rustc_borrowck: suggest_ampmut(): Just rename some variables

By making the variable names more descriptive it becomes easier to
understand the code. Especially with the more complicated code in the
next commit.

Author: Enselic

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -1474,16 +1474,16 @@ fn suggest_ampmut<'tcx>(
     // let x: &i32 = &'a 5;
     //                ^^ lifetime annotation not allowed
     //
-    if let Some(assignment_rhs_span) = opt_assignment_rhs_span
-        && let Ok(src) = tcx.sess.source_map().span_to_snippet(assignment_rhs_span)
-        && let Some(stripped) = src.strip_prefix('&')
+    if let Some(rhs_span) = opt_assignment_rhs_span
+        && let Ok(rhs_str) = tcx.sess.source_map().span_to_snippet(rhs_span)
+        && let Some(rhs_str_no_amp) = rhs_str.strip_prefix('&')
     {
-        let is_raw_ref = stripped.trim_start().starts_with("raw ");
+        let is_raw_ref = rhs_str_no_amp.trim_start().starts_with("raw ");
         // We don't support raw refs yet
         if is_raw_ref {
             return None;
         }
-        let is_mut = if let Some(rest) = stripped.trim_start().strip_prefix("mut") {
+        let is_mut = if let Some(rest) = rhs_str_no_amp.trim_start().strip_prefix("mut") {
             match rest.chars().next() {
                 // e.g. `&mut x`
                 Some(c) if c.is_whitespace() => true,
@@ -1500,7 +1500,7 @@ fn suggest_ampmut<'tcx>(
         // if the reference is already mutable then there is nothing we can do
         // here.
         if !is_mut {
-            let span = assignment_rhs_span;
+            let span = rhs_span;
             // shrink the span to just after the `&` in `&variable`
             let span = span.with_lo(span.lo() + BytePos(1)).shrink_to_lo();