Auto merge of #139949 - matthiaskrgr:rollup-pxc5tsx, r=matthiaskrgr

Rollup of 8 pull requests

Successful merges:

 - #138632 (Stabilize `cfg_boolean_literals`)
 - #139416 (unstable book; document `macro_metavar_expr_concat`)
 - #139782 (Consistent with treating Ctor Call as Struct in liveness analysis)
 - #139885 (document RUSTC_BOOTSTRAP, RUSTC_OVERRIDE_VERSION_STRING, and -Z allow-features in the unstable book)
 - #139904 (Explicitly annotate edition for `unpretty=expanded` and `unpretty=hir` tests)
 - #139932 (transmutability: Refactor tests for simplicity)
 - #139944 (Move eager translation to a method on Diag)
 - #139948 (git: ignore `60600a6fa4` for blame purposes)

r? `@ghost`
`@rustbot` modify labels: rollup

src/doc/unstable-book/src/compiler-flags/allow-features.md

@@ -0,0 +1,14 @@
+# `allow-features`
+
+This feature is perma-unstable and has no tracking issue.
+
+----
+
+This flag allows limiting the features which can be enabled with `#![feature(...)]` attributes.
+By default, all features are allowed on nightly and no features are allowed on stable or beta (but see [`RUSTC_BOOTSTRAP`]).
+
+Features are comma-separated, for example `-Z allow-features=ffi_pure,f16`.
+If the flag is present, any feature listed will be allowed and any feature not listed will be disallowed.
+Any unrecognized feature is ignored.
+
+[`RUSTC_BOOTSTRAP`]: ./rustc-bootstrap.html

src/doc/unstable-book/src/compiler-flags/rustc-bootstrap.md

@@ -0,0 +1,56 @@
+# `RUSTC_BOOTSTRAP`
+
+This feature is perma-unstable and has no tracking issue.
+
+----
+
+The `RUSTC_BOOTSTRAP` environment variable tells rustc to act as if it is a nightly compiler;
+in particular, it allows `#![feature(...)]` attributes and `-Z` flags even on the stable release channel.
+
+Setting `RUSTC_BOOTSTRAP=1` instructs rustc to enable this for all crates.
+Setting `RUSTC_BOOTSTRAP=crate_name` instructs rustc to only apply this to crates named `crate_name`.
+Setting `RUSTC_BOOTSTRAP=-1` instructs rustc to act as if it is a stable compiler, even on the nightly release channel.
+Cargo disallows setting `cargo::rustc-env=RUSTC_BOOTSTRAP` in build scripts.
+Build systems can limit the features they enable with [`-Z allow-features=feature1,feature2`][Z-allow-features].
+Crates can fully opt out of unstable features by using [`#![forbid(unstable_features)]`][unstable-features] at the crate root (or any other way of enabling lints, such as `-F unstable-features`).
+
+[Z-allow-features]: ./allow-features.html
+[unstable-features]: ../../rustc/lints/listing/allowed-by-default.html#unstable-features
+
+## Why does this environment variable exist?
+
+`RUSTC_BOOTSTRAP`, as the name suggests, is used for bootstrapping the compiler from an earlier version.
+In particular, nightly is built with beta, and beta is built with stable.
+Since the standard library and compiler both use unstable features, `RUSTC_BOOTSTRAP` is required so that we can use the previous version to build them.
+
+## Why is this environment variable so easy to use for people not in the rust project?
+
+Originally, `RUSTC_BOOTSTRAP` required passing in a hash of the previous compiler version, to discourage using it for any purpose other than bootstrapping.
+That constraint was later relaxed; see <https://github.com/rust-lang/rust/issues/36548> for the discussion that happened at that time.
+
+People have at various times proposed re-adding the technical constraints.
+However, doing so is extremely disruptive for several major projects that we very much want to keep using the latest stable toolchain version, such as Firefox, Rust for Linux, and Chromium.
+We continue to allow `RUSTC_BOOTSTRAP` until we can come up with an alternative that does not disrupt our largest constituents.
+
+## Stability policy
+
+Despite being usable on stable, this is an unstable feature.
+Like any other unstable feature, we reserve the right to change or remove this feature in the future, as well as any other unstable feature that it enables.
+Using this feature opts you out of the normal stability/backwards compatibility guarantee of stable.
+
+Although we do not take technical measures to prevent it from being used, we strongly discourage using this feature.
+If at all possible, please contribute to stabilizing the features you care about instead of bypassing the Rust project's stability policy.
+
+For library crates, we especially discourage the use of this feature.
+The crates depending on you do not know that you use this feature, have little recourse if it breaks, and can be used in contexts that are hard to predict.
+
+For libraries that do use this feature, please document the versions you support (including a *maximum* as well as minimum version), and a mechanism to disable it.
+If you do not have a mechanism to disable the use of `RUSTC_BOOTSTRAP`, consider removing its use altogether, such that people can only use your library if they are already using a nightly toolchain.
+This leaves the choice of whether to opt-out of Rust's stability guarantees up to the end user building their code.
+
+## History
+
+- [Allowed without a hash](https://github.com/rust-lang/rust/pull/37265) ([discussion](https://github.com/rust-lang/rust/issues/36548))
+- [Extended to crate names](https://github.com/rust-lang/rust/pull/77802) ([discussion](https://github.com/rust-lang/cargo/issues/7088))
+- [Disallowed for build scripts](https://github.com/rust-lang/cargo/pull/9181) ([discussion](https://github.com/rust-lang/compiler-team/issues/350))
+- [Extended to emulate stable](https://github.com/rust-lang/rust/pull/132993) ([discussion](https://github.com/rust-lang/rust/issues/123404))

