Fix clippy expect_used complains for derive(FromRow) macro (scylladb#1077)

Get rid of `.expect` usage in `FromRow` macro by using arrays to detect problems during compile time.

Author: Mr-Leshiy

scylla-cql/src/frame/response/cql_to_rust.rs

@@ -935,6 +935,11 @@ mod tests {
         );
     }
 
+    // Enabling `expect_used` clippy lint,
+    // validates that `derive(FromRow)` macro definition does do not violates such rule under the hood.
+    // Could be removed after such rule will be applied for the whole crate.
+    // <https://rust-lang.github.io/rust-clippy/master/index.html#/expect_used>
+    #[deny(clippy::expect_used)]
     #[test]
     fn struct_from_row() {
         #[derive(FromRow)]
@@ -959,6 +964,11 @@ mod tests {
         assert_eq!(my_row.c, Some(vec![1, 2]));
     }
 
+    // Enabling `expect_used` clippy lint,
+    // validates that `derive(FromRow)` macro definition does do not violates such rule under the hood.
+    // Could be removed after such rule will be applied for the whole crate.
+    // <https://rust-lang.github.io/rust-clippy/master/index.html#/expect_used>
+    #[deny(clippy::expect_used)]
     #[test]
     fn struct_from_row_wrong_size() {
         #[derive(FromRow, PartialEq, Eq, Debug)]
@@ -998,6 +1008,11 @@ mod tests {
         );
     }
 
+    // Enabling `expect_used` clippy lint,
+    // validates that `derive(FromRow)` macro definition does do not violates such rule under the hood.
+    // Could be removed after such rule will be applied for the whole crate.
+    // <https://rust-lang.github.io/rust-clippy/master/index.html#/expect_used>
+    #[deny(clippy::expect_used)]
     #[test]
     fn unnamed_struct_from_row() {
         #[derive(FromRow)]

scylla-macros/src/from_row.rs

@@ -14,19 +14,20 @@ pub(crate) fn from_row_derive(tokens_input: TokenStream) -> Result<TokenStream,
     // Generates a token that sets the values of struct fields: field_type::from_cql(...)
     let (fill_struct_code, fields_count) = match struct_fields {
         crate::parser::StructFields::Named(fields) => {
-            let set_fields_code = fields.named.iter().map(|field| {
+            let set_fields_code = fields.named.iter().enumerate().map(|(col_ix, field)| {
                 let field_name = &field.ident;
                 let field_type = &field.ty;
 
                 quote_spanned! {field.span() =>
                     #field_name: {
-                        let (col_ix, col_value) = vals_iter
-                            .next()
-                            .expect("BUG: Size validated iterator did not contain the expected number of values"); 
+                        // To avoid unnecessary copy `std::mem::take` is used.
+                        // Using explicit indexing operation is safe because `row_columns` is an array and `col_ix` is a litteral.
+                        // <https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/builtin/static.UNCONDITIONAL_PANIC.html>
+                        let col_value = ::std::mem::take(&mut row_columns[#col_ix]);
                         <#field_type as FromCqlVal<::std::option::Option<CqlValue>>>::from_cql(col_value)
                             .map_err(|e| FromRowError::BadCqlVal {
                                 err: e,
-                                column: col_ix,
+                                column: #col_ix,
                             })?
                     },
                 }
@@ -39,19 +40,19 @@ pub(crate) fn from_row_derive(tokens_input: TokenStream) -> Result<TokenStream,
             (fill_struct_code, fields.named.len())
         }
         crate::parser::StructFields::Unnamed(fields) => {
-            let set_fields_code = fields.unnamed.iter().map(|field| {
+            let set_fields_code = fields.unnamed.iter().enumerate().map(|(col_ix, field)| {
                 let field_type = &field.ty;
 
                 quote_spanned! {field.span() =>
                     {
-                        let (col_ix, col_value) = vals_iter
-                            .next()
-                            .expect("BUG: Size validated iterator did not contain the expected number of values"); 
-
+                        // To avoid unnecessary copy `std::mem::take` is used.
+                        // Using explicit indexing operation is safe because `row_columns` is an array and `col_ix` is a litteral.
+                        // <https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/builtin/static.UNCONDITIONAL_PANIC.html>
+                        let col_value = ::std::mem::take(&mut row_columns[#col_ix]);
                         <#field_type as FromCqlVal<::std::option::Option<CqlValue>>>::from_cql(col_value)
                             .map_err(|e| FromRowError::BadCqlVal {
                                 err: e,
-                                column: col_ix,
+                                column: #col_ix,
                             })?
                     },
                 }
@@ -71,15 +72,17 @@ pub(crate) fn from_row_derive(tokens_input: TokenStream) -> Result<TokenStream,
             -> ::std::result::Result<Self, #path::FromRowError> {
                 use #path::{CqlValue, FromCqlVal, FromRow, FromRowError};
                 use ::std::result::Result::{Ok, Err};
+                use ::std::convert::TryInto;
                 use ::std::iter::{Iterator, IntoIterator};
 
-                if #fields_count != row.columns.len() {
-                    return Err(FromRowError::WrongRowSize {
-                        expected: #fields_count,
-                        actual: row.columns.len(),
-                    });
-                }
-                let mut vals_iter = row.columns.into_iter().enumerate();
+
+                let row_columns_len = row.columns.len();
+                // An array used, to enable [uncoditional paniking](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/builtin/static.UNCONDITIONAL_PANIC.html)
+                // for "index out of range" issues and be able to catch them during the compile time.
+                let mut row_columns: [_; #fields_count] = row.columns.try_into().map_err(|_| FromRowError::WrongRowSize {
+                    expected: #fields_count,
+                        actual: row_columns_len,
+                })?;
 
                 Ok(#struct_name #fill_struct_code)
             }