Implement black hole register (helix-editor#1165)

book/src/usage.md

@@ -23,8 +23,10 @@ If there is a selected register before invoking a change or delete command, the
 | `/`                | Last search           |
 | `:`                | Last executed command |
 | `"`                | Last yanked text      |
+| `_`                | Black hole            |
 
 > There is no special register for copying to system clipboard, instead special commands and keybindings are provided. See the [keymap](keymap.md#space-mode) for the specifics.
+> The black hole register works as a no-op register, meaning no data will be written to / read from it.
 
 ## Surround
 

helix-core/src/register.rs

@@ -15,7 +15,11 @@ impl Register {
     }
 
     pub fn new_with_values(name: char, values: Vec<String>) -> Self {
-        Self { name, values }
+        if name == '_' {
+            Self::new(name)
+        } else {
+            Self { name, values }
+        }
     }
 
     pub const fn name(&self) -> char {
@@ -27,11 +31,15 @@ impl Register {
     }
 
     pub fn write(&mut self, values: Vec<String>) {
-        self.values = values;
+        if self.name != '_' {
+            self.values = values;
+        }
     }
 
     pub fn push(&mut self, value: String) {
-        self.values.push(value);
+        if self.name != '_' {
+            self.values.push(value);
+        }
     }
 }