src/doc/unstable-book/src/compiler-flags/rustc-override-version-string.md

@@ -0,0 +1,39 @@
+# `RUSTC_OVERRIDE_VERSION_STRING`
+
+This feature is perma-unstable and has no tracking issue.
+
+----
+
+The `RUSTC_OVERRIDE_VERSION_STRING` environment variable overrides the version reported by `rustc --version`. For example:
+
+```console
+$ rustc --version
+rustc 1.87.0-nightly (43f0014ef 2025-03-25)
+$ env RUSTC_OVERRIDE_VERSION_STRING=1.81.0-nightly rustc --version
+rustc 1.81.0-nightly
+```
+
+Note that the version string is completely overwritten; i.e. rustc discards commit hash and commit date information unless it is explicitly included in the environment variable. The string only applies to the "release" part of the version; for example:
+```console
+$ RUSTC_OVERRIDE_VERSION_STRING="1.81.0-nightly (aaaaaaaaa 2025-03-22)" rustc -vV
+rustc 1.81.0-nightly (aaaaaaaaa 2025-03-22)
+binary: rustc
+commit-hash: 43f0014ef0f242418674f49052ed39b70f73bc1c
+commit-date: 2025-03-25
+host: x86_64-unknown-linux-gnu
+release: 1.81.0-nightly (aaaaaaaaa 2025-03-22)
+LLVM version: 20.1.1
+```
+
+Note here that `commit-hash` and `commit-date` do not match the values in the string, and `release` includes the fake hash and date.
+
+This variable has no effect on whether or not unstable features are allowed to be used. It only affects the output of `--version`.
+
+## Why does this environment variable exist?
+
+Various library crates have incomplete or incorrect feature detection.
+This environment variable allows bisecting crates that do incorrect detection with `version_check::supports_feature`.
+
+This is not intended to be used for any other case (and, except for bisection, is not particularly useful).
+
+See <https://github.com/rust-lang/rust/pull/124339> for further discussion.

src/doc/unstable-book/src/language-features/cfg-boolean-literals.md

@@ -1,22 +0,0 @@
-# `cfg_boolean_literals`
-
-The tracking issue for this feature is: [#131204]
-
-[#131204]: https://github.com/rust-lang/rust/issues/131204
-
-------------------------
-
-The `cfg_boolean_literals` feature makes it possible to use the `true`/`false`
-literal as cfg predicate. They always evaluate to true/false respectively.
-
-## Examples
-
-```rust
-#![feature(cfg_boolean_literals)]
-
-#[cfg(true)]
-const A: i32 = 5;
-
-#[cfg(all(false))]
-const A: i32 = 58 * 89;
-```

