Convert $IFS to a read-specific thing

This removes $IFS globally (which prevents fish internals from breaking when IFS
is manipulated) but keeps it as a legacy/deprecated mode of specifying `read`'s
`--delimiter` value for compatibility with (ba)sh and legacy fish scripts.

src/builtins/read.rs

@@ -9,6 +9,7 @@ use crate::common::unescape_string;
 use crate::common::valid_var_name;
 use crate::common::UnescapeStringStyle;
 use crate::env::EnvMode;
+use crate::env::Environment;
 use crate::env::READ_BYTE_LIMIT;
 use crate::env::{EnvVar, EnvVarFlags};
 use crate::input_common::terminal_protocols_disable_ifn;
@@ -688,11 +689,18 @@ pub fn read(parser: &Parser, streams: &mut IoStreams, argv: &mut [&wstr]) -> Opt
             continue;
         }
 
-        // todo!("don't clone")
+        // Order of precedence: --delimiter, an explicitly set $IFS, the default IFS constant above.
+        // An empty --delimiter or $IFS is a special mode that splits by character (i.e. does not
+        // fall back to the next candidate).
+        let mut ifs_var = None;
         let delimiter = opts
             .delimiter
             .as_ref()
-            .map(|delim| delim.as_utfstr())
+            .or_else(|| {
+                ifs_var = Some(parser.vars().get(L!("IFS")).map(|ifs| ifs.as_string()));
+                ifs_var.as_ref().unwrap().as_ref()
+            })
+            .map(|s| s.as_utfstr())
             .unwrap_or(IFS);
 
         if delimiter.is_empty() {

tests/checks/read.fish

@@ -43,6 +43,31 @@ echo -n a | read -l one
 echo "$status $one"
 #CHECK: 0 a
 
+# Test splitting input with IFS empty
+set -l IFS
+echo hello | read -l one
+print_vars one
+#CHECK: 1 'hello'
+echo hello | read -l one two
+print_vars one two
+#CHECK: 1 'h' 1 'ello'
+echo hello | read -l one two three
+print_vars one two three
+#CHECK: 1 'h' 1 'e' 1 'llo'
+echo '' | read -l one
+print_vars one
+#CHECK: 0
+echo t | read -l one two
+print_vars one two
+#CHECK: 1 't' 0
+echo t | read -l one two three
+print_vars one two three
+#CHECK: 1 't' 0 0
+echo ' t' | read -l one two
+print_vars one two
+#CHECK: 1 ' ' 1 't'
+set -le IFS
+
 echo 'hello there' | read -la ary
 print_vars ary
 #CHECK: 2 'hello' 'there'
@@ -59,6 +84,18 @@ echo '' | read -la ary
 print_vars ary
 #CHECK: 0
 
+set -l IFS
+echo hello | read -la ary
+print_vars ary
+#CHECK: 5 'h' 'e' 'l' 'l' 'o'
+echo h | read -la ary
+print_vars ary
+#CHECK: 1 'h'
+echo '' | read -la ary
+print_vars ary
+#CHECK: 0
+set -le IFS
+
 # read -n tests
 echo testing | read -n 3 foo
 echo $foo
@@ -190,7 +227,7 @@ echo abc\ndef | $fish -i -c 'read a; read b; set --show a; set --show b' | $filt
 #CHECK: $b: set in global scope, unexported, with 1 elements
 #CHECK: $b[1]: |def|
 
-# Test --delimiter
+# Test --delimiter (and $IFS, for now)
 echo a=b | read -l foo bar
 echo $foo
 echo $bar
@@ -212,7 +249,18 @@ echo $bar
 echo $baz
 #CHECK: b
 
-# Default behavior
+# IFS empty string
+set -l IFS ''
+echo a=b | read -l foo bar baz
+echo $foo
+#CHECK: a
+echo $bar
+#CHECK: =
+echo $baz
+#CHECK: b
+
+# IFS unset
+set -e IFS
 echo a=b | read -l foo bar baz
 echo $foo
 #CHECK: a=b
@@ -238,6 +286,17 @@ echo $b
 #CHECK: b
 echo $c
 #CHECK: c
+# Multi-char delimiters with IFS
+begin
+    set -l IFS "..."
+    echo a...b...c | read -l a b c
+    echo $a
+    echo $b
+    echo $c
+end
+#CHECK: a
+#CHECK: b
+#CHECK: ..c
 
 # At one point, whatever was read was printed _before_ banana
 echo banana (echo sausage | read)