Auto merge of #26087 - fitzgen:improve-suggestion-hueristics, r=Aatch

This makes the maximum edit distance of typo suggestions a function of the typo'd name's length. FWIW, clang uses this same hueristic, and I've found their suggestions to be better than rustc's. Without something like this, you end up with suggestions that aren't related at all when there are short variable names.

See also https://github.com/rust-lang/rust/issues/20028#issuecomment-109767159

src/librustc_resolve/lib.rs

@@ -3206,14 +3206,11 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         NoSuggestion
     }
 
-    fn find_best_match_for_name(&mut self, name: &str, max_distance: usize)
-                                -> Option<String> {
-        let this = &mut *self;
-
+    fn find_best_match_for_name(&mut self, name: &str) -> Option<String> {
         let mut maybes: Vec<token::InternedString> = Vec::new();
         let mut values: Vec<usize> = Vec::new();
 
-        for rib in this.value_ribs.iter().rev() {
+        for rib in self.value_ribs.iter().rev() {
             for (&k, _) in &rib.bindings {
                 maybes.push(token::get_name(k));
                 values.push(usize::MAX);
@@ -3229,9 +3226,12 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             }
         }
 
+        // As a loose rule to avoid obviously incorrect suggestions, clamp the
+        // maximum edit distance we will accept for a suggestion to one third of
+        // the typo'd name's length.
+        let max_distance = std::cmp::max(name.len(), 3) / 3;
+
         if !values.is_empty() &&
-            values[smallest] != usize::MAX &&
-            values[smallest] < name.len() + 2 &&
             values[smallest] <= max_distance &&
             name != &maybes[smallest][..] {
 
@@ -3357,7 +3357,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                                     NoSuggestion => {
                                         // limit search to 5 to reduce the number
                                         // of stupid suggestions
-                                        self.find_best_match_for_name(&path_name, 5)
+                                        self.find_best_match_for_name(&path_name)
                                                             .map_or("".to_string(),
                                                                     |x| format!("`{}`", x))
                                     }

src/test/compile-fail/bad-expr-path.rs

@@ -8,8 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern: unresolved name `m1::a`. Did you mean `args`?
+// error-pattern: unresolved name `m1::arguments`. Did you mean `arguments`?
 
 mod m1 {}
 
-fn main(args: Vec<String>) { log(debug, m1::a); }
+fn main(arguments: Vec<String>) { log(debug, m1::arguments); }

src/test/compile-fail/bad-expr-path2.rs

@@ -8,12 +8,12 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern: unresolved name `m1::a`. Did you mean `args`?
+// error-pattern: unresolved name `m1::arguments`. Did you mean `arguments`?
 
 mod m1 {
-    pub mod a {}
+    pub mod arguments {}
 }
 
-fn main(args: Vec<String>) {
-    log(debug, m1::a);
+fn main(arguments: Vec<String>) {
+    log(debug, m1::arguments);
 }

src/test/compile-fail/typo-suggestion.rs

@@ -0,0 +1,21 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    let foo = 1;
+
+    // `foo` shouldn't be suggested, it is too dissimilar from `bar`.
+    println!("Hello {}", bar);
+    //~^ ERROR: unresolved name `bar`
+
+    // But this is close enough.
+    println!("Hello {}", fob);
+    //~^ ERROR: unresolved name `fob`. Did you mean `foo`?
+}