Deduplicate needed parentheses suggestion code

Author: estebank

src/librustc_typeck/check/mod.rs

@@ -4177,17 +4177,11 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                                 if let Some(sp) = tcx.sess.parse_sess.abiguous_block_expr_parse
                                     .borrow().get(&sp)
                                 {
-                                    if let Ok(snippet) = tcx.sess.source_map()
-                                        .span_to_snippet(*sp)
-                                    {
-                                        err.span_suggestion(
-                                            *sp,
-                                            "parentheses are required to parse this \
-                                             as an expression",
-                                            format!("({})", snippet),
-                                            Applicability::MachineApplicable,
-                                        );
-                                    }
+                                    tcx.sess.parse_sess.expr_parentheses_needed(
+                                        &mut err,
+                                        *sp,
+                                        None,
+                                    );
                                 }
                                 err.emit();
                                 oprnd_t = tcx.types.err;

src/libsyntax/parse/mod.rs

@@ -11,7 +11,7 @@ use crate::tokenstream::{TokenStream, TokenTree};
 use crate::diagnostics::plugin::ErrorMap;
 use crate::print::pprust::token_to_string;
 
-use errors::{FatalError, Level, Handler, ColorConfig, Diagnostic, DiagnosticBuilder};
+use errors::{Applicability, FatalError, Level, Handler, ColorConfig, Diagnostic, DiagnosticBuilder};
 use rustc_data_structures::sync::{Lrc, Lock};
 use syntax_pos::{Span, SourceFile, FileName, MultiSpan};
 use log::debug;
@@ -47,6 +47,9 @@ pub struct ParseSess {
     included_mod_stack: Lock<Vec<PathBuf>>,
     source_map: Lrc<SourceMap>,
     pub buffered_lints: Lock<Vec<BufferedEarlyLint>>,
+    /// Contains the spans of block expressions that could have been incomplete based on the
+    /// operation token that followed it, but that the parser cannot identify without further
+    /// analysis.
     pub abiguous_block_expr_parse: Lock<FxHashMap<Span, Span>>,
 }
 
@@ -95,6 +98,24 @@ impl ParseSess {
             });
         });
     }
+
+    /// Extend an error with a suggestion to wrap an expression with parentheses to allow the
+    /// parser to continue parsing the following operation as part of the same expression.
+    pub fn expr_parentheses_needed(
+        &self,
+        err: &mut DiagnosticBuilder<'_>,
+        span: Span,
+        alt_snippet: Option<String>,
+    ) {
+        if let Some(snippet) = self.source_map().span_to_snippet(span).ok().or(alt_snippet) {
+            err.span_suggestion(
+                span,
+                "parentheses are required to parse this as an expression",
+                format!("({})", snippet),
+                Applicability::MachineApplicable,
+            );
+        }
+    }
 }
 
 #[derive(Clone)]

src/libsyntax/parse/parser.rs

@@ -2931,14 +2931,7 @@ impl<'a> Parser<'a> {
                             if let Some(sp) = self.sess.abiguous_block_expr_parse.borrow()
                                 .get(&sp)
                             {
-                                if let Ok(snippet) = self.sess.source_map().span_to_snippet(*sp) {
-                                    err.span_suggestion(
-                                        *sp,
-                                        "parentheses are required to parse this as an expression",
-                                        format!("({})", snippet),
-                                        Applicability::MachineApplicable,
-                                    );
-                                }
+                                self.sess.expr_parentheses_needed(&mut err, *sp, None);
                             }
                             err.span_label(self.span, "expected expression");
                             return Err(err);
@@ -3657,14 +3650,11 @@ impl<'a> Parser<'a> {
                     pprust::token_to_string(&self.token),
                 ));
                 err.span_label(self.span, "expected expression");
-                let snippet = self.sess.source_map().span_to_snippet(lhs.span)
-                    .unwrap_or_else(|_| pprust::expr_to_string(&lhs));
-                err.span_suggestion(
+                self.sess.expr_parentheses_needed(
+                    &mut err,
                     lhs.span,
-                    "parentheses are required to parse this as an expression",
-                    format!("({})", snippet),
-                    Applicability::MachineApplicable,
-                );
+                    Some(pprust::expr_to_string(&lhs),
+                ));
                 err.emit();
             }
         }
@@ -4979,14 +4969,7 @@ impl<'a> Parser<'a> {
                         err.span_label(self.span, format!("expected {}", expected));
                         let sp = self.sess.source_map().start_point(self.span);
                         if let Some(sp) = self.sess.abiguous_block_expr_parse.borrow().get(&sp) {
-                            if let Ok(snippet) = self.sess.source_map().span_to_snippet(*sp) {
-                                err.span_suggestion(
-                                    *sp,
-                                    "parentheses are required to parse this as an expression",
-                                    format!("({})", snippet),
-                                    Applicability::MachineApplicable,
-                                );
-                            }
+                            self.sess.expr_parentheses_needed(&mut err, *sp, None);
                         }
                         return Err(err);
                     }