Do not treat \: or \= as file completion anchor

Partially reapplies f7dac82ed (Escape separators (colon and equals) to
improve completion, 2019-08-23) which has been reverted.

src/complete.rs

@@ -9,7 +9,11 @@ use std::{
     time::{Duration, Instant},
 };
 
-use crate::{common::charptr2wcstring, util::wcsfilecmp};
+use crate::{
+    common::charptr2wcstring,
+    reader::{get_quote, is_backslashed},
+    util::wcsfilecmp,
+};
 use bitflags::bitflags;
 use once_cell::sync::Lazy;
 use printf_compat::sprintf;
@@ -1522,6 +1526,7 @@ impl<'ctx> Completer<'ctx> {
             flags -= ExpandFlags::GEN_DESCRIPTIONS;
         }
 
+        // Expand words separated by '=' separately, unless '=' is escaped or quoted.
         // We have the following cases:
         //
         // --foo=bar => expand just bar
@@ -1529,13 +1534,25 @@ impl<'ctx> Completer<'ctx> {
         // foo=bar => expand the whole thing, and also just bar
         //
         // We also support colon separator (#2178). If there's more than one, prefer the last one.
-        let sep_index = s.chars().rposition(|c| c == '=' || c == ':');
+        let sep_index = if get_quote(s, s.len()).is_some() {
+            None
+        } else {
+            let mut end = s.len();
+            loop {
+                match s[..end].chars().rposition(|c| c == '=' || c == ':') {
+                    Some(pos) => {
+                        if !is_backslashed(s, pos) {
+                            break Some(pos);
+                        }
+                        end = pos;
+                    }
+                    None => break None,
+                }
+            }
+        };
         let complete_from_start = sep_index.is_none() || !string_prefixes_string(L!("-"), s);
 
         if let Some(sep_index) = sep_index {
-            // FIXME: This just cuts the token,
-            // so any quoting or braces gets lost.
-            // See #4954.
             let sep_string = s.slice_from(sep_index + 1);
             let mut local_completions = Vec::new();
             if expand_string(

src/reader.rs

@@ -5043,7 +5043,7 @@ fn try_expand_wildcard(
 
 /// Test if the specified character in the specified string is backslashed. pos may be at the end of
 /// the string, which indicates if there is a trailing backslash.
-fn is_backslashed(s: &wstr, pos: usize) -> bool {
+pub(crate) fn is_backslashed(s: &wstr, pos: usize) -> bool {
     // note pos == str.size() is OK.
     if pos > s.len() {
         return false;
@@ -5093,7 +5093,7 @@ fn replace_line_at_cursor(
     text[..start].to_owned() + replacement + &text[end..]
 }
 
-fn get_quote(cmd_str: &wstr, len: usize) -> Option<char> {
+pub(crate) fn get_quote(cmd_str: &wstr, len: usize) -> Option<char> {
     let cmd = cmd_str.as_char_slice();
     let mut i = 0;
     while i < cmd.len() {