src/doc/unstable-book/src/language-features/macro-metavar-expr-concat.md

@@ -0,0 +1,133 @@
+# `macro_metavar_expr_concat`
+
+The tracking issue for this feature is: [#124225]
+
+------------------------
+
+In stable Rust, there is no way to create new identifiers by joining identifiers to literals or other identifiers without using procedural macros such as [`paste`].
+ `#![feature(macro_metavar_expr_concat)]` introduces a way to do this, using the concat metavariable expression.
+
+> This feature uses the syntax from [`macro_metavar_expr`] but is otherwise
+> independent. It replaces the old unstable feature [`concat_idents`].
+
+> This is an experimental feature; it and its syntax will require a RFC before stabilization.
+
+
+### Overview
+
+`#![feature(macro_metavar_expr_concat)]` provides the `concat` metavariable expression for creating new identifiers:
+
+```rust
+#![feature(macro_metavar_expr_concat)]
+
+macro_rules! create_some_structs {
+    ($name:ident) => {
+        pub struct ${ concat(First, $name) };
+        pub struct ${ concat(Second, $name) };
+        pub struct ${ concat(Third, $name) };
+    }
+}
+
+create_some_structs!(Thing);
+```
+
+This macro invocation expands to:
+
+```rust
+pub struct FirstThing;
+pub struct SecondThing;
+pub struct ThirdThing;
+```
+
+### Syntax
+
+This feature builds upon the metavariable expression syntax `${ .. }` as specified in [RFC 3086] ([`macro_metavar_expr`]).
+ `concat` is available like `${ concat(items) }`, where `items` is a comma separated sequence of idents and/or literals.
+
+### Examples
+
+#### Create a function or method with a concatenated name
+
+```rust
+#![feature(macro_metavar_expr_concat)]
+
+macro_rules! make_getter {
+    ($name:ident, $field: ident, $ret:ty) => {
+        impl $name {
+            pub fn ${ concat(get_, $field) }(&self) -> &$ret {
+                &self.$field
+            }
+        }
+    }
+}
+
+pub struct Thing {
+    description: String,
+}
+
+make_getter!(Thing, description, String);
+```
+
+This expands to:
+
+```rust
+pub struct Thing {
+    description: String,
+}
+
+impl Thing {
+    pub fn get_description(&self) -> &String {
+        &self.description
+    }
+}
+```
+
+#### Create names for macro generated tests
+
+```rust
+#![feature(macro_metavar_expr_concat)]
+
+macro_rules! test_math {
+    ($integer:ident) => {
+        #[test]
+        fn ${ concat(test_, $integer, _, addition) } () {
+            let a: $integer = 73;
+            let b: $integer = 42;
+            assert_eq!(a + b, 115)
+        }
+
+        #[test]
+        fn ${ concat(test_, $integer, _, subtraction) } () {
+            let a: $integer = 73;
+            let b: $integer = 42;
+            assert_eq!(a - b, 31)
+        }
+    }
+}
+
+test_math!(i32);
+test_math!(u64);
+test_math!(u128);
+```
+
+Running this returns the following output:
+
+```text
+running 6 tests
+test test_i32_subtraction ... ok
+test test_i32_addition ... ok
+test test_u128_addition ... ok
+test test_u128_subtraction ... ok
+test test_u64_addition ... ok
+test test_u64_subtraction ... ok
+
+test result: ok. 6 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
+```
+
+[`paste`]: https://crates.io/crates/paste
+[RFC 3086]: https://rust-lang.github.io/rfcs/3086-macro-metavar-expr.html
+[`concat_idents!`]: https://doc.rust-lang.org/nightly/std/macro.concat_idents.html
+[`macro_metavar_expr`]: ../language-features/macro-metavar-expr.md
+[`concat_idents`]: ../library-features/concat-idents.md
+[#124225]: https://github.com/rust-lang/rust/issues/124225
+[declarative macros]: https://doc.rust-lang.org/stable/reference/macros-by-example.html

src/doc/unstable-book/src/language-features/macro-metavar-expr.md

@@ -0,0 +1,10 @@
+# `macro_metavar_expr`
+
+The tracking issue for this feature is: [#83527]
+
+------------------------
+
+> This feature is not to be confused with [`macro_metavar_expr_concat`].
+
+[`macro_metavar_expr_concat`]: ./macro-metavar-expr-concat.md
+[#83527]: https://github.com/rust-lang/rust/issues/83527

src/doc/unstable-book/src/library-features/concat-idents.md

@@ -6,6 +6,8 @@ The tracking issue for this feature is: [#29599]
 
 ------------------------
 
+> This feature is expected to be superseded by [`macro_metavar_expr_concat`](../language-features/macro-metavar-expr-concat.md).
+
 The `concat_idents` feature adds a macro for concatenating multiple identifiers
 into one identifier.