review comments

src/cargo/core/compiler/mod.rs

@@ -40,6 +40,7 @@ pub use self::layout::is_bad_artifact_name;
 use self::output_depinfo::output_depinfo;
 use self::unit_dependencies::UnitDep;
 pub use crate::core::compiler::unit::{Unit, UnitInterner};
+use crate::core::features::nightly_features_allowed;
 use crate::core::manifest::TargetSourcePath;
 use crate::core::profiles::{Lto, PanicStrategy, Profile};
 use crate::core::Feature;
@@ -714,6 +715,14 @@ fn add_error_format_and_color(
     } else {
         let mut color = true;
         match cx.bcx.build_config.message_format {
+            MessageFormat::Human 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));
+                }
+            }
             MessageFormat::Human => (),
             MessageFormat::Json {
                 ansi,

src/cargo/core/features.rs

@@ -337,6 +337,7 @@ pub struct CliUnstable {
     pub build_std: Option<Vec<String>>,
     pub timings: Option<Vec<String>>,
     pub doctest_xcompile: bool,
+    pub terminal_width: Option<usize>,
 }
 
 impl CliUnstable {
@@ -376,6 +377,16 @@ impl CliUnstable {
             }
         }
 
+        fn parse_usize_opt(value: Option<&str>) -> CargoResult<Option<usize>> {
+            Ok(match value {
+                Some(value) => match value.parse::<usize>() {
+                    Ok(value) => Some(value),
+                    Err(e) => failure::bail!("expected a number, found: {}", e),
+                },
+                None => None,
+            })
+        }
+
         match k {
             "print-im-a-teapot" => self.print_im_a_teapot = parse_bool(v)?,
             "unstable-options" => self.unstable_options = true,
@@ -395,6 +406,7 @@ impl CliUnstable {
             }
             "timings" => self.timings = Some(parse_timings(v)),
             "doctest-xcompile" => self.doctest_xcompile = true,
+            "terminal-width" => self.terminal_width = parse_usize_opt(v)?,
             _ => failure::bail!("unknown `-Z` flag specified: {}", k),
         }
 

src/cargo/core/shell.rs

@@ -136,6 +136,14 @@ impl Shell {
         }
     }
 
+    /// Returns the width of the terminal in spaces, if any. Always `None` in Windows.
+    pub fn accurate_err_width(&self) -> Option<usize> {
+        match self.err {
+            ShellOut::Stream { tty: true, .. } => imp::accurate_stderr_width(),
+            _ => None,
+        }
+    }
+
     /// Returns `true` if stderr is a tty.
     pub fn is_err_tty(&self) -> bool {
         match self.err {
@@ -383,6 +391,10 @@ mod imp {
 
     use super::Shell;
 
+    pub fn accurate_stderr_width() -> Option<usize> {
+        stderr_width()
+    }
+
     pub fn stderr_width() -> Option<usize> {
         unsafe {
             let mut winsize: libc::winsize = mem::zeroed();
@@ -414,6 +426,10 @@ mod imp {
 mod imp {
     pub(super) use super::default_err_erase_line as err_erase_line;
 
+    pub fn accurate_stderr_width() -> Option<usize> {
+        None
+    }
+
     pub fn stderr_width() -> Option<usize> {
         None
     }
@@ -431,6 +447,10 @@ mod imp {
 
     pub(super) use super::default_err_erase_line as err_erase_line;
 
+    pub fn accurate_stderr_width() -> Option<usize> {
+        None
+    }
+
     pub fn stderr_width() -> Option<usize> {
         unsafe {
             let stdout = GetStdHandle(STD_ERROR_HANDLE);

src/cargo/ops/cargo_compile.rs

@@ -426,12 +426,6 @@ pub fn compile_ws<'a>(
             }
         }
     }
-    if let Some(width) = config.shell().err_width() {
-        for unit in &units {
-            bcx.extra_compiler_args
-                .insert(*unit, vec![format!("-Zterminal-width={}", width)]);
-        }
-    }
 
     let unit_dependencies = build_unit_dependencies(
         &bcx,

tests/testsuite/build.rs

@@ -2,8 +2,8 @@ use cargo::util::paths::dylib_path_envvar;
 use cargo_test_support::paths::{root, CargoPathExt};
 use cargo_test_support::registry::Package;
 use cargo_test_support::{
-    basic_bin_manifest, basic_lib_manifest, basic_manifest, main_file, project, rustc_host,
-    sleep_ms, symlink_supported, t, Execs, ProjectBuilder,
+    basic_bin_manifest, basic_lib_manifest, basic_manifest, is_nightly, main_file, project,
+    rustc_host, sleep_ms, symlink_supported, t, Execs, ProjectBuilder,
 };
 use std::env;
 use std::fs::{self, File};
@@ -4723,3 +4723,34 @@ d
         )
         .run();
 }
+
+#[cargo_test]
+fn simple_terminal_width() {
+    if !is_nightly() {
+        // --terminal-width is unstable
+        return;
+    }
+    let p = project()
+        .file(
+            "src/lib.rs",
+            "
+                fn a() {
+/**/                    /**/ \
+/**/                    /**/ \
+/**/                    /**/ \
+/**/                    /**/ \
+/**/                    /**/ \
+/**/                    /**/ \
+/**/                    /**/ \
+                    let _: () = 42;
+                }
+            ",
+        )
+        .build();
+
+    p.cargo("build -Zterminal-width=60")
+        .masquerade_as_nightly_cargo()
+        .with_status(101)
+        .with_stderr_contains("3 | ...t _: () = 42;")
+        .run();
+}