Fix cast_possible_truncation should not suggest inside const context (#15164)

Closes #15163

This lint should not give suggestions in const context for now, since a
lot of the commonly used method of `Result` is not const.

changelog: [`cast_possible_truncation`] fix improperly give suggestions
inside const context

Author: samueltardieu

clippy_lints/src/casts/cast_possible_truncation.rs

@@ -3,7 +3,7 @@ use clippy_utils::diagnostics::{span_lint, span_lint_and_then};
 use clippy_utils::source::snippet;
 use clippy_utils::sugg::Sugg;
 use clippy_utils::ty::{get_discriminant_value, is_isize_or_usize};
-use clippy_utils::{expr_or_init, sym};
+use clippy_utils::{expr_or_init, is_in_const_context, sym};
 use rustc_abi::IntegerType;
 use rustc_errors::{Applicability, Diag};
 use rustc_hir::def::{DefKind, Res};
@@ -168,7 +168,9 @@ pub(super) fn check(
 
     span_lint_and_then(cx, CAST_POSSIBLE_TRUNCATION, expr.span, msg, |diag| {
         diag.help("if this is intentional allow the lint with `#[allow(clippy::cast_possible_truncation)]` ...");
-        if !cast_from.is_floating_point() {
+        // TODO: Remove the condition for const contexts when `try_from` and other commonly used methods
+        // become const fn.
+        if !is_in_const_context(cx) && !cast_from.is_floating_point() {
             offer_suggestion(cx, expr, cast_expr, cast_to_span, diag);
         }
     });

tests/ui/cast_size.32bit.stderr

@@ -177,6 +177,14 @@ error: casting `usize` to `f64` causes a loss of precision on targets with 64-bi
 LL |     9_999_999_999_999_999usize as f64;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+error: casting `usize` to `u16` may truncate the value
+  --> tests/ui/cast_size.rs:71:20
+   |
+LL |     const N: u16 = M as u16;
+   |                    ^^^^^^^^
+   |
+   = help: if this is intentional allow the lint with `#[allow(clippy::cast_possible_truncation)]` ...
+
 error: literal out of range for `usize`
   --> tests/ui/cast_size.rs:63:5
    |
@@ -186,5 +194,5 @@ LL |     9_999_999_999_999_999usize as f64;
    = note: the literal `9_999_999_999_999_999usize` does not fit into the type `usize` whose range is `0..=4294967295`
    = note: `#[deny(overflowing_literals)]` on by default
 
-error: aborting due to 19 previous errors
+error: aborting due to 20 previous errors
 

tests/ui/cast_size.64bit.stderr

@@ -177,5 +177,13 @@ error: casting `usize` to `f64` causes a loss of precision on targets with 64-bi
 LL |     9_999_999_999_999_999usize as f64;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 18 previous errors
+error: casting `usize` to `u16` may truncate the value
+  --> tests/ui/cast_size.rs:71:20
+   |
+LL |     const N: u16 = M as u16;
+   |                    ^^^^^^^^
+   |
+   = help: if this is intentional allow the lint with `#[allow(clippy::cast_possible_truncation)]` ...
+
+error: aborting due to 19 previous errors
 

tests/ui/cast_size.rs

@@ -65,3 +65,9 @@ fn main() {
     //~[32bit]^^ ERROR: literal out of range for `usize`
     // 999_999_999_999_999_999_999_999_999_999u128 as f128;
 }
+
+fn issue15163() {
+    const M: usize = 100;
+    const N: u16 = M as u16;
+    //~^ cast_possible_truncation
+}