Make -Z terminal-width opt-in.

This commit modifies the parsing of `-Z terminal-width` so that it can
optionally take a value and only uses `accurate_err_width` when the user
does not provide a value - therefore making the emission of `-Z
terminal-width` opt-in.

Signed-off-by: David Wood <[email protected]>

Author: davidtwco

src/cargo/core/compiler/mod.rs

@@ -720,11 +720,18 @@ fn add_error_format_and_color(
     cmd.arg(json);
 
     if nightly_features_allowed() {
-        if let (Some(width), _) | (_, Some(width)) = (
-            cx.bcx.config.cli_unstable().terminal_width,
-            cx.bcx.config.shell().accurate_err_width(),
-        ) {
-            cmd.arg(format!("-Zterminal-width={}", width));
+        let config = cx.bcx.config;
+        match (config.cli_unstable().terminal_width, config.shell().accurate_err_width()) {
+            // Terminal width explicitly provided - only useful for testing.
+            (Some(Some(width)), _) => {
+                cmd.arg(format!("-Zterminal-width={}", width));
+            }
+            // Terminal width was not explicitly provided but flag was provided - common case.
+            (Some(None), Some(width)) => {
+                cmd.arg(format!("-Zterminal-width={}", width));
+            }
+            // User didn't opt-in.
+            _ => (),
         }
     }
 

src/cargo/core/features.rs

@@ -357,7 +357,7 @@ pub struct CliUnstable {
     pub separate_nightlies: bool,
     pub multitarget: bool,
     pub rustdoc_map: bool,
-    pub terminal_width: Option<usize>,
+    pub terminal_width: Option<Option<usize>>,
 }
 
 impl CliUnstable {
@@ -448,7 +448,7 @@ impl CliUnstable {
             "separate-nightlies" => self.separate_nightlies = parse_empty(k, v)?,
             "multitarget" => self.multitarget = parse_empty(k, v)?,
             "rustdoc-map" => self.rustdoc_map = parse_empty(k, v)?,
-            "terminal-width" => self.terminal_width = parse_usize_opt(v)?,
+            "terminal-width" => self.terminal_width = Some(parse_usize_opt(v)?),
             _ => bail!("unknown `-Z` flag specified: {}", k),
         }