beginning of the span and an index in the slice for the end of the span.
     pub fn span_to_source<F, T>(&self, sp: Span, extract_source: F) -> Result<T, SpanSnippetError>
     where
-        F: Fn(&str, usize, usize) -> Result<T, SpanSnippetError>,
+        F: FnOnce(&str, usize, usize) -> Result<T, SpanSnippetError>,
     {
         let local_begin = self.lookup_byte_offset(sp.lo());
         let local_end = self.lookup_byte_offset(sp.hi());
@@ -724,7 +724,7 @@ impl SourceMap {
     pub fn span_extend_prev_while(
         &self,
         span: Span,
-        f: impl Fn(char) -> bool,
+        mut f: impl FnMut(char) -> bool,
     ) -> Result<Span, SpanSnippetError> {
         self.span_to_source(span, |s, start, _end| {
             let n = s[..start]
@@ -735,6 +735,12 @@ impl SourceMap {
         })
     }
 
+    /// Extends the given `Span` to include the previous n characters
+    pub fn span_extend_prev_n_chars(&self, span: Span, n: usize) -> Result<Span, SpanSnippetError> {
+        let mut iter = 0..n;
+        self.span_extend_prev_while(span, |_c| iter.next().is_some())
+    }
+
     /// Extends the given `Span` to just before the next occurrence of `c`.
     pub fn span_extend_to_next_char(&self, sp: Span, c: char, accept_newlines: bool) -> Span {
         if let Ok(next_source) = self.span_to_next_source(sp) {

tests/ui/coercion/coerce-block-tail.stderr

@@ -6,10 +6,11 @@ LL |     let _: &i32 = & { Box::new(1i32) };
    |
    = note: expected type `i32`
             found struct `Box<i32>`
-help: consider unboxing the value
+help: consider removing the Box
+   |
+LL -     let _: &i32 = & { Box::new(1i32) };
+LL +     let _: &i32 = & { 1i32 };
    |
-LL |     let _: &i32 = & { *Box::new(1i32) };
-   |                       +
 
 error: aborting due to 1 previous error
 

tests/ui/coercion/coerce-box-new-to-unboxed.rs

@@ -0,0 +1,4 @@
+fn main() {
+    let _: String = Box::new(String::new());
+    //~^ ERROR mismatched types
+}

tests/ui/coercion/coerce-box-new-to-unboxed.stderr

@@ -0,0 +1,19 @@
+error[E0308]: mismatched types
+  --> $DIR/coerce-box-new-to-unboxed.rs:2:21
+   |
+LL |     let _: String = Box::new(String::new());
+   |            ------   ^^^^^^^^^^^^^^^^^^^^^^^ expected `String`, found `Box<String>`
+   |            |
+   |            expected due to this
+   |
+   = note: expected struct `String`
+              found struct `Box<String>`
+help: consider removing the Box
+   |
+LL -     let _: String = Box::new(String::new());
+LL +     let _: String = String::new();
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.