Rollup merge of #113780 - dtolnay:printkindpath, r=b-naber

Support `--print KIND=PATH` command line syntax

As is already done for `--emit KIND=PATH` and `-L KIND=PATH`.

In the discussion of #110785, it was pointed out that `--print KIND=PATH` is nicer than trying to apply the single global `-o` path to `--print`'s output, because in general there can be multiple print requests within a single rustc invocation, and anyway `-o` would already be used for a different meaning in the case of `link-args` and `native-static-libs`.

I am interested in using `--print cfg=PATH` in Buck2. Currently Buck2 works around the lack of support for `--print KIND=PATH` by [indirecting through a Python wrapper script](https://github.com/facebook/buck2/blob/d43cf3a51a31f00be2c2248e78271b0fef0452b4/prelude/rust/tools/get_rustc_cfg.py) to redirect rustc's stdout into the location dictated by the build system.

From skimming Cargo's usages of `--print`, it definitely seems like it would benefit from `--print KIND=PATH` too. Currently it is working around the lack of this by inserting `--crate-name=___ --print=crate-name` so that it can look for a line containing `___` as a delimiter between the 2 other `--print` informations it actually cares about. This is commented as a "HACK" and "abuse". https://github.com/rust-lang/cargo/blob/31eda6f7c360d9911f853b3014e057db61238f3e/src/cargo/core/compiler/build_context/target_info.rs#L242 (FYI `@weihanglo` as you dealt with this recently in https://github.com/rust-lang/cargo/pull/11633.)

Mentioning reviewers active in #110785: `@fee1-dead` `@jyn514` `@bjorn3`

3 files changed, 87 insertions, 37 deletions

diff --git a/compiler/rustc_session/messages.ftl b/compiler/rustc_session/messages.ftl
index 4897bd8d5da..ee24c6d902f 100644
--- a/compiler/rustc_session/messages.ftl
+++ b/compiler/rustc_session/messages.ftl
@@ -26,6 +26,8 @@ session_feature_gate_error = {$explain}
 
 session_file_is_not_writeable = output file {$file} is not writeable -- check its permissions
 
+session_file_write_fail = failed to write `{$path}` due to error `{$err}`
+
 session_hexadecimal_float_literal_not_supported = hexadecimal float literal is not supported
 
 session_incompatible_linker_flavor = linker flavor `{$flavor}` is incompatible with the current target
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index 593983f117d..a8147ede970 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -3,6 +3,7 @@
 
 pub use crate::options::*;
 
+use crate::errors::FileWriteFail;
 use crate::search_paths::SearchPath;
 use crate::utils::{CanonicalizedPath, NativeLib, NativeLibKind};
 use crate::{lint, HashStableContext};
@@ -31,6 +32,7 @@ use std::collections::btree_map::{
 use std::collections::{BTreeMap, BTreeSet};
 use std::ffi::OsStr;
 use std::fmt;
+use std::fs;
 use std::hash::Hash;
 use std::iter;
 use std::path::{Path, PathBuf};
@@ -710,8 +712,14 @@ impl ExternEntry {
     }
 }
 
+#[derive(Clone, PartialEq, Debug)]
+pub struct PrintRequest {
+    pub kind: PrintKind,
+    pub out: OutFileName,
+}
+
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
-pub enum PrintRequest {
+pub enum PrintKind {
     FileNames,
     Sysroot,
     TargetLibdir,
@@ -855,6 +863,17 @@ impl OutFileName {
             OutFileName::Stdout => outputs.temp_path(flavor, codegen_unit_name),
         }
     }
+
+    pub fn overwrite(&self, content: &str, sess: &Session) {
+        match self {
+            OutFileName::Stdout => print!("{content}"),
+            OutFileName::Real(path) => {
+                if let Err(e) = fs::write(path, content) {
+                    sess.emit_fatal(FileWriteFail { path, err: e.to_string() });
+                }
+            }
+        }
+    }
 }
 
