Simplify Emitter::fix_multispan_in_std_macros

1. Rewrite `if let` into `match` to return earl and avoid indenting giant block 
2. Assign `spans_updated` only once

Author: AnthonyMikh

src/librustc_errors/emitter.rs

@@ -295,81 +295,82 @@ pub trait Emitter {
                                    source_map: &Option<Lrc<SourceMapperDyn>>,
                                    span: &mut MultiSpan,
                                    always_backtrace: bool) -> bool {
-        let mut spans_updated = false;
+        let sm = match source_map {
+            Some(ref sm) => sm,
+            None => return false,
+        };
 
-        if let Some(ref sm) = source_map {
-            let mut before_after: Vec<(Span, Span)> = vec![];
-            let mut new_labels: Vec<(Span, String)> = vec![];
+        let mut before_after: Vec<(Span, Span)> = vec![];
+        let mut new_labels: Vec<(Span, String)> = vec![];
 
-            // First, find all the spans in <*macros> and point instead at their use site
-            for sp in span.primary_spans() {
-                if sp.is_dummy() {
+        // First, find all the spans in <*macros> and point instead at their use site
+        for sp in span.primary_spans() {
+            if sp.is_dummy() {
+                continue;
+            }
+            let call_sp = sm.call_span_if_macro(*sp);
+            if call_sp != *sp && !always_backtrace {
+                before_after.push((*sp, call_sp));
+            }
+            let backtrace_len = sp.macro_backtrace().len();
+            for (i, trace) in sp.macro_backtrace().iter().rev().enumerate() {
+                // Only show macro locations that are local
+                // and display them like a span_note
+                if trace.def_site_span.is_dummy() {
                     continue;
                 }
-                let call_sp = sm.call_span_if_macro(*sp);
-                if call_sp != *sp && !always_backtrace {
-                    before_after.push((*sp, call_sp));
+                if always_backtrace {
+                    new_labels.push((trace.def_site_span,
+                                        format!("in this expansion of `{}`{}",
+                                                trace.macro_decl_name,
+                                                if backtrace_len > 2 {
+                                                    // if backtrace_len == 1 it'll be pointed
+                                                    // at by "in this macro invocation"
+                                                    format!(" (#{})", i + 1)
+                                                } else {
+                                                    String::new()
+                                                })));
                 }
-                let backtrace_len = sp.macro_backtrace().len();
-                for (i, trace) in sp.macro_backtrace().iter().rev().enumerate() {
-                    // Only show macro locations that are local
-                    // and display them like a span_note
-                    if trace.def_site_span.is_dummy() {
-                        continue;
-                    }
-                    if always_backtrace {
-                        new_labels.push((trace.def_site_span,
-                                            format!("in this expansion of `{}`{}",
-                                                    trace.macro_decl_name,
-                                                    if backtrace_len > 2 {
-                                                        // if backtrace_len == 1 it'll be pointed
-                                                        // at by "in this macro invocation"
-                                                        format!(" (#{})", i + 1)
-                                                    } else {
-                                                        String::new()
-                                                    })));
-                    }
-                    // Check to make sure we're not in any <*macros>
-                    if !sm.span_to_filename(trace.def_site_span).is_macros() &&
-                        !trace.macro_decl_name.starts_with("desugaring of ") &&
-                        !trace.macro_decl_name.starts_with("#[") ||
-                        always_backtrace {
-                        new_labels.push((trace.call_site,
-                                            format!("in this macro invocation{}",
-                                                    if backtrace_len > 2 && always_backtrace {
-                                                        // only specify order when the macro
-                                                        // backtrace is multiple levels deep
-                                                        format!(" (#{})", i + 1)
-                                                    } else {
-                                                        String::new()
-                                                    })));
-                        if !always_backtrace {
-                            break;
-                        }
+                // Check to make sure we're not in any <*macros>
+                if !sm.span_to_filename(trace.def_site_span).is_macros() &&
+                    !trace.macro_decl_name.starts_with("desugaring of ") &&
+                    !trace.macro_decl_name.starts_with("#[") ||
+                    always_backtrace {
+                    new_labels.push((trace.call_site,
+                                        format!("in this macro invocation{}",
+                                                if backtrace_len > 2 && always_backtrace {
+                                                    // only specify order when the macro
+                                                    // backtrace is multiple levels deep
+                                                    format!(" (#{})", i + 1)
+                                                } else {
+                                                    String::new()
+                                                })));
+                    if !always_backtrace {
+                        break;
                     }
                 }
             }
-            for (label_span, label_text) in new_labels {
-                span.push_span_label(label_span, label_text);
+        }
+        for (label_span, label_text) in new_labels {
+            span.push_span_label(label_span, label_text);
+        }
+        for sp_label in span.span_labels() {
+            if sp_label.span.is_dummy() {
+                continue;
             }
-            for sp_label in span.span_labels() {
-                if sp_label.span.is_dummy() {
-                    continue;
-                }
-                if sm.span_to_filename(sp_label.span.clone()).is_macros() &&
-                    !always_backtrace
-                {
-                    let v = sp_label.span.macro_backtrace();
-                    if let Some(use_site) = v.last() {
-                        before_after.push((sp_label.span.clone(), use_site.call_site.clone()));
-                    }
+            if sm.span_to_filename(sp_label.span.clone()).is_macros() &&
+                !always_backtrace
+            {
+                let v = sp_label.span.macro_backtrace();
+                if let Some(use_site) = v.last() {
+                    before_after.push((sp_label.span.clone(), use_site.call_site.clone()));
                 }
             }
-            // After we have them, make sure we replace these 'bad' def sites with their use sites
-            for (before, after) in before_after {
-                span.replace(before, after);
-                spans_updated = true;
-            }
+        }
+        // After we have them, make sure we replace these 'bad' def sites with their use sites
+        let spans_updated = !before_after.is_empty();
+        for (before, after) in before_after {
+            span.replace(before, after);
         }
 
         spans_updated