Add SEMICOLON_IN_EXPRESSIONS_FROM_MACROS lint

cc #79813

This PR adds an allow-by-default future-compatibility lint
`SEMICOLON_IN_EXPRESSIONS_FROM_MACROS`. It fires when a trailing semicolon in a
macro body is ignored due to the macro being used in expression
position:

```rust
macro_rules! foo {
    () => {
        true; // WARN
    }
}

fn main() {
    let val = match true {
        true => false,
        _ => foo!()
    };
}
```

The lint takes its level from the macro call site, and
can be allowed for a particular macro by adding
`#[allow(semicolon_in_expressions_from_macros)]`.

The lint is set to warn for all internal rustc crates (when being built
by a stage1 compiler). After the next beta bump, we can enable
the lint for the bootstrap compiler as well.

src/bootstrap/bootstrap.py

@@ -833,6 +833,7 @@ class RustBuild(object):
         target_linker = self.get_toml("linker", build_section)
         if target_linker is not None:
             env["RUSTFLAGS"] += " -C linker=" + target_linker
+        # cfg(bootstrap): Add `-Wsemicolon_in_expressions_from_macros` after the next beta bump
         env["RUSTFLAGS"] += " -Wrust_2018_idioms -Wunused_lifetimes"
         if self.get_toml("deny-warnings", "rust") != "false":
             env["RUSTFLAGS"] += " -Dwarnings"

src/bootstrap/builder.rs

@@ -1250,6 +1250,12 @@ impl<'a> Builder<'a> {
             // some code doesn't go through this `rustc` wrapper.
             lint_flags.push("-Wrust_2018_idioms");
             lint_flags.push("-Wunused_lifetimes");
+            // cfg(bootstrap): unconditionally enable this warning after the next beta bump
+            // This is currently disabled for the stage1 libstd, since build scripts
+            // will end up using the bootstrap compiler (which doesn't yet support this lint)
+            if compiler.stage != 0 && mode != Mode::Std {
+                lint_flags.push("-Wsemicolon_in_expressions_from_macros");
+            }
 
             if self.config.deny_warnings {
                 lint_flags.push("-Dwarnings");

src/test/ui/lint/semicolon-in-expressions-from-macros/allow-semicolon-in-expressions-from-macros.rs

@@ -0,0 +1,15 @@
+// check-pass
+// Ensure that trailing semicolons are allowed by default
+
+macro_rules! foo {
+    () => {
+        true;
+    }
+}
+
+fn main() {
+    let val = match true {
+        true => false,
+        _ => foo!()
+    };
+}

src/test/ui/lint/semicolon-in-expressions-from-macros/semicolon-in-expressions-from-macros.rs

@@ -0,0 +1,30 @@
+// check-pass
+#![warn(semicolon_in_expressions_from_macros)]
+
+#[allow(dead_code)]
+macro_rules! foo {
+    ($val:ident) => {
+        true; //~ WARN trailing
+              //~| WARN this was previously
+              //~| WARN trailing
+              //~| WARN this was previously
+    }
+}
+
+fn main() {
+    // This `allow` doesn't work
+    #[allow(semicolon_in_expressions_from_macros)]
+    let _ = {
+        foo!(first)
+    };
+
+    // This 'allow' doesn't work either
+    #[allow(semicolon_in_expressions_from_macros)]
+    let _ = foo!(second);
+
+    // But this 'allow' does
+    #[allow(semicolon_in_expressions_from_macros)]
+    fn inner() {
+        let _ = foo!(third);
+    }
+}

src/test/ui/lint/semicolon-in-expressions-from-macros/semicolon-in-expressions-from-macros.stderr

@@ -0,0 +1,33 @@
+warning: trailing semicolon in macro used in expression position
+  --> $DIR/semicolon-in-expressions-from-macros.rs:7:13
+   |
+LL |         true;
+   |             ^
+...
+LL |         foo!(first)
+   |         ----------- in this macro invocation
+   |
+note: the lint level is defined here
+  --> $DIR/semicolon-in-expressions-from-macros.rs:2:9
+   |
+LL | #![warn(semicolon_in_expressions_from_macros)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #79813 <https://github.com/rust-lang/rust/issues/79813>
+   = note: this warning originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+warning: trailing semicolon in macro used in expression position
+  --> $DIR/semicolon-in-expressions-from-macros.rs:7:13
+   |
+LL |         true;
+   |             ^
+...
+LL |     let _ = foo!(second);
+   |             ------------ in this macro invocation
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #79813 <https://github.com/rust-lang/rust/issues/79813>
+   = note: this warning originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+warning: 2 warnings emitted
+