 #[derive(Clone, Hash, Debug, HashStable_Generic)]
@@ -2005,13 +2024,7 @@ fn parse_output_types(
     if !unstable_opts.parse_only {
         for list in matches.opt_strs("emit") {
             for output_type in list.split(',') {
-                let (shorthand, path) = match output_type.split_once('=') {
-                    None => (output_type, None),
-                    Some((shorthand, "-")) => (shorthand, Some(OutFileName::Stdout)),
-                    Some((shorthand, path)) => {
-                        (shorthand, Some(OutFileName::Real(PathBuf::from(path))))
-                    }
-                };
+                let (shorthand, path) = split_out_file_name(output_type);
                 let output_type = OutputType::from_shorthand(shorthand).unwrap_or_else(|| {
                     handler.early_error(format!(
                         "unknown emission type: `{shorthand}` - expected one of: {display}",
@@ -2028,6 +2041,14 @@ fn parse_output_types(
     OutputTypes(output_types)
 }
 
+fn split_out_file_name(arg: &str) -> (&str, Option<OutFileName>) {
+    match arg.split_once('=') {
+        None => (arg, None),
+        Some((kind, "-")) => (kind, Some(OutFileName::Stdout)),
+        Some((kind, path)) => (kind, Some(OutFileName::Real(PathBuf::from(path)))),
+    }
+}
+
 fn should_override_cgus_and_disable_thinlto(
     handler: &EarlyErrorHandler,
     output_types: &OutputTypes,
@@ -2091,41 +2112,49 @@ fn collect_print_requests(
 ) -> Vec<PrintRequest> {
     let mut prints = Vec::<PrintRequest>::new();
     if cg.target_cpu.as_ref().is_some_and(|s| s == "help") {
-        prints.push(PrintRequest::TargetCPUs);
+        prints.push(PrintRequest { kind: PrintKind::TargetCPUs, out: OutFileName::Stdout });
         cg.target_cpu = None;
     };
     if cg.target_feature == "help" {
-        prints.push(PrintRequest::TargetFeatures);
+        prints.push(PrintRequest { kind: PrintKind::TargetFeatures, out: OutFileName::Stdout });
         cg.target_feature = String::new();
     }
 
-    const PRINT_REQUESTS: &[(&str, PrintRequest)] = &[
-        ("crate-name", PrintRequest::CrateName),
-        ("file-names", PrintRequest::FileNames),
-        ("sysroot", PrintRequest::Sysroot),
-        ("target-libdir", PrintRequest::TargetLibdir),
-        ("cfg", PrintRequest::Cfg),
-        ("calling-conventions", PrintRequest::CallingConventions),
-        ("target-list", PrintRequest::TargetList),
-        ("target-cpus", PrintRequest::TargetCPUs),
-        ("target-features", PrintRequest::TargetFeatures),
-        ("relocation-models", PrintRequest::RelocationModels),
-        ("code-models", PrintRequest::CodeModels),
-        ("tls-models", PrintRequest::TlsModels),
-        ("native-static-libs", PrintRequest::NativeStaticLibs),
-        ("stack-protector-strategies", PrintRequest::StackProtectorStrategies),
-        ("target-spec-json", PrintRequest::TargetSpec),
-        ("all-target-specs-json", PrintRequest::AllTargetSpecs),
-        ("link-args", PrintRequest::LinkArgs),
-        ("split-debuginfo", PrintRequest::SplitDebuginfo),
-        ("deployment-target", PrintRequest::DeploymentTarget),
+    const PRINT_KINDS: &[(&str, PrintKind)] = &[
+        ("crate-name", PrintKind::CrateName),
+        ("file-names", PrintKind::FileNames),
+        ("sysroot", PrintKind::Sysroot),
+        ("target-libdir", PrintKind::TargetLibdir),
+        ("cfg", PrintKind::Cfg),
+        ("calling-conventions", PrintKind::CallingConventions),
+        ("target-list", PrintKind::TargetList),
+        ("target-cpus", PrintKind::TargetCPUs),
+        ("target-features", PrintKind::TargetFeatures),
+        ("relocation-models", PrintKind::RelocationModels),
+        ("code-models", PrintKind::CodeModels),
+        ("tls-models", PrintKind::TlsModels),
+        ("native-static-libs", PrintKind::NativeStaticLibs),
+        ("stack-protector-strategies", PrintKind::StackProtectorStrategies),
+        ("target-spec-json", PrintKind::TargetSpec),
+        ("all-target-specs-json", PrintKind::AllTargetSpecs),
+        ("link-args", PrintKind::LinkArgs),
+        ("split-debuginfo", PrintKind::SplitDebuginfo),
+        ("deployment-target", PrintKind::DeploymentTarget),
     ];
 
