Add a --edition-idioms flag to cargo fix

src/bin/cargo/commands/fix.rs

@@ -51,6 +51,11 @@ pub fn cli() -> App {
                 .conflicts_with("edition")
                 .hidden(true),
         )
+        .arg(
+            Arg::with_name("idioms")
+                .long("edition-idioms")
+                .help("Fix warnings to migrate to the idioms of an edition")
+        )
         .arg(
             Arg::with_name("allow-no-vcs")
                 .long("allow-no-vcs")
@@ -126,6 +131,7 @@ pub fn exec(config: &mut Config, args: &ArgMatches) -> CliResult {
     ops::fix(&ws, &mut ops::FixOptions {
         edition: args.is_present("edition"),
         prepare_for: args.value_of("prepare-for"),
+        idioms: args.is_present("idioms"),
         compile_opts: opts,
         allow_dirty: args.is_present("allow-dirty"),
         allow_no_vcs: args.is_present("allow-no-vcs"),

src/cargo/ops/fix.rs

@@ -24,9 +24,12 @@ const BROKEN_CODE_ENV: &str = "__CARGO_FIX_BROKEN_CODE";
 const PREPARE_FOR_ENV: &str = "__CARGO_FIX_PREPARE_FOR";
 const EDITION_ENV: &str = "__CARGO_FIX_EDITION";
 
+const IDIOMS_ENV: &str = "__CARGO_FIX_IDIOMS";
+
 pub struct FixOptions<'a> {
     pub edition: bool,
     pub prepare_for: Option<&'a str>,
+    pub idioms: bool,
     pub compile_opts: CompileOptions<'a>,
     pub allow_dirty: bool,
     pub allow_no_vcs: bool,
@@ -59,6 +62,12 @@ pub fn fix(ws: &Workspace, opts: &mut FixOptions) -> CargoResult<()> {
             edition.to_string(),
         ));
     }
+    if opts.idioms {
+        opts.compile_opts.build_config.extra_rustc_env.push((
+            IDIOMS_ENV.to_string(),
+            "1".to_string(),
+        ));
+    }
     opts.compile_opts.build_config.cargo_as_rustc_wrapper = true;
     *opts.compile_opts.build_config.rustfix_diagnostic_server.borrow_mut() =
         Some(RustfixDiagnosticServer::new()?);
@@ -471,6 +480,7 @@ fn log_failed_fix(stderr: &[u8]) -> Result<(), Error> {
 struct FixArgs {
     file: Option<PathBuf>,
     prepare_for_edition: PrepareFor,
+    idioms: bool,
     enabled_edition: Option<String>,
     other: Vec<OsString>,
 }
@@ -512,6 +522,7 @@ impl FixArgs {
         } else if env::var(EDITION_ENV).is_ok() {
             ret.prepare_for_edition = PrepareFor::Next;
         }
+        ret.idioms = env::var(IDIOMS_ENV).is_ok();
         return ret
     }
 
@@ -523,6 +534,12 @@ impl FixArgs {
             .arg("--cap-lints=warn");
         if let Some(edition) = &self.enabled_edition {
             cmd.arg("--edition").arg(edition);
+            if self.idioms {
+                match &edition[..] {
+                    "2018" => { cmd.arg("-Wrust-2018-idioms"); }
+                    _ => {}
+                }
+            }
         }
         match &self.prepare_for_edition {
             PrepareFor::Edition(edition) => {
@@ -569,6 +586,12 @@ impl FixArgs {
         process::exit(1);
     }
 
+    /// If we're preparing for an edition and we *don't* find the
+    /// `rust_2018_preview` feature, for example, in the entry point file then
+    /// it probably means that the edition isn't actually enabled, so we can't
+    /// actually fix anything.
+    ///
+    /// If this is the case, issue a warning.
     fn warn_if_preparing_probably_inert(&self) -> CargoResult<()> {
         let edition = match &self.prepare_for_edition {
             PrepareFor::Edition(s) => s,

src/cargo/util/diagnostic_server.rs

@@ -48,6 +48,11 @@ pub enum Message {
         file: String,
         edition: String,
     },
+    IdiomEditionMismatch {
+        file: String,
+        idioms: String,
+        edition: Option<String>,
+    },
 }
 
 impl Message {
@@ -78,6 +83,7 @@ pub struct DiagnosticPrinter<'a> {
     config: &'a Config,
     preview_not_found: HashSet<String>,
     edition_already_enabled: HashSet<String>,
+    idiom_mismatch: HashSet<String>,
 }
 
 impl<'a> DiagnosticPrinter<'a> {
@@ -86,6 +92,7 @@ impl<'a> DiagnosticPrinter<'a> {
             config,
             preview_not_found: HashSet::new(),
             edition_already_enabled: HashSet::new(),
+            idiom_mismatch: HashSet::new(),
         }
     }
 
@@ -172,8 +179,32 @@ information about transitioning to the {0} edition see:
                 self.config.shell().error(&msg)?;
                 Ok(())
             }
-        }
+            Message::IdiomEditionMismatch { file, idioms, edition } => {
+                // Same as above
+                if !self.idiom_mismatch.insert(file.clone()) {
+                    return Ok(())
+                }
+                self.config.shell().error(&format!(
+                    "\
+cannot migrate to the idioms of the {} edition for `{}`
+because it is compiled {}, which doesn't match {0}
+
+consider migrating to the {0} edition by adding `edition = '{0}'` to
+`Cargo.toml` and then rerunning this command; a more detailed transition
+guide can be found at
 
+  https://rust-lang-nursery.github.io/edition-guide/editions/transitioning.html
+",
+                    idioms,
+                    file,
+                    match edition {
+                        Some(s) => format!("with the {} edition", s),
+                        None => format!("without an edition"),
+                    },
+                ))?;
+                Ok(())
+            }
+        }
     }
 }
 

tests/testsuite/fix.rs

@@ -966,6 +966,62 @@ fn fix_overlapping() {
     assert!(contents.contains("crate::foo::<crate::A>()"));
 }
 
+#[test]
+fn fix_idioms() {
+    if !is_nightly() {
+        return
+    }
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                cargo-features = ['edition']
+                [package]
+                name = 'foo'
+                version = '0.1.0'
+                edition = '2018'
+            "#,
+        )
+        .file(
+            "src/lib.rs",
+            r#"
+                use std::any::Any;
+                pub fn foo() {
+                    let _x: Box<Any> = Box::new(3);
+                }
+            "#
+        )
+        .build();
+
+    let stderr = "\
+[CHECKING] foo [..]
+[FIXING] src/lib.rs (1 fix)
+[FINISHED] [..]
+";
+    assert_that(
+        p.cargo("fix --edition-idioms --allow-no-vcs")
+            .masquerade_as_nightly_cargo(),
+        execs()
+            .with_stderr(stderr)
+            .with_status(0),
+    );
+
+    assert!(p.read_file("src/lib.rs").contains("Box<dyn Any>"));
+}
+
+#[test]
+fn idioms_2015_ok() {
+    let p = project()
+        .file("src/lib.rs", "")
+        .build();
+
+    assert_that(
+        p.cargo("fix --edition-idioms --allow-no-vcs")
+            .masquerade_as_nightly_cargo(),
+        execs().with_status(0),
+    );
+}
+
 #[test]
 fn both_edition_migrate_flags() {
     if !is_nightly() {