Resolve needless_borrow clippy lints

    error: this expression borrows a reference (`&ast::Field`) that is immediately dereferenced by the compiler
      --> impl/src/prop.rs:68:25
       |
    68 |             return Some(&field);
       |                         ^^^^^^ help: change this to: `field`
       |
       = note: `-D clippy::needless-borrow` implied by `-D clippy::all`
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

    error: this expression borrows a reference (`&ast::Field`) that is immediately dereferenced by the compiler
      --> impl/src/prop.rs:77:25
       |
    77 |             return Some(&field);
       |                         ^^^^^^ help: change this to: `field`
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

    error: this expression borrows a reference (`&ast::Field`) that is immediately dereferenced by the compiler
      --> impl/src/prop.rs:82:70
       |
    82 |             Member::Named(ident) if ident == "source" => return Some(&field),
       |                                                                      ^^^^^^ help: change this to: `field`
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

    error: this expression borrows a reference (`&ast::Field`) that is immediately dereferenced by the compiler
      --> impl/src/prop.rs:92:25
       |
    92 |             return Some(&field);
       |                         ^^^^^^ help: change this to: `field`
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

    error: this expression borrows a reference (`&ast::Field`) that is immediately dereferenced by the compiler
      --> impl/src/prop.rs:97:25
       |
    97 |             return Some(&field);
       |                         ^^^^^^ help: change this to: `field`
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

    error: this expression borrows a reference (`&syn::Type`) that is immediately dereferenced by the compiler
       --> impl/src/valid.rs:191:41
        |
    191 |         if contains_non_static_lifetime(&source_field.ty) {
        |                                         ^^^^^^^^^^^^^^^^ help: change this to: `source_field.ty`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

impl/src/prop.rs

@@ -65,7 +65,7 @@ impl Field<'_> {
 fn from_field<'a, 'b>(fields: &'a [Field<'b>]) -> Option<&'a Field<'b>> {
     for field in fields {
         if field.attrs.from.is_some() {
-            return Some(&field);
+            return Some(field);
         }
     }
     None
@@ -74,12 +74,12 @@ fn from_field<'a, 'b>(fields: &'a [Field<'b>]) -> Option<&'a Field<'b>> {
 fn source_field<'a, 'b>(fields: &'a [Field<'b>]) -> Option<&'a Field<'b>> {
     for field in fields {
         if field.attrs.from.is_some() || field.attrs.source.is_some() {
-            return Some(&field);
+            return Some(field);
         }
     }
     for field in fields {
         match &field.member {
-            Member::Named(ident) if ident == "source" => return Some(&field),
+            Member::Named(ident) if ident == "source" => return Some(field),
             _ => {}
         }
     }
@@ -89,12 +89,12 @@ fn source_field<'a, 'b>(fields: &'a [Field<'b>]) -> Option<&'a Field<'b>> {
 fn backtrace_field<'a, 'b>(fields: &'a [Field<'b>]) -> Option<&'a Field<'b>> {
     for field in fields {
         if field.attrs.backtrace.is_some() {
-            return Some(&field);
+            return Some(field);
         }
     }
     for field in fields {
         if field.is_backtrace() {
-            return Some(&field);
+            return Some(field);
         }
     }
     None

impl/src/valid.rs

@@ -188,7 +188,7 @@ fn check_field_attrs(fields: &[Field]) -> Result<()> {
         }
     }
     if let Some(source_field) = source_field.or(from_field) {
-        if contains_non_static_lifetime(&source_field.ty) {
+        if contains_non_static_lifetime(source_field.ty) {
             return Err(Error::new_spanned(
                 &source_field.original.ty,
                 "non-static lifetimes are not allowed in the source of an error, because std::error::Error requires the source is dyn Error + 'static",