+    // We disallow reusing the same path in multiple prints, such as `--print
+    // cfg=output.txt --print link-args=output.txt`, because outputs are printed
+    // by disparate pieces of the compiler, and keeping track of which files
+    // need to be overwritten vs appended to is annoying.
+    let mut printed_paths = FxHashSet::default();
+
     prints.extend(matches.opt_strs("print").into_iter().map(|req| {
-        match PRINT_REQUESTS.iter().find(|&&(name, _)| name == req) {
-            Some((_, PrintRequest::TargetSpec)) => {
+        let (req, out) = split_out_file_name(&req);
+
+        let kind = match PRINT_KINDS.iter().find(|&&(name, _)| name == req) {
+            Some((_, PrintKind::TargetSpec)) => {
                 if unstable_opts.unstable_options {
-                    PrintRequest::TargetSpec
+                    PrintKind::TargetSpec
                 } else {
                     handler.early_error(
                         "the `-Z unstable-options` flag must also be passed to \
@@ -2133,9 +2162,9 @@ fn collect_print_requests(
                     );
                 }
             }
-            Some((_, PrintRequest::AllTargetSpecs)) => {
+            Some((_, PrintKind::AllTargetSpecs)) => {
                 if unstable_opts.unstable_options {
-                    PrintRequest::AllTargetSpecs
+                    PrintKind::AllTargetSpecs
                 } else {
                     handler.early_error(
                         "the `-Z unstable-options` flag must also be passed to \
@@ -2143,16 +2172,28 @@ fn collect_print_requests(
                     );
                 }
             }
-            Some(&(_, print_request)) => print_request,
+            Some(&(_, print_kind)) => print_kind,
             None => {
                 let prints =
-                    PRINT_REQUESTS.iter().map(|(name, _)| format!("`{name}`")).collect::<Vec<_>>();
+                    PRINT_KINDS.iter().map(|(name, _)| format!("`{name}`")).collect::<Vec<_>>();
                 let prints = prints.join(", ");
                 handler.early_error(format!(
                     "unknown print request `{req}`. Valid print requests are: {prints}"
                 ));
             }
+        };
+
+        let out = out.unwrap_or(OutFileName::Stdout);
+        if let OutFileName::Real(path) = &out {
+            if !printed_paths.insert(path.clone()) {
+                handler.early_error(format!(
+                    "cannot print multiple outputs to the same path: {}",
+                    path.display(),
+                ));
+            }
         }
+
+        PrintRequest { kind, out }
     }));
 
     prints
diff --git a/compiler/rustc_session/src/errors.rs b/compiler/rustc_session/src/errors.rs
index 4a3e668da11..dd15ad45145 100644
--- a/compiler/rustc_session/src/errors.rs
+++ b/compiler/rustc_session/src/errors.rs
@@ -164,6 +164,13 @@ pub struct FileIsNotWriteable<'a> {
 }
 
 #[derive(Diagnostic)]
+#[diag(session_file_write_fail)]
+pub(crate) struct FileWriteFail<'a> {
+    pub path: &'a std::path::Path,
+    pub err: String,
+}
+
+#[derive(Diagnostic)]
 #[diag(session_crate_name_does_not_match)]
 pub struct CrateNameDoesNotMatch {
     #[primary_span]